United States Patent 3610800

In an electronic organ, the actuation of keys in accordance with corresponding, audible tones to be reproduced effects the gating of pulses into time slots of a time decision multiplexed signal, the time slots of the multiplexed signal being structured in accordance with a desired assignment sequence to correspond to the keys and to be representative thereof for identifying each note capable of being generated by the organ. A set of note, or tone, generators with availability assignment control means for capturing a pulse in the multiplexed signal are each rendered responsive to a given captured pulse for generating the tone represented by that pulse. Automatic transposition of notes, by a specified number of half steps higher or lower than the note played, is selectively effected by a time shift of pulses in the multiplexed signal by one time slot per half note to be transposed. In this manner, when an organist plays a musical selection in an original musical key, the organ produces the audible musical output in the selected, transposed musical key.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
84/657, 984/323, 984/332, 984/338, 984/392
International Classes:
G06F1/02; G06F1/03; G10H1/057; G10H1/18; G10H1/20; G10H7/04; (IPC1-7): G10H1/18; G10H1/057; G10H1/20; G10H7/02; G06F1/03; G10H7/04; G06F1/02; G01h001/02
Field of Search:
View Patent Images:
US Patent References:
3515792DIGITAL ORGAN1970-06-02Deutsch
3482027AUTOMATIC RHYTHM INSTRUMENT1969-12-02Okamoto et al.
3383452Musical instrument1968-05-14Park et al.
3358068Automatic rhythm device1967-12-12Campbell
3316341Electrical musical instruments1967-04-25Peterson
3255296Player controlled dynamic variation of pitch and/or timbre1966-06-07Peterson
3007362Combination random-probability system1961-11-07Olson et al.
2989885Electronic musical instrument and method1961-06-27Pearson
2855816Music synthesizer1958-10-14Olson et al.
2601265Electronic musical instrument1952-06-24Davis

Primary Examiner:
Hirshfield, Milton O.
Assistant Examiner:
Witkowski, Stanley J.
What is claimed is

1. An electronic musical instrument, comprising:

2. The electronic musical instrument according to claim 1 wherein successive note assignment positions of the digital signal correspond to notes of successive, different pitches, and wherein said transposing controlling means selectively shifts the note assignments corresponding to the actuated keys through a desired number of positions in said signal in accordance with the amount of transposition desired.

3. The electronic musical instrument according to claim 2 wherein said transposing controlling means selectively shifts all of said note assignments an equal number of positions in said signal for any selected amount of transposition.

4. The electronic musical instrument according to claim 3 wherein said transposing controlling means effects an appropriate delay of said digital signal relative to a predetermined reference time with which said signal is normally synchronized to introduce the desired transposition shift.

5. The electronic musical instrument according to claim 2 wherein there is further provided means for selectively introducing assignments of notes into said digital signal in an octave differing from the octave for the notes actually called for by actuation of said keys, to produce those notes otherwise outside the range of note generation of said instrument as a result of the amount of transposition selected, in that different octave and thus within the said range.

6. The electronic musical instrument according to claim 5 wherein said means for selectively introducing assignments of notes into a different octave produces a displacement of only one octave from the octave for the corresponding notes actually called for by actuation of said keys.

7. The electronic musical instrument according to claim 6 wherein said one octave displacement is below the octave for the notes actually called for by actuation of said keys.

8. The electronic musical instrument according to claim 5 wherein said means for selectively introducing assignments of notes into a different octave comprises switch means for transferring signals representative of actuation of said keys which due to the transposition selected call for the generation of notes at a pitch exceeding the range of note generation of said instrument, to positions normally occupied by signals representative of the keying of the corresponding notes in said different octave lying within the range of note generation, to be detected during said scanning of said keys.

9. The electronic musical instrument according to claim 1 wherein said transposition controlling means is responsive to said transposition selection means for weighting the note assignments to which said note generating means is responsive by a factor calculated to introduce the amount of transposition desired in the generated note.

10. The electronic musical instrument according to claim 9 wherein said weighting factor is the common ratio of adjacent notes in a musical scale of equal temperament, said factor being varied in accordance with the number of semitones of transposition desired within said musical scale.

11. The electronic musical instrument according to claim 9 wherein there is further provided means for selectively introducing assignments of notes into said digital signal in an octave differing from the octave for the corresponding notes actually called for by actuation of said keys, to produce those notes otherwise outside the range of note generation of said instrument as a result of the amount of transposition selected in that different octave and thus within the said range.

12. The electronic musical instrument according to claim 11 wherein said means for selectively introducing assignments of notes into a different octave produces a displacement of only one octave from the octave actually called for by actuation of said keys.

13. The electronic musical instrument according to claim 12 wherein said means for selectively introducing assignments of notes into a different octave comprises switch means for transferring signals representative of actuation of said keys to positions normally occupied by signals representative of the keying of corresponding notes in said different octave, to be detected during said scanning of said keys.

14. An electronic musical instrument, comprising:

15. The invention defined by claim 14 wherein said instrument is a keyboard instrument.

16. The invention defined by claim 15 wherein said instrument is an organ.

17. The electronic musical instrument according to claim 14 wherein said transposition controlling means comprises means for selectively shifting the positions of said signals in said multiplexed waveform by a number of positions based on the amount of transposition desired.

18. The electronic musical instrument according to claim 14 wherein

19. An automatic transposition system for an electrical musical instrument for automatically and selectively transposing the notes selected by actuation of keys of that instrument to higher and lower pitches than those normally corresponding to the respectively actuated keys, comprising:

20. An automatic transposition system as recited in claim 19 wherein said transposition control means comprises:

21. An automatic transposition system as recited in claim 19 wherein there is further provided:

22. An automatic transposition system as recited in claim 19 wherein said note producing means includes:


1. Field of the Invention

This invention resides broadly in the field of electronic musical instruments and is particularly adaptable for use in an electronic organ as a digital selection system for calling forth desired tones from those available to be produced by the organ. The term "organ" is used throughout the specification and claims in a generic sense (as well as in a specific sense) to include any electronic musical instrument having a keyboard such as electronic organs, electric pianos and accordions, and the principles of the present invention are, in fact, applicable to any musical instrument in which musical sounds are generated in response to the actuation of key switches, regardless of whether those switches are actuated directly, i.e., by the musician's fingers, or indirectly, e.g., by the plucking of strings. The term "key" is also used in a generic sense, to include depressible levers, actuable on-off switches, touch- or proximity-responsive (e.g., capacitance- or inductance-operated) devices, closable apertures (e.g., a hole in a "keyboard" of holes which when covered by the musician's finger closes or opens a fluidic circuit to produce a tonal response), and so forth.

2. Description of the Prior Art

The function of an electronic organ is to faithfully reproduce, or to simulate by electronic means, the sounds or tones developed by a true pipe organ in response to playing of the organ by the organist in the selection of notes, and voices and other characteristics of those notes. It is frequently desirable to transpose the musical selection being played, whether on an electronic organ, an electronic piano, or any other electronic musical keyboard instrument, from one musical key to another, and for a skilled musician this may not present any great problem. The typical home organist, however, may play for his own personal enjoyment and usually does not possess the skill required to freely and comfortably transpose keys. Accordingly, it is desirable to provide the instrument with means for automatically transposing between musical keys, as preselected by the musician, while permitting the musician to play the musical piece by keying the instrument as though the piece were being played in the the original key.

Even the professional musician can be substantially assisted by an automatic transposition capability. Frequently, a singer will ask the accompanying musician to play a full tone higher or possibly one and one-half tones lower so that the range of a particular song can be adapted to the singer's range. The same transposition requirement may exist when a solo instrument such as a B-flat trumpet or clarinet is to be played with an organ or piano accompaniment, in cases where the selected musical piece has not been transcribed to sheet music to meet the requirements of the solo instrument range and is scored for a concert key instrument. One possible solution is for the soloist to play each note a full tone higher than it appears in the sheet music. A solution preferred by most soloists is for the organist or pianist to play each note a full tone lower. Clearly, the provision of automatic transposition in a keyboard instrument can solve such problems in a quick and efficient manner. The terminology "automatic transpostion" is used in a generic sense to denote a method by which the instrument itself can be preset to cause each key on the keyboard to sound a note frequency other than that normally associated with that key when the key is depressed. For example, the instrument may be implemented to cause each actuated key to sound a semitone higher, e.g., depressing the key for C4 actually causes C-4 to sound.

While the concept of automatic transposition for keyboard instruments dates back many years, the problems of cast and complexity of implementing ans servicing the prior art configurations for producing automatic transposition have discouraged any common usage of the concept. In one prior art arrangement a single manual organ having no pedal board was provided with keys resting on pegs which in turn controlled the airflow to the organ reeds. A lever was attached to the key mechanism in such a manner that shifting the lever to the right or left caused the keys to shift to the right or left respectively by a corresponding amount. Thus, by shifting the lever the distance of one tone on the keyboard, the entire keyboard was shifted to operate the pegs controlling tones one halftone higher or lower than was the case in the normal position of the keys. This arrangement, of course, was limited as to the amount of transposition available, and usually provided the organ with the capability to transpose up to 12 halftones. This device has not survived to the extent of use in modern instruments. One significant disadvantage was its lack of adaptability to the requirements of large organs or pianos. For example, there is apparently no simple method of expanding the scheme to simultaneously transpose keys throughout several keyboards of a single instrument, e.g., the manuals and pedal boards of an organ. Moreover, the arrangement requiring a physical sliding of keys from a normal position is an undesirable feature.

Another more recent automatic transposition scheme for larger organs requires individual switching of each of the key switches on each manual. Since a typical manual has 61 keys, a 61 pole, 12-position switch would be required for each manual. In addition, each of these manual switches must be ganged together for common operation, so that all manuals are transposed simultaneously. Here again, cost and complexity of the arrangement has resulted in infrequent use of the system.

In the copending application of G. A. Watson entitled "multiplexing System For Selection Of Notes And Voices In An Electronic Musical Instrument" filed on even date herewith, and assigned to the same assignee as the present invention, there is described an electronic organ in which every key of every keyboard is scanned in cyclic sequence. The actuation of a key or keys on any keyboard is entered as information in a parallel digital format developed by the scanning of the keyboards, the information indicating the order and combination, as well as each individual one, of the keys that have been actuated and deactuated. THis parallel format is continuously converted to a serial format to provide information regarding key actuation in the form of pulses in appropriate time slots of the time division multiplexed signal which is supplied to the tone generating section of the organ. Each time slot of the multiplexed signal is representative of a specific key to permit identification of the notes associated with the respective keys and thereby to result in the appropriate sounds being generated in response to the playing of the keys.

The tone generators which respond to the incoming multiplexed signal to bring forth the appropriate tones corresponding to those keys that have been actuated, in the order and combination of such actuation, produce digital amplitude samples of a waveform of the desired sound at a frequency corresponding to the desired note frequency.

Such an arrangement permits reduction of complexity that is usually found in electronic organs, and in particular permits elimination of a substantial number of wires and cables that are usually required between the keyboards and the tome generators. Furthermore, since the digital electronic organ of the aforementioned Watson application provides simple and efficient assignment of a small number of tone generators, relative to the number of keys available, to the keys which have been actuated, there is a further reduction in complexity of mapping the subset of depressed keys into the available tone generators, over conventional requirements. Still further, the digital electronic organ overcomes such difficulties as may occur when a key switch has faulty or dirty contacts, a situation that would ordinarily lead to intermittent electrical contact and discontinuity of tone. By use of a multiplexed signal, the presence of a pulse in a particular time slot of a repeating signal is sufficient to represent the actuation of the corresponding key, and less than perfect contact is required to produce that pulse.

Each of the limited number of tone generators provided in the digital electronic organ of the aforementioned Watson application is associated with generator assignment logic constructed and arranged to assign an available tone generator to an incoming pulse in the multiplexed signal which has not yet captured to tone generator. Each tone generator includes a memory means storing digital representations of amplitudes of the wave shape to be synthesized at a large number of sample points. When the tone generator is captured by a pulse, the memory means associated with that tone generator is accessed to read out amplitude samples in accordance with the frequency of the tone to be generated.

Effectively, each tone generator constitutes a master oscillator from which all of the musical frequencies encompassed by the organ are obtained (in digital format) by frequency synthesis. The present invention takes advantage of this fact to provide a relatively simple technique for implementing an automatic transposition system within the digital electronic organ, or other keyboard instrument, this being the primary object of the invention.


Briefly, in accordance with the present invention, musical keys are automatically transposed in an electronic digital keyboard instrument in which key switch operation (note selection) information is entered as respective pulses into preassigned time slots of a multiplexed signal, by shifting (or effectively shifting) the pulses by one time slot per semitone of desired transposition.

In one specific form of implementation, the original keyboard multiplexed signal or pulse train is subjected to delay to the extent necessary to provide the desired time shift and hence the desired transposition. For example, the multiplexed information may be applied serially to a 12-bit shift register which acts effectively as a 12-bit tapped delay line, each successive tap of the shift register constituting an additional one-bit delay so that a delay up to 12 bits in length (one octave) may be selected according to the tap from which the output is taken. Of course, only one tap, or output terminal, serves to provide an output for any given transposition, the entire multiplexed waveform emanating from that tap with the desired time shift.

The direction of shift, upward or downward in key, depends on the direction in which the keyboards are scanned to provide the multiplexed waveform. That is to say, the direction of shift depends on whether the scanning is from the lowest to the highest frequency for each keyboard or from the highest to the lowest frequency. It is immaterial to the present invention in which direction the keyboards are scanned, but if it is from the highest to the lowest frequency then clearly a 12-bit maximum delay will permit only a downward shift in key; that is, a transposition to notes of lower frequency. In a similar manner, scanning in the opposite direction will permit only an upward shift in key, for a 12-bit maximum delay. To obtain an upward shift in the highest to lowest frequency scanning arrangement, or what is effectively an upward shift, the delay may span almost, but not quite, a complete cycle of the multiplexed waveform. Specifically, the delay should extend to that time slot preceding, by a desired time shift, the time slot normally occupied by the pulse in question.

In a second specific implementation an approach is taken which is actually merely a substitution of operation in the frequency domain for operation in the time domain, by exercising the desired shift at the tone generation end of the organ electronics, rather than producing a shift of the pulses in the time division multiplexed waveform.


In describing the present invention, reference will be made to the accompanying Figures of drawings in which:

FIG. 1 is a simplified block diagram of a system for producing a time division multiplexed signal containing a recycling sequence of time slots each associated with a particular key of the organ and in which each time slot containing a pulse is indicative of the actuation of the associated key;

FIG. 2 is a circuit diagram of an exemplary decoder for use in the system of FIG. 1;

FIG. 3 is a more detailed circuit diagram of the switching array and encoder used in the system of FIG. 1;

FIG. 3A is a circuit diagram of an alternative encoder to that shown in FIG. 3, for use in the system of FIG. 1;

FIG. 4 is a circuit diagram of the input-output bus connecting means at each intersection of the switching array of FIG. 3;

FIG. 5 is illustrative of a multiplex waveform developed by the system of FIG. 1 is response to actuation of selected keys;

FIG. 6 is a simplified block diagram of generator assignment and tone generating apparatus for processing the multiplexed signal produced by the system of FIG. 1 to develop the desired tones as an audible output of the organ;

FIG. 7A and 7B together constitute a circuit diagram of one embodiment of the tone generator assignment logic for the system of FIG. 6;

FIG. 8 is a block diagram of tone generator suitable for synthesizing the frequency of every note capable of being played in the organ, for use with the assignment logic of FIGS. 7A and 7B in the system of FIG. 6;

FIG. 9 is illustrative of a complex waveshape of the type produced by a pipe organ, and of the sample points at which amplitude values are taken, for simulation at selected note frequencies;

FIG. 10 is a block diagram of an embodiment of an attack and decay control unit for use in an electronic digital musical instrument of the type shown and described with reference to the preceding figures of drawing;

FIG. 11 is a representation of the multiplexed signal showing the appearance of three consecutive note assignments in the repetitive signal;

FIG. 12 is a simplified block diagram of one embodiment of the present invention;

FIG. 13 is a more detailed circuit diagram of the embodiment of FIG. 12;

FIG. 14 is a circuit diagram of a system for providing octave folding during transposition; and

FIG. 15 is a block diagram of another embodiment of the invention for use in the tone generating apparatus of FIG. 8.


Referring to FIG. 1, the keyboard multiplexing system or note selection system includes a keyboard counter 1 which is implemented to provide a specified count for each key of each keyboard (including manuals and pedal divisions) of the organ. If, for example, the electronic organ in which the multiplexing system is used has four keyboards, such as three manuals and a pedal board, each encompassing up to eight octaves, then keyboard counter 1 should have the capability of generating 4×8×12=384 separate counts (digital words). It is essential that the counter be capable of developing a count representative of every key on every keyboard of the organ; however, it may be desirable to provide a counter that can produce a count greater than the number of available keys in order to have available certain redundant counts not associated with any keys. Such redundancy is readily provided by simply utilizing a counter of greater capacity than the minimum required count.

Keyboard counter 1 is divided into three separate sections (or separate counters) designated 2, 3 and 4. The first section (designated 2) is constructed to count modulo 12 so as to designate each of the 12 keys associated with the 12 notes in any octave. The second section (designated 3) is adapted to count modulo 8, to specify each of the eight octaves encompassed by any of the four keyboards. The last section (designated 4) is designed to count modulo 4 to specify each keyboard of the organ. Therefore, the overall keyboard counter is arranged to count modulo 384, in that at the conclusion of every 384 counts, the entire set of keyboards have been covered (scanned) and the count repeats itself. To that end, each counter section may be composed of a separate conventional ring counter, the three counters being connected in the typical cascaded configuration such that when section 2 reacts its maximum count it advances the count of counter section 3 by one, and will automatically initiate a repetition of its own count. Similarly, attainment of its maximum count by counter section 3 is accompanied by advancement of the count of section 4 by one.

Advancement of the count of counter 2 is accomplished by application of clock pluses thereto from a master clock source 5 which delivers clock pluses at a sufficiently rapid repetition rate (frequency) to ensure resolution of depression (actuation) and release (deactuation) of any key on any keyboard, i.e., to supply a pulse at the instant of either of these events. Scanning of all keyboards of the organ at a rate of 200 or more times a second is deemed quite adequate to obtain this desirable resolution. For the exemplary keyboard counter set forth above, this is equivalent to a minimum of 200×384=76,800 counts per second, so that a master clock delivering clock pulses at a rate of 100 kc./s. is quite suitable.

A total of four lines emanate from counter 4, one line connected to each ring counter stage, to permit sensing of the specific keyboard which is presently being scanned. Similarly, stages, respectively of octave counter 3 to detect the octave presently being scanned. Thus, a total of 12 lines extend from counters 3 and 4, and these twelve lines can carry signals indicative of 32 (8×4) possible states of the keyboard counter. The specific one of the 32 states, representative of a particular octave on a particular keyboard, which is presently being scanned is determined by use of a decoder circuit 7 composed of 32 AND gates designated 8-1, 8-2, 8-3, ..., 8-32 (FIG. 2), each with two input terminals and an output terminal. The gates are arranged in four groups of eight each, with every gate of a particular group having one of its two input terminals (ports) connected to one of the four lines of counter 4. Distinct and different ones of the eight lines from counter 3 are connected to the other input terminal of respective ones of the eight AND gates of that group. A corresponding situation exists for each group of AND gates, with the only difference being that each group is associated with a different output line of counter section 4. Using this arrangement, the decoder logic designates every octave of keys in the organ by a respective driver pulse when a count corresponding to that octave is presently contained in the counter.

The output pluses deriving from the AND gates (or drivers) of decoder circuit 7 are supplied on respective ones of 32 bus bars (or simply, buses), generally designated by reference numeral 10, to a keyboard switching array 11. From the preceding description, then, it will be clear that array 11 has one input bus 10 for every octave of keys in the organ (including every octave on every keyboard), and that a drive pulse will appear on each input bus approximately 200 times per second, the exemplary rate of scan of the keyboards, as noted above, for obtaining adequate resolution of operation of the keys. Switching array 11 also has 12 output buses, generally designated by reference number 12, each to be associated with a respective on of the 12 notes (and hence, the 12 keys) in any given octave.

Array 11 is basically a diode switching matrix, in which spaced input buses 10 and spaced output buses 12 are orthogonally arranged so that an intersection or crossing occurs between each input bus and each output bus (see FIG. 3), for a total of 384 intersections, one for each count of the keyboard counter 1. As is typical in this type of matrix, the crossed lines or buses are not directly interconnected. Instead, a "jump" diode, such as that designated by reference number 13 in FIG. 4, is connected between the input bus 10 and the output bus 12 at each intersection, the diode poled for forward conduction (anode-to-cathode) in the direction from an input bus 10 to an output bus 12. Wired in series circuit or series connection with each diode 13 is a respective switch 14 which is normally open circuited and is associated with a distinct respective one of the keys of the organ, such that depression of the associated key produces closure (close circuiting) of the switch 14 whereas release of the associated key results in return of the switch to its open state. Alternatively, each of switches 14 may itself constitute a respective key of the various keyboards of the organ.

While switch 14 is shown schematically as being of mechanical single pole, single throw (SPST) structure, it will be understood that any form of switch, electronic, electromechanical, electromagnetic, and so forth, may be utilized, the exact nature of the switch depending primarily upon the nature of the energization produced upon operation of the associated key. Switch 14, then, is adapted to respond to the particular form of energization or actuation produced upon operation of a key on any keyboard (or, as observed above, may itself constitute the key), to complete the circuit connecting associated diode 13 between a respective input bus 10 and a respective output bus 12 at the intersection of those buses, when the key is depressed, and to open the circuit connecting the diode between respective input and output buses at that intersection when the key is released. Positive pulses occurring at the rate of approximately 200 per second, for example, according to the timing established by master clock 5, are transferred from input bus 10 to output bus 12 via the respective diode 13 and closed switch 14 when the associated key is depressed. While a switch alone (i.e., without the series connected diode) would serve the basic purpose of transferring a signal between the input and output lines of array 11, the diode provides a greater degree of isolation form sources of possible interference (noise) and acts to prevent feedback from output to input lines.

In FIG. 3, the output buses 12 from switching array 11 are connected to an encoder circuit 15 to which are also connected the 12 output lines, generally designated by reference number 16, from keyboard counter section 2. To produce an orderly arrangement in which each key of the organ is assigned a distinct and different time slot in a time-division multiplex waveform, the switches 14 associated with the respective keys are conveniently arranged in a specific sequence in the switching array 11. Assume, for example, that a specific output bus 17 of the switching array is to be associated with note A of any octave, a second output bus 18 is to be associated with note B of any octave, and so forth. Then switches 14 in the row corresponding to output bus 17 in array or matrix 11 are associated with the keys corresponding to the note A in each octave of keys in the organ. The column position of each switch 14 in matrix 11 corresponds to a specific octave of keys in the organ, and hence, to a specific octave encompassed by a specific keyboard of the organ.

Each of the output buses 12, including 17, 18, and so forth, is connected to one of the two input ports or terminals of a respective AND gate of the 12 AND gates 20-1, 20-2, 20-3, ..., 20-12, of encoder circuit 15. An output lead 16 of counter section 2 associated with the ring counter stage designating the count for a particular note (key) in a given octave is connected to the remaining port of an encoder circuit AND gate having as its other input a pulse on the output bus 12 associated with that same note. A similar arrangement is provided for each of the remaining 11 output lines 16 of counter section 2 with respect to the AND gates 20 and the output buses 12. Thus, for example, if output bus 17 (associated with the row of switches 14 in matrix 11 for note A) is connected to one input terminal of AND gate 20-1, then output line 22 from the stage of counter 2 designating the count associated with note A is connected to the remaining input terminal of gate 20-1. The output terminal of each of AND gates 20 is connected to a respective input terminal of OR gate 23, the output of the OR gate constituting the output signal of the encoder circuit. By virtue of its structure, encoder circuit 15 is effective to convert the parallel output of array 11 to a serial output signal in accordance with the scanning of output buses 12 as provided by the advancing and repeating count sensed in the form of pulses (at a rate of about 200 per second) appearing on output lines 16. The end result of this circuitry is the production of a time-division multiplex (TDM) signal on a single conductor 25 emanating from encoder 15.

As an alternative to the specific logic construction shown for encoder 15 in FIG. 3, the encoder may have the circuit configuration exemplified by FIG. 3A. Referring to the latter Figure, the encoder includes a shift register 80 having 12 cascaded stages designated SR1, SR2, SR3, ..., SR12, each connected to a respective output bus 12 of switching matrix 11 to receive a respective output pulse appearing thereon. The shift register stages are loaded in parallel with the data read from switching array 11 on output buses 12, in response to each of the pulses appearing (i.e., each time a pulse appears) on one of the 12 output leads 16 of note counter 2. That one output of the note counter which is to supply the load command for all 12 stages of shift register 80 is selected to permit the maximum amount of settling time to elapse between each advance of octave counter 3 and keyboard counter 4 and the loading of the shift register. In other words, it is extremely desirable that the data to be entered into the shift register from the switching array be stabilized to the greatest possible extent, and this is achieved by allowing the counters whose scanning develops this data, to settle at least immediately prior to loading. Thus, the first note counter stage, or one of the early stages, is selected to provide "load" pulses to shift register 80.

"Shift" pulses are supplied to the shift register by master clock 5, which also supplies note counter 2, to shift the contents of each shift register stage to the next succeeding stage except during those bit times when the shift pulse is preempted by a load pulse from the note counter. Accordingly, shift register 80 is parallel loaded, and the data contents of the register are then shifted out of the register in serial format on encoder output line 25 until a one-bit pause occurs when another set of data is parallel loaded into the shift register, followed again by serial readout on line 25. This serial pulse train constitutes the time division multiplexed output signal of encoder 15 just as in the embodiment of FIG. 3, except that with the FIG. 3A configuration, decoder 7 (and the counters 3 and 4 supplying pulses thereto) undergo a greater amount of settling time.

It will be observed that this operation constitutes a parallel-to-serial conversion of the information on output buses 12 to a time-division multiplexed waveform on the output line 25 of encoder 15.

In the TDM signal, each key has a designated time slot in the 384 time slots constituting one complete scan of every keyboard of the organ. In the specific example of the time base provided by master clock 5, the TDM waveform (shown by way of example in FIG. 5) is initiated about 200 times per second. The waveform contains all of the note selection information, in serial digital form on a single output line, that had heretofore required complex wiring arrangements. This waveform development will be more clearly understood from an example of the operation of the circuitry thus far discussed. It should be observed first, however, that all of the counter and logic circuitry described up to this point can be accommodated within a very small volume of space by fabrication in integrated circuit form using conventional microelectronic manufacturing techniques.

When the main power switch for the electronic organ is turned on, all components are energized to an operational state, the master clock delivering pulses to keyboard counter 1 at the aforementioned rate. Upon depression of a key on any keyboard of the organ, including the manuals and pedal divisions, a respective switch 14 associated in series connection with a diode 13 at the intersection between the appropriate input bus 10 and output bus 12 of the switching array 11 is closed, thereby connecting the two buses to supply pulses appearing on a given bus 10 from decoder 7, to the appropriately connected output bus 12 for application to encoder 15. If, for example, the key that was depressed is associated with note C in the second octave, C2 appears in the appropriate time slot of the multiplexed signal emanating from encoder 15 and will repetitively appear in that time slot in each scan of the keyboards of the organ as long as that key is depressed. That is to say, a pulse appears on output line 10 of decoder 7 associated with the second octave in the manual being played, in accordance with the scan provided by master clock 5, as the counter stage associated with that octave is energized in keyboard counter octave section 3 and the counter stage associated with that manual is energized in section 4 of the keyboard counter. The connection between the appropriate input bus 10 and output bus 12 of switching array 11 for the particular octave and keyboard under consideration is effected by the depression and continued operation of the key associated with the switch 14 for that intersection in the array. Since, as previously stated, each switch is associated with a particular note (key) and is positioned in a specific row of the switching array, a signal level is thereby supplied to the appropriate output bus 12 of the switching array arranged to be associated with that note. Each time the specified note, here the note C, is scanned in the sequence of count in the note section 2 of the keyboard counter, a second input is provided to the AND gate 20 receiving the signal level on output bus 12, and a pulse is delivered to OR gate 23. By virtue of this operation, the pulse which appears at the output of OR gate 23 always appears in the identical specified time slot in the multiplexed signal for a specific note associated with a particular key on a particular keyboard of the organ.

If more than one key is depressed, regardless of whether one or more keyboards is involved, operation corresponding to that described above for a single depressed key is effected for every operated key. Thus, for example, assume that the key associated with note C2 is played on one manual, the note B4 is played on a second manual, and the notes D5, E5, and G5 are played on a third manual, the associated keys being depressed substantially simultaneously to produce desired simultaneous reproduction of all notes as the audio output of the organ. Under these conditions, the associated switches 14 in the switching array 11 are closed to provide through connections between the respective input buses 10 and output buses 12 for the specific octaves and manuals involved. As the appropriate AND gates 20 in encoder 15 are supplied with gating signals from the sequentially energized counter stages of note section 2, during the scanning operation provided by that keyboard counter section, pulse levels appearing on output buses 12 for which switches 14 have been closed are gated in the appropriate time slots of the multiplex signal on the output lead 25 from OR gate 23 of encoder 15, for the specific notes involved.

An example of the multiplex signal waveform thus generated is shown in FIG. 5. While the pulses appearing in the time slots associated with the specific notes mentioned above are in a serial format or sequential order, their appearance is repetitive during the interval in which the respective keys are actuated. Hence, the effect is to produce a simultaneous reproduction of the notes as an audio output of the organ, as will be explained in more detail in connection with the description of operation of the tone generation section.

Referring now to FIG. 6, the multiplexed signal arriving from encoder 15 is supplied to generator assignment logic network 26 which functions to assign a tone generator 28 to a depressed key (and hence, to generate a particular note) when the associated pulse first appears in its respective time slot in the multiplexed signal supplied to the assignment logic. If only 12 tone generators 28 are available in the particular organ under consideration, for example, the assignments are to be effected in sequence (order of availability), and once particular pulses have been directed to all of the available generators (i.e., all available tone generators have been "captured" by respective note assignments), the organ is in a state of saturation. Thereafter, no further assignments can be made until one of more of the tone generators is released. The availability of 12 (or more) tone generators, however, renders it extremely unlikely that the organ would ever reach a state of saturation since it is quite improbable that more than 12 keys would be depressed in any given instant of time during performance of a musical selection. The output waveforms from the captured tone generators at the proper frequencies for the notes being played, are supplied as outputs to appropriate waveshaping and amplification networks and thence to the acoustical output speakers of the organ. If the tone generators 28 supply a digital representation of the desired waveform, as is the case in one embodiment to be described, then the digital format is supplied to an appropriate digital-to-analog converter, which is turn supplies an output to the waveshaping network.

At any given instant of time, each tone generator 28 may be in only one of three possible states, although the concurrent states of the tone generators may differ from one tone generator to the next. These three states are as follows:

(1) a particular note represented by a specific pulse in the multiplexed signal has captured (i.e., claimed) the tone generator;

(2) the tone generator is presently uncaptured (i.e., unclaimed or available), but will be captured by the next incoming pulse in the multiplexed signal associated with a note which is not presently a tone generator captor; and

(3) the tone generator is presently available, and will not be captured by the next incoming pulse. It should be apparent from this delineation of possible states that any number of the tone generators provided (12, is this particular example) may be in one or the other of the states designated (1) and (3), above, but that only one of the tone generators can be in state (2) during a given instant of time. That is, one and only one generator can is the next generator to be claimed. When the specific tone generator in state (2) is claimed by an incoming pulse, the next incoming pulse which is not presently claiming a tone generator is to be assigned to the generator that has now assumed state (2). For example, if the third tone generator (-3) of the 12 generators is captured by an incoming pulse (note representation) and the fourth generator (-4) was and still is captured by a previous note selection, then tone generator -4 is unavailable to the next incoming pulse, and the privilege of capture must pass to the next tone generator which is not presently is a state of capture. If all of the tone generators are captured, that is, all are in state (1) as described above, then the organ is saturated and not further notes can be played until at least on of the tone generators is released. As previously observed, however, the saturation of an organ having 12 (or more) tone generators is highly unlikely.

Generator assignment system 26 is utilized to implement the logic leading to the desired assignment of the tone generators 28, and thus to the three states of operation described above. An exemplary embodiment of the generator assignment logic is shown in FIGS. 7A and 7B. Referring to FIG. 7A, a ring counter 30, or a 12-bit recirculating shift register in which one and only one bit position is a logical "1" at any one time, is used to introduce a claim selection, i.e., to initiate the capture, of the next available tone generator in the set of tone generators 28 provided in the organ. A shift signal appearing on line 32 advances the "1" bit from one register or counter stage to the next, i.e., shifts the "1" to the next bit position. Each bit position is associated with and corresponds to a particular tone generator, so that the presence of the logical "1" in a particular bit position indicates selection of the tone generator to be claimed next, provided that it is not already claimed.

Each time the logical "1" appears in a stage of shift register 30, a "claim select" signal appears on the respective output line 34 associated with the stage. This "claim select" signal is supplied in parallel to one input of a respective one of AND gates 35, on line 36, and to further logic circuitry (to be described presently with reference to FIG. 7B), on line 37. The output line of each of AND gates 35 is connected to a separate and distinct input line of an OR gate 40 which, in turn, supplies an input to an AND gate 42 whose other input constitutes pulses from the master clock 5.

In operation of the portion of the generator assignment logic shown in FIG. 7A, assume that shift register stage -2 contains the logical "1." That stage therefore supplies "claim select 2" signal to the respectively associated AND gate 35 and, as well, to further logic circuitry on line 37. If this further logic circuitry determines that the associated note generator may be claimed, a "claimed" signal is applied as the second input to the respectively associated AND gate 35. Since both inputs of that AND gate are now "true", an output pulse is furnished via OR gate 40 to the synchronization gate 42. The latter gate produces a "shift" pulse on line 32 upon simultaneous occurrence of the output pulse from OR gate 40 and a clock pulse from master clock 5. Accordingly, the logical "1" is advanced one-bit position, from stage -2 to stage -3 of shift register 30, in preparation for the claiming of the next tone generator.

Suppose, however, that the tone generator 28 corresponding to stage -3 is already claimed by a previous note pulse in the multiplexed signal. In that event a "claimed" signal appears as one input to the associated AND gate 35, and with the "claim select" signal appearing as the other input to that gate by virtue of stage -3 containing the single logical "1," another shift pulse is immediately generated on line 32 to advance the logical "1" to stage -4 of the shift register. Similar advancement of bit position of the "1" continues until an unclaimed tone generator is selected. If it should happen that no note is presently being selected on a keyboard of the organ at the time when an unclaimed tone generator is selected, the "1" bit remains in the shift register stage associated with the selected tone generator until such time as a "claimed" signal is concurrently applied to the respective AND gate 35, i.e., until the selected tone generator is claimed, because until that time no further shift signals can occur.

Referring now to FIG. 7B, each tone generator also has associated therewith a respective portion of the generator assignment logic as shown in the figure, In other words, the circuitry of FIG. 7B, with minor exceptions to be noted in the ensuring description, is associated with the i'th tone generator (where i=1, 2, 3, ..., 12), and since each of these portions of the assignment logic is identical, a single showing and description will suffice for all. An AND gate 50 has three inputs, one of which is the multiplexed signal deriving from encoder 15 (this being supplied in parallel to the AND gates 50 of the remaining identical portions of the assignment logic for the other tone generators, as well), a second of which is the "claim select" signal appearing on line 37 associated with the i'th stage of shift register 30 (FIG. 7A), and the third of which is a signal, on line 52, indicating that the pulse in the multiplexed signal has not captured any tone generator as yet. Of course, these signals are not present unless the respective events which produce them are actually occurring, but if all three signals are simultaneously presented as inputs to AND gate 50, a "set" signal is applied to a claim flip-flop 53 to switch that flip-flop to the "claimed" state and simultaneously therewith to supply a "claimed" signal to the AND gate 35 associated with the i'th stage of shift register 30 and to the respectively associated tone generator 28.

A modulo 384 counter 55 is employed to permit recognition by the respective portion of the generator assignment logic of the continued existence in the multiplexed signal of the pulse (time slot) which resulted in the capture of the associated tone generator. To that end, counter 55 is synchronized with keyboard counter 1 (also a modulo 384 counter) by simultaneous application thereto of clock pluses from master clock 5. The count of each counter 55 associated with an uncaptured tone generator is maintained in synchronism with the count of keyboard counter 1 by application of a reset signal to an AND gate 58 each time the keyboard counter assumes a zero count; i.e., each time the count of the keyboard counter repeats. However, that reset signal is effective to reset counter 55 only if the associated tone generator is uncaptured. The latter information is provided by the state of flip-flop 53, i.e., a "not claimed" signal is supplied as a second input to AND gate 58 whenever flip-flop 53 is in the "unclaimed" state.

When the flip-flop (and hence, the associated tone generator) is claimed, however, it is desirable to indicate the time slot occupied by the pulse which effected the capture, and for that reason a "reset" signal is applied to counter 55 at any time that an output signal is derived from AND gate 50. Thus, in the captured state, the zero count of counter 55 occurs with each repetition of the "capturing" pulse in the TDM waveform. Such information is valuable for a variety of reasons; for example, to prevent capture of an already captured tone generator when the zero count continues to appear simultaneously with a pulse in the TDM waveform, and to provide a "key released" indication when the zero count is no longer accompanied by a pulse in the TDM waveform Capture prevention is effected by feeding a signal representative of zero count from counter 55 to the appropriate input terminal of an OR gate 60 associated with all of the tone generators and their respective generator assignment logic. The logical "1" supplied to OR gate 60 is inverted so that simultaneous identical logical inputs cannot be presented to AND gate 50. On the other hand, when the zero count is merely synchronized with the zero count of the keyboard counter and is not the result of capture of the associated tone generator id does not interfere with subsequent capture of that tone generator since it does not occur simultaneously with a pulse in the TDM signal. A "key release" indication is obtained by supplying the "zero count" signal to an AND gate 62 to which is also supplied any signal deriving from an inverter 63 connected to receive inputs from the TDM signal. If the zero count coincides with a pulse in the multiplexed signal, the inversion of the latter pulse prevents an output from AND gate 62, and this is proper because the coincidence of the zero count and the TDM pulse is indicative of continuing depression of the key which has captured the generator. Lack of coincidence is indicative that the key has been released, and results in the "key release" signal. Scanning of the keyboards is sufficiently rapid that any delay which might exist between actual key release and initiation of the "key release" signal is negligible, and in any event is undetectable by the human senses. Furthermore, the generation of a false "key release" signal when the tone generator is presently unclaimed, as a result of the occurrence of a zero count from counter 55 synchronized with the zero count of the keyboard counter and the simultaneous absence of a pulse in the TDM signal, can have no effect on the audio output of the organ since the associated tone generator is not captured and is therefore not generating any tone. In any case, the "key release" signal deriving from AND gate 62 is supplied at attack/delay logic of the tone generator to initiate the decay of the generated tone.

The "set claim" signal output of AND gate 50 that occurs with the simultaneous appearance of the three input signals to that gate is utilized to provide a "key depressed" indication to the attack/decay circuitry of the tone generator (and to percussive controls, if desired), as well as to provide its previously recited functions of "setting" flip-flop 53 and "resetting" counter 55.

The assignment logic embodiment of FIGS. 7A and 7B may be associated with only a small number of tone generators (12, in the example previously given), the exact number being selected in view of the cost limitations and the likely maximum number of keys that normally may be actuated simultaneously. In that case, each tone generator must supply every desired frequency corresponding to every not in every octave that may be played on the electronic organ. To that end, a digital tone generator of the exemplary configuration shown in block diagrammatic form in FIG. 8 is employed.

Before describing the cooperative structural and functional relationships between the elements of the tone generator shown in FIG. 8, it is instructive to consider some of the available alternatives in the construction and operation of digital tone generators for ultimately generating a desired audio frequency for a note corresponding to an actuated key. When a key is depressed on any keyboard of the digital electronic organ, a waveform is to be generated with a periodicity corresponding to the desired note frequency in the audible range. The waveform is computed in digital format consisting of a series of numbers (digital words) which represent the magnitude of the waveform at a series, or sequence, of uniformly spaced sample points. The digital sample point values thus generated are subsequently converted to analog form.

The sample points are preferably uniformly spaced because such a format permits the most direct analysis, and therefore the most direct synthesis, of the desired waveform. If desired, the uniform spacing of sample points may be such that there is provided an integral number of samples per cycle for each note frequency to be generated. Such a technique requires a sampling rate that varies directly with the frequency. Alternatively, the samples may be spaced uniformly in time, in which case the phase angle between samples points varies with the frequency of the note to be generated. Although the synthesis of a multiplicity of note frequencies can be implemented for either technique, using a single clock frequency, the preferred frequency synthesis technique is that in which the phase angle between the sample points varies with frequency, i.e., in which the sampling rate is fixed for all note frequencies to be generated, and the various generated note frequencies are produced as a result of the different phase angles.

FIG. 8 shows, in block diagram form, a specific exemplary structure of a tone generator for generating the required note frequencies of the organ from a memory containing amplitude samples of the desired waveform obtained at uniformly spaced points in time. The sample points are accessed at a fixed, single clock frequency for all note frequencies to be generated and the phase angle between the sample points thereby varies with the frequency of the note to be generated. The tone generator includes, as basic components, a phase angle register 101, a sample point address resister 102, a read-only memory 103, an address decoder 103d, an accumulator 104, a sampling clock 105, and a comparator 107. As will be apparent hereafter, the phase angle calculator 100 and the read-only memory 103 may be shared by all of the tone generators 28. In addition, each tone generator is addressed or accessed individually and in sequence and thus once in each cycle of addressing all tone generators. For that reason, the sampling clock 105 may comprise a clock rate provided by a master sampling clock, successive clock pulses of which are directed to the series of tone generators. The sampling clock addressed to a given tone generator is thus at a rate comprising the pulse repetition rate of the master sampling clock divided by the number of tone generators provided in the system. Further, since the same read-only memory may be addressed by all tone generators, the accumulator 104 may be a composite structure associated with appropriate gating circuitry related to each tone generator for accumulating the information read from the memory 103 in response to accessing thereof by a given tone generator.

When a claim flip-flop of the tone generator assignment logic, such as flip-flop 53 (FIG. 7B), is switched to the claimed state in accordance with capturing of a pulse in the incoming multiplexed waveform by a given tone generator 28, the phase angle calculator 100 is instructed to determined the appropriate phase angle for the frequency of the note to be reproduced as identified by the captured pulse. A determination of the value of the phase angle constant, and hence, of the particular not corresponding to the key that has been actuated, is initiated by supplying both the count from the main keyboard counter 1 and the count of the modulo 384 counter 55 (e.g., of FIG. 7B) associated with the captured flip-flop, and which is reset to zero upon that capture, to a count comparator 107. Comparator 107 subtracts the count of counter 55 from the count of the keyboard counter 1 and supplies a number representative of the difference, and hence, representative of the time slot position corresponding to a particular note (i.e., that note which captured the flip-flop), to phase angle calculator 100. The difference computed by comparator 107 will always be positive, or zero, because the computation is elicited from the comparator only when the associated flip-flop 53 is captured and at that moment counter 55 is reset to zero, whereas the keyboard counter probably has some greater count or contains a least count, i.e., zero.

On the basis of the difference count supplied by comparator 107, calculator 100 is informed as to the note for which the phase angle calculation is to be performed, i.e., the note and thus the frequency to be produced by the tone generator. The calculator 100 may compute the phase angle as a function of the frequency of the note to be reproduced and of the number of memory sampling points of the waveform in storage and thus as approximately equal to the phase angle of the fundamental between adjacent memory sampling points for the frequency to be produced. An alternative embodiment of the phase angle calculator 100 is a conventional storage unit with look-up capabilities, or simply a memory from which the correct phase angle is extracted when the memory is suitably addressed with the identification of the count of the captured pulse. Alternatively, a combination of a memory with look-up capabilities and of a calculator capable of computation for determination of the phase angles may be employed. The synthesis of note frequencies in accordance with the digitally stored waveform sample points may be arbitrarily as accurate as desired and, in effect, provides a true equally tempered scale of the synthesized not frequencies wherein the notes within the scale differ by the power of 2 1/12. The degree of accuracy in a practical system, however, must be realized within a finite maximum information content and thus the stored phase angles are quantized and rounded off.

The phase angle thus developed is supplied to and stored in the phase angle register 101. Thus, upon capture of a given tone generator, a command control means such as flip-flop 53 which establishes the captured state of the tone generator controls the operation of the comparator 107 and, in turn, the phase angle determination function of the phase angle calculator 100 for the given note frequency to be generated, for supply of that phase angle to the register 101. Since this operation must preceded the addressing function, a delay may be provided (as by use of a delay multivibrator 106) to actuate a switch 108 for passage of pulses from the sampling clock source 105 (which may be an appropriately gated pulse from a master sampling clock source) to registers 101 and 102.

If desired, the sample point address register 102 may be cleared when claim flip-flop 53 reverts to a noncaptured state, so that it is prepared for entry of information from the phase angle register 101 upon each calculation. However, it is important to note that during accessing of the memory it is the rate at which the value of register 102 increases and not the absolute value thereof which is significant in the control of the rate of read out of the memory 103 and thus the cyclic frequency of read out of the memory and, ultimately, the frequency of the note reproduced by the give tone generator.

Once each sampling clock time as determined by the sampling clock source 105, the phase angle value stored in phase angle register 101 is added to the previously stored value of the sample point address register 102. An address decoder 103a decodes preselected bit positions of the count established in register 102 to effect accessing, or addressing, of the memory, 103. The transfer from the register 101 to the register 102 is a nondestructive transfer such that the phase angle value is maintained in the register 101 as long as that tone generator is captured by a given pulse.

Thus, once each clock time, the phase angle register value, comprising a digital binary word, is added to the sample point address register value and correspondingly, for each such clock time, the memory location corresponding to the sample point address then existing is the register 102 is accessed. As a practical matter, only a relatively small, finite set of amplitudes can be stored in the memory 103, because of practical limitations on its capacity, and thus only a finite number of addresses are available. Furthermore, the registers such as 101 and 102 must be of a finite, practical length. In particular, the length of the phase angle register 101 is determined by the accuracy with which the frequency of the note is to be generated. The frequency actually produced will be exactly the value of the phase angle in register 101 times the memory sampling rate. The sample point address register 102, on the other hand, must be sufficiently long to accept data from the phase angle register 101. The register 102, however, preferably includes additional bit positions which are not used, or not used at all times, for accessing the memory. In this respect, it will be apparent that one-bit position in the register 102 is scaled at one cycle of the fundamental of the frequency of the note to be generated. A set of next successive less significant bits may therefore specify the sample point address in accordance with the function of the decoder 103a. The more significant bits of the register 102 may be used to count numbers of cycles of the waveform for various control functions not here pertinent. In addition, by selecting appropriate bit positions by means of decoder 103a, the frequency of the note reproduced may be readily adjusted to different octaves. That is, a one-bit positional shift constitutes division or multiplication by two, depending upon direction of shift. For example, if the most significant bit is numbered 1 and thus bit positions 2 through 6 comprise the sample point address bits normally used for an 8 foot voice, then a 16 foot voice can be obtained by using bits 1 through 5 as the sample point address source. Correspondingly, a 4 foot voice can be obtained by using bits 3 through 7 as the sample point address bits.

The read-only memory 103 contains digital amplitude values of a single cycle of the complex periodic waveform to be reproduced for all note frequencies. That is to say, the same complex periodic waveform is to be reproduced for each note played, the only difference being the frequency at which the complex waveform is reproduced.

Referring to FIG. 9, illustrating a typical complex waveshape 110 of the type that may be produced by a pipe organ, the wave may be sampled at a multiplicity of points, shown as vertical lines in the FIG., to provide the amplitude data for storage in memory 103. If absolute amplitude data is stored in memory 103, then the data accessed is the actual amplitude of the output waveform at the respective sample points (i.e., with respect to a "zero" level at time axis 111). In that event, the digital amplitude data successively read from the memory may be applied directly to an appropriate digital-to-analog conversion system. On the other hand, if incremental amplitude information (i.e., simply the difference in amplitude between the present sample and the immediately preceding sample) is stored in memory 103, then the data accessed must be added to an accumulator (e.g., 104 in FIG. 8) to provide the absolute amplitude information at each sample point prior to digital-to-analog conversion. Each of the sample points of the memory 103 may comprise a digital word of approximately seven or eight bits.

The digital words thus read out from the memory 103 are supplied to the accumulator 104 which provides a digital representation of the waveform at selected sample points over a cycle of the waveform and at a frequency corresponding to the note to be reproduced. As above described, this digital waveform representation may itself be operated upon for waveshape control, e.g., attack and decay, and subsequently is supplied to a digital-to-analog converter for producing an analog signal suitable for driving the acoustical output means, such as audio speakers, of the organ.

Memory 103 may be a microminiature diode array of the type disclosed by R. M. Ashby et al. in U.S. Pat. No. 3,377,513, issued Apr. 9, 1968, and assigned to the same assignee as is the present invention. The array may, for example, contain an amplitude representation of the desired waveform in the form of an eight-bit binary ward at each of 48 or more ample points. Such a capacity permits the storage of up to 128 amplitude levels in addition to a polarity (algebraic sign) bit. In any event, the capacity of memory 103 should be sufficient to allow faithful reproduction of note frequencies.

If whole values of amplitude levels at the sample points of the waveform are read from memory 103 in the embodiment of FIG. 8, the same sample point may be addressed several times in succession. This is the result of the requirement that the memory be accessed at a fixed rate for every note frequency, a requirement which implies that for decreasing note frequencies an increasing number of sample points must be read out during each cycle; and since the number of sample points is fixed and no sample points can be skipped regardless of note frequency, this simply means repetition of the same sample point possibly several times in succession. This does not undesirably affect the ultimate waveform generated, however, because there is consistent plural sampling of each point of the stored waveform.

On the other hand, if incremental values of the waveform have been stored in memory 103, each increment can be read out only once during each cycle of the waveform. THis is because an accumulation of incremental values is required, and repetition will produce a significant error in the accumulation and the ultimate waveform to be generated, regardless of the note frequency. Since the same sample point may be read out of memory 103 several times in succession depending upon the note frequency to be produced, just as in the whole value sample point case noted above, for incremental values all but one readout for each sample point must be inhibited to prevent repetitive application to accumulator 104. To that end, a gate 103b (shown dotted in FIG. 8) is positioned in the output line of memory 103 preceding accumulator 104 if incremental values are utilized. Gate 103b is preferably enabled to pass the sample value being read from the memory only when the least significant bit in address register 102 changes. Since such change occurs upon a "carry" into that position, indicating advancement to the next memory address, a bit change sensor 102a many be used to detect the change and to enable gate 103b at each advancement to a new address. The same sample point may still be accessed several times in succession, but only one such value will be "read out" (i.e., will be passed by the gate since it is disabled at all other times).

The phase angle calculations should be such that the highest note playable is that note for which a sample point value is read out each time the memory is addressed. Since the ratio between adjacent notes on the equally tempered musical scale is an irrational number, it is preferable that the largest number is the phase angle register be slightly smaller then the least significant bit in the address register. If the phase angle number were larger, it would be necessary to occasionally skip a sample point and this would lead to inconsistency in the note frequency, whereas if the phase angle number were equal to the least significant bit in the address register the note frequency would be slightly higher (i.e., about one-half of a halftone higher) than the highest note that can be played. By requiring the phase angle number to be slightly smaller, the highest note capability of the instrument will not be exceeded.

The same read-only memory 103 may be shared by all of the tone generators 28 if the data words (amplitude values of sample points) read therefrom are gated to respective wave shapers in synchronism with the addressing of the memory for the respective notes being played. In other words, simultaneous or concurrent play to two or more notes requires that these be distinguished as separate sets of sample points, if a single memory is to be shared for all tone generators.

In the present embodiment, however, it is assumed that each tone generator has its own memory (and, incidentally, memories composed of microminiature diode arrays of the type disclosed in the aforementioned Ashby et al. patent are readily fabricated with more than 5,000 diode elements per square inch), which supplies its digital output to a respectively associated attack and decay control unit. The binary-valued amplitude samples are applied directly to the attack and decay circuitry if each sample is a whole value, or may be applied via an accumulator 104 if each sample is an incremental value. Alternatively, accumulation of incremental values may be preformed after shaping, if desired.

Referring the FIG. 10, an embodiment of the attack and decay unit associated with each tone generator included a multiplier 120 to which the sample values from memory 103 are applied for multiplication by an appropriate scale factor or control the leading the trailing portions of the note waveform envelope. As is well know, the faithful simulation of true pipe organ sounds by an electronic organ requires that the latter be provided with the capability to shape each tone envelope to produce other than an abrupt rise and fall. Without special attack and decay control, the note waveform produced by an electronic organ normally rises sharply to full intensity immediately upon depression of the respective key, and ceases abruptly when that key is released. At times, this may be a desirable effect to maintain during the play of a musical selection. In those cases, the attack and decay controls may be avoided entirely, or the scale factor supplied to multiplier 120, and with which the amplitude samples are to be multiplied, may be set at unity. More often, however, attack and/or decay are desirable for or in conjunction with special effects such as percussion, sustain, and so forth.

The multiplying scale factor is varied as a function of time to correspondingly vary the magnitude of the digital samples, with which it is multiplied, on a progressive basis to simulate attack and/or decay. In the embodiment of FIG. 10, the total time duration and the time constant(s) for the attack or decay are controlled by a counter 122 which may be selectively supplied with uniformly timed pulses that are independent of the specific note frequency under consideration, such as pulses obtained or derived from the master clock, or with pulses having a repetition rate representative of or proportional to the note frequency. In this respect, the counter 122 may be considered as determining the abscissa of a graph of envelope amplitude versus time and representative of the attack or decay. The ordinate or amplitude scale of the graph is represented by the series of scale factors stored in a read-only memory 125 to be accessed by the counter itself, or by an address decoder 126 which addresses the memory for readout of scale factors on the basis of each count (or timed, separated counts) of counter 122.

The counter may be of the reversible, up-down (forward-backward) type in which it is responsive to incoming pulses to count upwardly when its "up" (here, attack) terminal is activated, and to count downwardly when its "down" (here, decay) terminal is activated. The attack mode of the overall control unit is entered when the associated tone generator is captured by a hitherto unclaimed note pulse in the multiplexed signal. The capture of a tone generator is accompanied by a signal indicative of a key having been depressed (see FIG. 7B), form the assignment logic, and it is this signal which initiates the attack count of counter 122. In particular the first "key depressed" signal (and possibly the only one) that occurs upon capture of a tone generator 28 is effective to produce a count in the first stage of ring counter 128, thereby supplying a trigger signal form that stage to a monostable delay multivibrator 130 which is set to have an ON time (delay time) of sufficient duration to ensure that the attack is completed despite release of the key prior to the normal end of the attack interval. It has been found that a delay time equal to or greater than approximately the time occupied by seven cycles (i.e., seven periods) of the lowest frequency note is quite adequate for multivibrator 130 to ensure this positive attack. During that interval, the "up" control of counter 122 is activated by the quasi-stable state of multivibrator 130 and the counter continues to count incoming pulses until the multivibrator spontaneously returns to its stable state, or until the note envelope reaches the full desired intensity (magnitude), if earlier. This full intensity value may be preset into the attack/decay control logic or it may be determined by logic circuitry responsive to such factors as the force with which the respective key is struck (i.e., to velocity-responsive or touch-responsive device outputs). In the embodiment shown in FIG. 10, the former arrangement is utilized in which a maximum desired count is set into a fixed counter 131 for continuous comparison in comparator 133 with the present count of up-down counter 122. If the latter exceeds the former, a "disable" command is applied to the counter to terminate the attack.

Pulses to be counted by counter 122 may be obtained at a rate which is a function of note frequency, as by supplying the output of phase angle calculator 100 to a phase-to-frequency converter 135, or at a rate based on the master clock rate, whichever is desired. Selection of either rate is accomplished by appropriately setting a switch 136 coupled to an associated switch or key on or adjacent to one of the keyboards.

In operation of the attack/decay control unit of FIG. 10, after switch 136 has been set at the desired position, the pulses to be counted appear at the input of counter 122 but no count is initiated until a key is depressed and the associated pulse in the multiplexed signal from the keyboard results in the capture of a tone generator 28. The "key depress" signal from the generator assignment logic initiates a count in ring counter 128, which bad been reset by completion of decay the immediately preceding time the attack/decay control unit had been used. Preferably, the latter reset signal is obtained upon switching of the claim flip-flop 53 in the assignment logic 26 to the "not claimed" (delay complete) state. The up count of counter 122 is thereby enabled and continues through completion of attack regardless of whether or not the key remains depressed. The duration of attack depends on whether the note frequency mode or the fixed time mode is employed.

With each count of counter 122 (or less frequently, by use of suitably timed "enabling" commands), address decoder 126 develops a related address code for accessing a digital scale factor stored in the appropriate address of read-only memory unit 125, to be combined as a product in multiplier 120 with the amplitude samples being read from tone generator 28 of FIG. 8. By presetting memory 125 such that the scale factors stored therein are logarithmically increasing (up to the equivalent of unity) with addresses decoded according to progressively increasing count in counter 122 (up to the maximum desired count, representing full note intensity), a logarithmic attack is provided in the note being played.

When the key is released, a "key release" signal is applied from AND gate 62 of assignment logic 26 (FIG. 7B) to a flip-flop 138 to initiate the decay mode of the attack/decay control unit by enabling the "decay" (down) count of counter 122. Accordingly, incoming pulses to the counter are counted downwardly from the count representative of full intensity, until a zero count is obtained unless decay is terminated earlier. As in the case of the attack mode, the count in counter 122 is periodically decoded (e.g., once each count) by unit 126 for addressing of memory 125, thereby supplying logarithmically decreasing scale factors, from unity to zero, for multiplication with amplitude samples form the tone generator in multiplier 120. This produces the desired fall in note intensity at the trailing portion of the note waveform.

If during decay the same note pulse should reappear in the multiplexed keyboard signal, a second "key depress" signal is applied to ring counter 128 thus increasing the count therein to the second stage and switching flip-flop 138 from the decay state to its other state, which reintroduces the attack mode. Since decay is incomplete in this particular instance, the count of counter 122 now proceeds upward from the minimum count which had been attained when decay was interrupted. If, however, the key is again released, prior to completion of attack, positive attack is no longer in effect and the flip-flop 138 reverts immediately to the decay state by virtue of application of the "key release" signal thereto.

To prevent flip-flop 138 from being in the "decay" state when the initial attack condition is established in counter 122 (by the quasi-stable state of delay MV 130 ), flip-flop 138 may be switched to its "attack" state upon full completion of decay, by the "not claimed" signal of associated flip-flop 53.

Upon completion of decay of a note whose representative pulse in the keyboard multiplexed signal resulted in capture of a tone generator, a "decay complete" signal is applied to the claim flip-flop 53 (FIG. 7B) of the respective assignment logic unit to cause that flip-flop to return to its "not claimed" state, and thereby to release the tone generator for claiming by another note.

With reference now to FIG. 11, the digital electronic organ thus far described is peculiarly suited to a simple automatic transposition technique by virtue of its keyboard multiplexing scheme. Each position, or time slot, in the multiplexed signal is assigned to a particular key (and the note associated therewith) on each keyboard. As was indicated earlier, the multiplexed signal is structured such that adjacent time slots therein correspond to adjacent semitones in the equally tempered musical scale. For example, pulses associated with notes C4, C-4, D4, will appear in successive positions in the order recited in the multiplexed signal, as shown for the single cycle of the multiplexed signal in FIG. 11, whenever the key switches associated with those notes are concurrently depressed.

The basic scheme of automatic transposition contemplated by the present invention is the shifting of note frequencies by a selected amount during play of the keys on each keyboard in the normal manner. One technique of accomplishing this objective is shown in simplified form in FIG. 12, where the multiplexed signal from encoder 15 (FIG. 1) is applied to a pulse delay device 150 which is preset, or which is adjustable, to introduce a delay into the multiplexed signal by a number of time slots, or pulse positions, equal to the number of halftone transpositions desired, prior to entry of the multiplexed signal into the tone generator assignment logic (FIG. 6). Thus, for example, if scanning of the keyboards is from low frequencies to high frequencies, original time slots 83 and 85 could be subjected to a one-pulse delay (a single halftone transposition) to accomplish a shift of those slots to time slots 84 and 86, respectively, in the delayed multiplexed signal leaving delay device 150.

A more detailed circuit diagram of the system of FIG. 12 is shown in FIG. 13. The multiplexed signal appearing on line 25 from encoder 15 is supplied, via a normally open gate 151 (i.e., a completed circuit path for passing signal), to a switch 152 having a switch arm 153 and a pair of contacts 154 and 155. Depending upon which of the contacts the switch arm is in electrical contact with, the system of FIG. 13 is capable of producing an upward shift in frequency or a downward shift in frequency. For the sake of example, it is assumed that the scanning of the keyboards is in a direction from the highest frequency to the lowest frequency. Again, this is immaterial to the present invention, but if the scanning is instead from the lowest frequency to the highest frequency of each keyboard, then a slight modification of the arrangement shown in FIG. 13 is required, a modification which will be perfectly obvious to one of ordinary skill in the art from a consideration of the implementation of the circuit of FIG. 13.

If transposition to a lower frequency is desired, any pulses occuring in the multiplexed signal are inserted into a 12-bit shift register 155 in their respective time slots, by positioning switch arm 153 against contact 154 as shown. Shift register 155 is effectively a 12-bit delay line with an output tap at each stage. If no transposition is desired, the output is taken from the first stage of the shift register since at that point no delay has been introduced into the multiplexed signal. Shifting of pulses through the register is effected by pulses from the master clock. A selector switch 157 may have a knob (not shown) positioned on or near the keyboards or in any position conveniently accessible to the organist for presetting the amount of delay, and thus the extent of transposition, into the system. To that end, switch 157 has a rotatable arm 158 connected to output line 159 of the transposition system and selectively movable against each contact associated with the output taps of the twelve stages of shift register 155. If, for example switch arm 158 is positioned against the contact associated with line 160 of the 12-bit shift register, as shown, then a one time slot delay is introduced into the multiplexed signal. Similarly, positioning of arm 158 against the contact associated with line 161 connected to the third stage of shift register 155 will introduce a two-time slot delay into the multiplexed signal, and so forth for each contact associated with each succeeding stage of the sift register.

Thus, the organist may select any desired delay up to and including one complete octave (i.e., 12 semitones) of the organ and thereby any note is audibly produced by the organ a respective number of halftones lower than the note with which the actuated key is normally associated. Of course, the organist selects the desired amount of transposition prior to playing the musical selection. When transposition from one key to another is desired, the transposition selection switch 157 is set to the appropriate stage of delay unit 155 that will produce the desired time delay, and thus the desired shift in frequency (downward, in the case of highest frequency to lowest frequency scanning) for each note played. Suppose, for example, that the organist desires to play a musical piece written in the key of C natural in the transposed key of F in the next lower octave. This requires a time shift of seven time slots in the multiplexed waveform since the notes B, A-, A, G-, G, and F- lie between C natural and F natural in the next lower octave. Accordingly, the arm 158 of transposition switch 157 is set at the contact connected to the eighth stage of 12-bit shift register 155. Thereafter each note played by the organist in the key of C natural is transposed automatically to the key of F natural, the transposed pulse train (i.e., delayed multiplexed signal) being supplied to input terminal 30 of the generator assignment logic circuitry 31 (FIG. 6).

If an increase in frequency is desired for the transposition under the stated conditions of scanning (multiplexing) from high to low notes, one suitable mechanization is to utilize a delay of N--n, where

N = total number of time slots in a complete cycle of the multiplex signal; and

n = number of halftones by which the musical selection is to be transposed above the nominal frequencies.

That is, if the organist desires to transpose from one musical key to another in the next higher octave, and continuing with the assumption that the keyboards are scanned from the highest frequency to the lowest frequency of the organ, then the time shift of pulses in the multiplexed waveform by one time slot per halftone to be transposed will require a sufficient delay to obtain access to time slots in the next higher octave in the next complete repetition of the cyclically repeating multiplexed signal. Thus, the selection for transposition to a key in the next higher octave will require a time shift of almost, but not quite, a complete number of time slots in the multiplexed signal. To that end, in the embodiment of FIG. 13 switch 152 may be set in the higher frequency mode in which arm 153 is placed against contact 155, so that the incoming multiplexed signal is supplied to a 372-bit delay line 165 (for the previous example of a multiplexed signal containing 384 time slots). In this manner, sufficient delay has been introduced prior to introduction of the multiplexed signal into shift register 155, to permit adjustment of the transposition selector switch 157 to transpose from the musical key in which the piece is to be played to the desired key in the next higher octave. In this instance, however the organist must place the switch arm 158 of the transposition selector switch 157 to the contact connected to an appropriate stage of shift register 155 that will provide automatic transposition according to a reversal of the previous computation process. For example, if the organist desires to play a selected musical piece in the note F natural by transposition from C natural in the same octave, then the shift must be five semitones, but in a backward or reverse direction, i.e., a delay of 372+ (12-5)= 372+ 7= 379 time slots. Therefore, the organist will set switch arm 158 against the contact connected to the fifth from the last stage of the 12-bit shift register 155.

When a particular musical piece is transposed in key by appropriate selection using transposition selector switch 157, it may happen that certain notes to be selected in the musical piece will be out of the range of the organ. That is, if the transposition is upward in frequency, the highest notes in the musical piece to be played may no longer be available to be sounded in the particular organ in which the digital multiplexing system is employed. This will depend, of course on the frequency range encompassed by the tone generators of the organ. If some redundancy has been provided in the time division multiplexed waveform, as was previously discussed, then this presents no particular problem. Thus, if say 10 to 12 time slots at the beginning and/or the end of the multiplexed signal are not associated with any keys of the organ, then the upward shift will place the unavailable notes into time slots in which provision has been made to prevent capture of any tone generator. Accordingly, no note is sounded when the keys representative of out of range notes are depressed. On the other hand, if no redundancy is provided in the TDM signal then some means should be employed to remove (blank) the pulses for notes which are out of the range of the organ, or to mute the tones produced as a result of those pulses. This is because these pulses will have been shifted to time slots which are representative of notes in the range of the organ but at the completely opposite end of the musical scale. The notes may lie in an octave several octaves below that in which the notes are to be sounded.

To prevent the improper notes from sounding, blanking pulses may be applied to the inhibit terminal of gate 151 in synchronism with the undesired pulses in the multiplexed signal. Synchronization of blanking pulses may be achieved by supplying a count representative of the extent of delay selected by transposition switch 157 to a pulse generator (not shown) which also receives pulses from the master clock and which is effective to generate the blanking pulses at the master clock rate upon attainment of a desired count.

A typical organ, for example, is exhausted of tone generators above note C7, and acceptance of such an upper limitation implies that, in turn, each of the keys in the upper octave sound blanks as the transposition is increased in steps to higher frequencies. A preferred alternative to this blanking of notes which has been described above, however, is what may be referred to as "octave folding." In the "octave folding" technique, the notes that would exceed the uppermost note (C7, in this example) during transposition are folded, or shifted, such that they sound in the next lower octave. An exemplary folding scheme is illustrated in the following table. ##SPC1## The top line in the above table illustrates the nominal tones of the sixth octave of the organ, assuming for the sake of example that this is the highest keyed octave on any organ manual. If a halftone increase is effected by transposition in the previously described manner, depressing of the key associated with note C6 will cause note C-6 to sound, and so forth. Similarly, depressing the key associated with note B6 will bring forth note C7, and at this point the uppermost note in the organ has been sounded. If the musical selection as written ranges upward to note C7, when the organist depresses the respective key the transposition would ordinarily demand that C-7 be sounded. The latter note, however, is unavailable. According to the "octave folding" feature of the present invention, when the key associated with note C7 is depressed the note in the next lower octave corresponding to the transposed note is the one that is sounded. In this instance, then, note C-6 rather than note C-7 (which is unavailable) is called forth. This, of course, is appropriate to the note requested, and is much more pleasing to the listener than would be the total inhibition (blanking) of a note during play of the piece.

The situation becomes more critical with increase in degree of transposition. Thus, referring again to the above table, a one and one-half tone transposition to higher frequencies will cause selection of note A6 to call forth note C7, the highest available note. This leaves the next three keys, for notes A-6, B6, and C7, subject to blanking were it not for the octave folding technique of the present invention. As a consequence of the latter, however, the notes C-6, D6, and D-6, respectively, will be sounded. It will be observed that each half-note increase in transposition requires that an additional note in the upper octave be brought into the octave folding scheme.

FIG. 14 shows one simple arrangement for implementing octave folding. Each key switch, such as 175, 176, 177, 190, 191, 192, on each manual of the organ is connected to a source of voltage so that upon depression of a key switch a keying signal is supplied to call forth the associated tone. In the keyboard multiplexer embodiment which has been described, the keying signals produced upon depression of key switches are supplied to switching array 11 (FIG. 1) to operate respective switches 14 (FIG. 4). In the uppermost octave, and any desired lower octave for that matter, the leads via which the keying signals are supplied to the overall multiplexed signal are wired through respective single pole, double throw (SPDT) octave folding switches. Lead 193 connects key switch 192 to octave folding switch 194, lead 195 connects key switch 191 to octave folding switch 196, lead 197 connects key switch 190 to octave folding switch 198, and so forth. As previously observed, each halftone increase in transposition brings an additional note into the octave folding scheme, in successive downward steps from the highest available note. Thus, the technique is readily extended beyond the three note involvement shown in FIG. 14, in the event that four or more halftone transpositions and related octave folding capabilities are desired to be made available.

In FIG. 14, when the musical instrument is operated without transposition, or the transposition is to the lower frequencies, each octave folding switch is maintained in what may be marked a "normal" position. Thus, each of switches 194, 196, and 198 has its switch arm resting against respective contact N when no octave folding is to be provided. If a halftone transposition in an upward frequency direction is introduced, the organist merely throws switch 194, conveniently located on one of the manuals and ganged with similar switches for the same note on other manuals, to the one-half tone contact. Thereafter, when key switch 192 (for note C7) is struck, the keying signal thereby developed is supplied along lead 193, through switch 194, and along lead 200, the latter lead normally associated with the key switch for note C6. Signal indicative of note C6 therefore appears in the multiplexed signal when note C7 is called for. However, a one-half tone transposition is in effect and thus note C-6 will be sounded. This is precisely the desired operation as will be observed by further reference to the table above.

As successively greater transposition is selectively instituted, successively more of the octave folding switches are to be thrown according to the amount of transposition in effect. The preceding octave folding switches are also operated to the "folding" position. For example, if a one and one-half tone transposition to higher frequencies is selected, switch 198 is thrown to the 11/2 tone position, and all preceding octave folding switches, here 196 and 194, are also operated to the octave folding position. If desired, the octave folding switches may be ganged to the transposition selection switch for introduction of the proper amount of octave folding simultaneously with selection of transposition.

As previously observed in connection with the description of FIG. 8, and as is generally well known in the art, the ratio of two adjacent note frequencies in the equally tempered musical scale is 21/12. This fact may be utilized to advantage to provide a further embodiment of an automatic transposition scheme other than that using a shift of the note assignments in the multiplexed waveform. While the effect is that of shifting pulses representing note assignments by one time slot per semitone of desired transposition, the means for achieving the transposition is different. In essence, the shift operation is performed in the frequency domain, rather than in the time domain as was described earlier, by acting on the selection of waveform sample addresses, and the rate of accessing, of the waveform memory unit in the tone generator.

The phase angle computed by calculator 100 of tone generator 28 in FIG. 8, on the basis of information furnished by comparator 107, is effectively a number representative of a particular ratio which has been assigned to each note of the scale and which is used to increment, or step, the address of wave shape memory 103 at a rate determined by the note being played. To obtain the desired transposition, instead of loading the ratio number calculated by calculator 100 into register 101, this nominal ratio number is appropriately varied by multiplying it by the transposition ratio

RT = 2n/12,

where n is the number of semitones of transposition desired, and n is a positive number if the shift is to be to a higher frequency, and a negative number if the shift is to be to a lower frequency. This arrangement is shown in FIG. 15, where a multiplier 210 is inserted into the line between calculator 100 and register 101, and is supplied with a second input (i.e., the ratio number furnished by calculator 100 constituting the other input) from a transposition ratio selector 212. The latter unit may simply comprise a set of recycling values of RT, any one of which may be selected according to desired transposition and which is read out in synchronization with the output generated by calculators 100 (e.g., using the master clock rate), to supply the value including the incremental frequency shift to register 101. In this manner, the rate of addressing of memory 103 is appropriately varied for accessing the digital amplitude samples of the wave shape stored in the memory at a rate consonant with the selected transposition.

The octave folding technique previously described in conjunction with the apparatus of FIG. 14 is usable with any embodiment of the invention introducing the desired transposition. That is, it may be employed with the exemplary embodiment of FIG. 13 or with the exemplary embodiment of FIG. 15.