05/447907

09/23/1975

03/04/1974

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Hammond Corporation (Chicago, IL)

84/618

84/DIG.020, 984/330, 84/DIG.002, 984/381, 84/684

G10H1/18; G10H5/06; G10H5/00; G10H1/00; G10H5/00

84/1.01,1.24,DIG.2,DIG.20

3509262	BASS REGISTER KEYING SYSTEM EMPLOYING PREFERENCE NETWORKS	April 1970	Munch, Jr.
3598892	CONTROLED SWITCHING OF OCTAVES IN AN ELECTRONIC MUSICAL INSTRUMENT	August 1971	Yamashita
3781450	SIGNAL-SELECTING SYSTEM FOR AN ELECTRONIC MUSICAL INSTRUMENT	December 1973	Nakajima
3801721	MONOPHONIC ELECTRONIC MUSIC SYSTEM WITH APPARATUS FOR SPECIAL EFFECT TONE SIMULATION	April 1974	Bunger
3806623	SINGLE NOTE SELECTING STORAGE CIRCUIT	April 1974	Yamada
3806624	DISCOVERY IN KEYING CIRCUIT FOR A MUSICAL INSTRUMENT	April 1974	Kniepkamp et al.
3808344	ELECTRONIC MUSICAL SYNTHESIZER	April 1974	Ippolito et al.
3828108	BINARY ORGAN AND CODING SYSTEM FOR OPERATING SAME	August 1974	Thompson
3836692	SIGNAL-SELECTING SYSTEM FOR A KEYBOARD TYPE ELECTRONIC MUSICAL INSTRUMENT	September 1974	Nakajima

Hartary, Joseph W.

Witkowski, Stanley J.

Bergstedt, Lowell C.

I claim

1. A monophonic electronic musical instrument comprising:

2. Apparatus as claimed in claim 1, further comprising octave lockout circuit means for locking out control signals from all but one octave of control elements to determine an active octave;

3. Apparatus as claimed in claim 2, wherein and of said bistable storage elements comprises a flip-flop circuit having set and reset input leads, and a single output lead; and further comprising a keydown detector circuit means for generating a keydown signal in response to a control signal from any one of said control elements;

4. Apparatus as claimed in claim 3, wherein said keydown detector circuit means comprises circuit means for summing the control signals on said common octave busses to produce said keydown signal each time a control signal appears on one of said common octave busses.

5. Apparatus as claimed in claim 1, further comprising:

6. Apparatus as claimed in claim 3, further comprising:

7. Apparatus as claimed in claim 6, wherein each of said gates is a transistor gate turned on by a gate operate signal to ground its associated junction between said voltage dropping elements.

8. A monophonic electronic musical instrument comprising:

9. Apparatus as claimed in claim 8, further comprising:

10. A monophonic electronic musical instrument comprising:

11. Apparatus as claimed in claim 10, further comprising:

12. A monophonic electronic musical instrument comprising:

FIELD OF THE INVENTION

This invention relates to monophonic electronic musical instruments and more particularly to electronic music synthesizers for simulating various orchestral instrument voices and for producing unique musical and non-musical sounds.

DESCRIPTION OF PRIOR ART

Electronic music synthesizers are typically monophonic instruments which involve generating a tone signal of a selected frequency and waveshape and subjecting the tone signal to controlled frequency modulation, controlled filtering, and controlled amplification to produce the desired musical effect. By providing a variety of waveshapes and dynamic changes in frequency, filtering, and amplification, as well as controlled introduction of noise, various types of orchestral instrument voices can be authentically simulated and unique sounds not made by familiar musical instruments can be generated.

The commercially available synthesizers marketed under the trade names "Moog" and "ARP" have generally similar system characteristics. A keyboard, which is generally similar to a piano or organ keyboard, is provided with keyswitches for each key having a plurality of contact pairs for different control functions. One contact pair per key is employed to ground a junction in a precision resistor divider string fed by a constant current source to develop a voltage at the output of the current source which is linearly related to the position of the actuated key on the keyboard. Other contact pairs are employed to produce a "keydown" signal, i.e., a signal that at least one key is depressed, and a "legato pulse" signal, i.e., a signal that a new effective key is depressed. The constant current source and precision resistor divider string comprise a "volts per octave" circuit which responds only to the lowest or highest key actuated depending upon whether the current source feeds the divider string from the low end or high end of the keyboard.

The output voltage signal from the volts per octave circuit is fed to a sample and hold circuit which functions under the control of the legato pulse generator to store or memorize the voltage signal so that it is available even if the actuated key is released. The memorized voltage signal is fed to a circuit which converts the linear volts per octave signal to an exponential signal. This exponentially varying signal has the proper characteristic to control a voltage-controlled oscillator which thus produces an output tone signal corresponding to the note associated with the actuated key on the keyboard. The output tone signal is fed to a voltage-controlled filter which may be programmed to have various frequency response characteristics including dynamic characteristics produced by a circuit which produces voltage control envelopes of various types. Then the filtered signal is further processed in a voltage-controlled amplifier which may be programmed via a circuit which produces various types of voltage control envelopes to amplitude modulate the signal. Moreover, the voltage-controlled oscillator itself may be subjected to various types of modulation to produce vibrato and other musical effects.

Several years ago, the Wurlitzer Company introduced a synthesizer as an optional add-on feature to several of its electronic organ models. The synthesizer was controlled via a two-octave keyboard separate from the solo keyboard of the organ and thus the player could not play the synthesizer integrally with upper manual solo voices. The Wurlitzer synthesizer employs a single oscillator-parallel divider chain approach to generating the top octave tone signals. These top octave tone signals are directly fed to a first priority latching network which is coupled to one octave of keyboard switches. The top octave tone signals are also sent through individual frequency dividers to generate the next lowest octave of tones and then are fed to a second priority latching network. A complex arrangement of parallel frequency dividers fed by the two priority latching networks is controlled by a steering circuit to provide selection between the two octaves.

The Baldwin Piano and Organ Company and Thomas Organ Company have within the past couple of years introduced organ models with built-in synthesizers functioning under the control of the upper keyboard of the organ. Both companies employ a contact pair per key in addition to regular organ keying contacts to generate a high select voltage signal to a sample and hold circuit for tuning a voltage-controlled oscillator. Thus, these companies have chosen to integrate the type of tone generation system used in Moog and ARP units and to control the tone generation system via additional contacts per key.

SUMMARY OF THE INVENTION

This invention proceeds from the system disclosed in co-pending Schrecongost patent application, Ser. No. 448,020, filed Mar. 4, 1974, and assigned to Hammond Corporation, the assignee of this Invention and provides an electronic music synthesizer in which collected note and octave keying signals are stored in special note and octave latches (flip-flop circuits) to retain the note and octave information after release of a keyswitch. The stored note and octave signals are used to gate note and octave tone signals as in the Schrecongost system, but are also used to derive a very stable volt per octave signal. This volt per octave signal is useful to produce pitch slide effects in the Schrecongost-type system but is also useful as a control signal to a voltage-controlled oscillator system as employed in Moog and ARP synthesizer systems. Separate voltage signals corresponding to the position of the selected high note and selected high octave, respectively, are produced by separate circuits each of which comprises a string of equal voltage dropping elements, such as diodes selected to have equal forward voltage drops, fed by a constant current source with transistor gates at junctions between diodes to ground out the string at a position corresponding to the highest associated latch which is in a set condition. The voltage at the high end of the string of elements fed by the constant current source is proportional in the note-related circuit to the high note position and in the octave-related circuit to the high octave position. Summing the high note and high octave signals in a 1:12 ratio produces a final signal which is directly proportional to the position of the highest keyswitch actuated.

A simple, yet quite functional, voltage-controlled filter is also disclosed as part of this synthesizer system. The filter includes a plurality of cascaded phase shift circuit stages in a negative feedback path of an amplifier with each stage comprising a series capacitor and a field effect transistor (FET) connected with its source-drain circuit providing a variable impedance shunt to ground controlled by a DC control voltage on its gate which is connected in common with gate electrodes of FETs in other stages. Preferably four stages are employed to give a 180° phase shift at a frequency selected by the control voltage at the FET gates to peak the gain of the amplifier at that frequency.

Some of the inventive concepts disclosed herein are also employed in certain specific embodiments of synthesizer systems disclosed in the copending Schreier patent application Ser. No. 447,905, filed Mar. 4, 1974, and assigned to Hammond Corporation, the assignee of this invention, and it is to be understood that the disclosed systems of that application represent additional preferred embodiments of this invention which are known to this applicant and are specifically incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block schematic diagram of an electronic music synthesizer of the type in which this invention is useful.

FIG. 2 is a block schematic diagram of one of the preferred embodiments of this invention.

FIGS. 3 through 6 together comprise a circuit schematic diagram of a preferred embodiment of this invention.

FIG. 7 illustrates how FIGS. 3 through 6 are to be assembled to form an overall interconnected schematic diagram.

FIG. 8 is a circuit schematic diagram of a voltagecontrolled filter according to this invention.

FIG. 9 is a graphical representation of the response of a voltage-controlled filter according to FIG. 7.

FIG. 1 illustrates a synthesizer system which includes the Schrecongost approach to collecting note and octave information separately and using that information to gate top octave tone signals and octave tone signals after frequency division. Keyboard 10 produces control signals which are fed via cable 20 to keying circuits 140 which are regular polyphonic organ tone signal keying circuits. Top octave tone generator 100 generates the highest octave of tone signals which are fed via cable 110 to frequency dividers 120 which comprise parallel chains of frequency dividers to generate other octaves of tone signals to be fed to keying circuits 140. Each of the actuated control elements in keyboard 10 operates one or more individual keying circuits in block 140 to produce polyphonic tone signal outputs on cable 150 as in a regular electronic organ system. Preferably the polyphonic organ system employs large scale integrated circuits to perform the top octave tone generation, frequency division, and D.C. keying as is characteristic of recent models of organs introduced by Hammond Organ Company. It would also be preferable to employ a separate oscillator and top octave tone generator to feed frequency dividers 120 so that animation of polyphonic organ signals will be independent of animation of monophonic synthesizer signals. U.S. Pat. Nos. 3,534,144 and 3,636,231 disclose integrated circuit approaches to stairstep synthesis keying for formant organ voices and drawbar synthesis keying for sine wave synthesis organ voices.

Keyboard 10 is preferably a single contact per key system and the D.C. keying control signals from actuated keys which are fed via cable 20 to organ keying circuits 140 are also sent via cable 40 through low octave lockout circuit 30 to note collect circuit 60 and octave collect circuit 70 via cable branches 42 and 41. The output signals from note collect circuit 60 are coupled via cable 90 to note preference circuit 160. The output signals from octave collect circuit 70 are fed via cable 50 to low octave lockout circuit 30 and to octave preference circuit 190. Signals from octave collect circuit 70 cause low octave lockout circuit 30 to lock out all control signals from keyboard 10 except those corresponding to the highest octave in which keys are actuated. This lockout is effective only for control signals fed to octave collect circuit 70 and note collect circuit 60 and does not affect the transmission of control signals to organ keying circuits 140 because of isolation resistors (not shown) within keyboard 10. As a result of low octave lockout circuit 30 only one octave of keys, namely that of the highest actuated key, is active with respect to the synthesizer portion of the system. This synthesizer system will be described in terms of a high select system which is considered to be more useful when the upper or solo keyboard of an organ is used to control the synthesizer since the melody note is usually the highest note played in polyphonic playing and the synthesizer is essentially a melody instrument. It should be readily apparent that a low select system could be provided for a stand-alone version of the synthesizer system and would be essentially the reverse of the approach to be described herein. It should also be apparent that a combined low and high select system could also be provided by duplicating the necessary circuitry.

Top octave tone generator 100 generates at least the top octave of twelve tone signals on cable 110. The highest C note may also be generated as a thirteenth tone signal. These tone signals on cable 110 feed note preference circuit 160 which is controlled by signals from note collect circuit 60 to gate onto output lead 161 only the tone signals corresponding to the highest note played in the active octave. Divider 170 divides the tone signal on lead 161 into octavely related tone signals on cable 180. Octave preference circuit 190 functions under the control of signals from octave collect circuit 70 to gate onto lead 191 the appropriate one of the octavely related tone signals from divider 170 corresponding to the octave in which the highest key is actuated. The tone signal on lead 191 thus corresponds to the highest key actuated in the active (highest) octave in which keys are actuated. Low octave lockout circuit 30 prevents any higher key actuated in a lower octave from affecting note preference circuit 161 and thereby precludes erroneous tone signal selection when plural keys in different octaves are actuated.

The high tone signal on lead 191 is fed to pitch and waveform circuits 200 wherein various different pitches may be selected and different waveforms produced. The selected tone signal of selected pitch and waveform is fed to a voltage-controlled filter 210, thence to a voltagecontrolled amplifier, and finally to an output speaker system. Pitch and waveform circuits 200, voltage-controlled filter 210, voltage-controlled amplifier 220, top octave tone generator 100, voltage-controlled oscillator 240, vibrato and portamento circuits 250, filter envelope generator 270, amplifier envelope generator 280, legato pulse generator 260, volts per octave circuit 230 and keydown detector 80 are discussed in detail below in conjunction with FIGS. 2-11.

FIG. 2 shows a block diagram of an embodiment of this invention in an overall synthesizer system. It shows the outputs of note collect circuit 61 and octave collect circuit 71 feeding into note latches 62 and octave latches 72, respectively. Outputs from note latches 62 and octave latches 72 feed volts per octave circuit 230. These portions of the overall synthesizer system shown together with a specific version of voltage-controlled filter 210 comprise this invention as will be understood from the detailed description of FIGS. 3, 4, 5, and 6 as assembled according to FIG. 7.

FIG. 3 illustrates, in detail, keyboard 10, low octave lockout circuit 30, octave collect circuit 60, /and/ note collect circuit 70 and keydown detector 80. Keyboard 10 comprises a typical one-contact-per-key organ keyboard such as is typically employed in modern organs of the D.C. keying variety. A D.C. keying bus 11 feeds a number of keyswitches 12 -- one for each key on the keyboard of the organ or stand-alone synthesizer unit. Two complete octaves of keyswitches are shown for the notes C through B2 and the first and last keyswitches only for octaves three through five and one keyswitch for C6. Keying bus 11 is coupled to a source of negative keying voltage -V1 which is typically -28 volts. The invention will be described in terms of negative D.C. keying signals, but it should be apparent that positive keying signals could also be employed if obvious adjustments are made in diode directions, transistor types, and bias voltage.

Diodes D1 comprise note collect circuit 60. Each keyswitch corresponding to a C note in each octave is coupled via a diode D1 to common C note bus NB1. Thus any one or more C note keyswitches will place a negative D.C. voltage on bus NB1 through a resistor R1. Corresponding all C- note keying signals are collected, through a resistor R1 and a diode D1, on bus NB2; all D note keying signals are collected on bus NB3, and so forth for all of the notes of the musical scale.

At the same time, cable 41 carries each of the keying signals to diodes D2 which comprise octave collect circuit 70. All of the keyswitches in the first keyboard octave are coupled through resistors R1 and diodes D2 to first octave bus OB1. Similarly, all keyswitches in the second through fifth keyboard octaves, respectively, are coupled to separate busses OB2 to OB5. Keyswitch C6 is a special case, and in this instance is considered part of the fifth octave and is collected on a separate note bus NB13.

In effect, diodes D1 comprise a plurality of logic OR gates for the notes of the musical scale and diodes D2 comprise a plurality of logic OR gates for the octaves of the keyboard. Also, diodes D1 isolate common note busses from keying signals on common octave busses and vice versa for diodes D2.

Diodes D3 comprise a logic OR gate fed by the five common octave busses which functions as a keydown detector 80. Lead 81 will have a negative D.C. voltage thereon whenever any one or more of the keyswitches 12 are actuated and zero volts when no keyswitches 12 are actuated.

Gating circuits 31 through 34 together with diodes D4 and D5 interconnected as shown comprise low octave lockout circuit 30. Transistor T1 in gating circuit 31 will be turned on to a saturate condition by a negative keying signal on common octave bus OB5. Ground reference on the emitter of transistor T1 will appear also at its collector and ground out bus OB4. Similar circuitry in blocks 32 to 34 will be operated by the negative keying voltage fed along a diode string comprising diodes D5, and will thus ground out busses OB1 to OB3. Thus, operation of any one or more of the keyswitches 12 associated with notes C5 through C6 will place a negative keying control signal on bus OB5 and lockout circuit 30 will thereupon ground out busses OB1 to OB4. The grounds on busses OB1 to OB4 are fed back through diodes D2 connecting those busses to the junctions of resistors R1 and diodes D1 associates with the first four octaves of keyswitches associated with notes C1 through B4. Consequently, if any one or more of those keyswitches is operated, they cannot place any keying potential on any of the common note busses because of grounded common octave busses. However, polyphonic organ keying circuits would be cabled into the keyswitch side of resistors R so that keying voltage developed across resistors R1 will operate corresponding D.C. keying circuits for any of the notes C1 through B4 whose keyswitches are actuated.

In a similar manner, if one or more keyswitches in the fourth keyboard octave C4 to B4 are actuated, but none of C5 to C6 are actuated, negative keying potential is fed to bus OB4. This negative voltage operates gating circuit 32 to ground out bus OB3 and is also fed to the left only through diodes D5 to operate circuits 33 and 34 to ground out busses OB1 and OB2. Under these conditions only keyboard octave four is active to produce keying signals on common note busses NB1-NB12 because all other keyboard octaves are locked out. Similar explanations hold for activating only octave three and octave two. In general, then, only the highest keyboard octave in which at least one keyswitch is actuated will be permitted to place keying signals on common note busses NB1 to NB13. This is necessary to provide an unambiguous high note select system as will be shown below by a detailed example. Generally, low octave lockout circuit 30 grounds out all octave busses except the one corresponding to the keyboard octave in which the highest note is played and locks out all keying signals from other octaves at the input to note collect circuit 60 so that only keyswitches in the active (highest) octave can put keying voltage on common note busses NB1 to NB13. Diodes D3 add the keying signals on common octave busses OB1 to OB5 to perform the keydown detector function. Lead 81 will have negative keying potential thereon whenever any one or more keyswitches is actuated.

Referring now to FIG. 4, it will be noted that each of the common note busses NB1 to NB13 is connected to the set lead of an associated one of the note latches FFN1 to FFN13 and each of the common octave busses OB1 to OB5 is connected to the set lead of an associated one of the octave latches FFO1 to FFO5. Only the circuitry of C latch, FFN1, is shown in detail since all other latches are identical. Keydown signal lead 81 feeds the reset leads of all note and octave latches as shown.

Assume for purposes of illustration that C3 is played on keyboard 10. As a result, a negative keydown signal appears on lead 81 and is fed to all latch reset leads. Negative keying signals also appear on busses NB1 and OB3 and feed set leads in C latch FFN1 and third octave latch FFO3. The negative keying signal on bus NB1 is coupled through diode D7 and resistor R45 in FFN1 to transistor T58, causing that transistor to saturate and thereby to shunt the negative keydown signal on the reset lead of FFN1 to ground. Accordingly, the negative keying signal on bus NB1 is also coupled through diodes D7 and D6 and resistor R44 to the base of transistor T59. Assuming the C latch had been in a reset state with transistor T60 on and transistor T59 off, the negative voltage placed on the base of transistor T59 turns it on. As the collector of transistor T59 rises toward ground potential, that voltage is fed via resistor R43 to the base of transistor T60 to turn transistor T60 off. This is a typical flip-flop operation except for the clamping circuit coupled between set and reset leads to provide the necessary set lead preference. As transistor T60 turns off, its collector voltage goes from ground to negative supply -V1 and thus the output on the Q lead of FFN1 switches from ground to -V1. The same operation occurs for third octave latch FFO3 to switch its Q output lead from ground to -V1 assuming FFO3 was previously in a reset state.

Referring to FIG. 5, it will be seen that the respective Q outputs of note latches and octave latches control note and octave preference gating circuits which are identical to those in the above-referenced Schrecongost application. In the Schrecongost circuit keying signals on common note and octave busses are coupled directly to transistor gates in note and octave preference gating arrangements; whereas in the circuitry according to this invention, the keying signals are first memorized in note and octave latches and corresponding note and octave latch outputs control individual transistor gates in note and octave preference gating arrangements.

Assume now that the C3 keyswitch is released and no other key is played immediately. The signals on busses NB1 and OB3 will disappear as will the signal on keydown signal lead 81. Accordingly FFN1 and FFO3 will remain in their set states, and will maintain transistors T15 and T33 in note and octave preference gating circuits in an on state. The high note tone signal fed through voltage-controlled filter 210 into voltage-controlled amplifier 220 (FIGS. 1 and 2) will be sustained and with an appropriate envelope signal to voltagecontrolled amplifier 220, an output tone signal to a speaker is sustained after keyswitch release.

Now assume that the D2 keyswitch is actuated. Keying signals appear on busses NB3 and OB2, and a keydown signal is produced on lead 81. This keydown signal resets FFN1 and FFO3 because of the absence of a keying signal on their set leads. FFN3 and FFO2 are set and transistors T17 and T32 turn on. Transistors T15 and T33 go off as FFN1 and FFO3 flip back to a reset condition to ground their Q output leads. Accordingly a new tone signal for the note D2 appears on high note tone signal lead 191.

Referring now to FIG. 6, it will be seen that the Q outputs from note latches FFN1 to FFN13 are coupled via cable 162 through resistors R23 to the bases of transistors T36 to T48. Each of the transistors T36, T48 is associated in note order with one of note latches FFN1 to FFN13; e.g., T36 is associated with FFN1. Diodes D9 comprise a string of equal voltage dropping elements and transistors T49 and T50 with related circuit components comprise a constant current source feeding the string of diodes D9. The highest one of the transistors T36 to T48 which is turned on by its associated note latch grounds out the string of diodes at a particular junction. The voltage drop across the diodes which then effectively remain in the string produces a negative voltage on the collector of T50 which is proportional to the position of the highest note played. Low notes produce a larger negative voltage than high notes. Thus the note voltage signal on the collector of transistor T50 is linearly related to the position of the highest note played on keyboard 10.

A similar set of five transistors T51 to T55 is controlled by octave latches OB1 to OB5. Transistors T56 and T57 with associated circuit components comprise a second constant current source which feeds a second string of diodes D9. In a similar manner to that described above, the octave voltage signal on the collector of transistor T57 is proportional to the highest octave in which a keyswitch is actuated to set a corresponding one of octave latches FFO1 to FFO5.

Transistors T61 and T62 and related circuit elements comprise emitter follower circuits which couple the note and octave voltage signals to a summing network comprising resistors R33 to R38 as shown. Resistors R33 and R34 and resistors R37 and R38 are selected to have values which will produce a 1:12 ratio of input signals for notes and octaves. In other words, since there are twelve notes per octave, a voltage change between the same note in two adjacent octaves must be twelve times the voltage change between two adjacent notes in the same octave. The note and octave voltage signals in 1:12 ratios are summed with a range adjust voltage fed through resistors R35 and R36 and form an input to the negative input lead of operational amplifier OA1. The gain of operational amplifier OA1 is set by the resistors R39 and R40 in a negative feedback loop such that the change in voltage for a one-octave change in high note position is the standard one volt which is employed in the Moog and ARP type of synthesizers.

This volt per octave signal is useful to produce a pitch slide effect in the type of synthesizer system which is shown in FIG. 2. However, it should be understood that this volt per octave circuit could also be employed in place of the volt per octave circuit in the Moog and ARP type of synthesizer system as described above. In other words, the circuitry shown in FIGS. 3, 4, and 6 herein could be used to produce a very stable volt per octave signal to be used to tune a voltage-controlled oscillator as a primary monophonic tone signal source in a synthesizer system. The need for a sample and hold circuit to hold the volt per octave signal after keyswitch release is eliminated since the note and octave latches which remain set after keyswitch release maintain the volt per octave output signal at a very constant level until a different high note keyswitch is actuated. This approach would thus be preferable in integrating the Moog and ARP type of synthesizer into an organ because it would eliminate the need to employ additional contact pairs in the organ keyboard to control the synthesizer system.

The circuitry shown in FIGS. 3 to 6 preserves all of the advantages of the Schrecongost system and dramatically enhances its functional capabilities. Unambiguous high note select in a stand-alone synthesizer version or an integrated organ-synthesizer version is provided with note and octave latching so that sustained tones after keyswitch release can be produced. The circuitry in FIGS. 3, 4, and 6, without FIG. 5, also preserves the unambiguous high note select function and provides a volt per octave output which is useful for tuning a voltage-controlled oscillator in a prior art type of synthesizer system of the Moog and ARP variety and which would improve the stability of tuning of the oscillator. The diodes D9 in the volts per octave circuit of FIG. 6 can readily be tightly selected to have equal forward voltage drops to ensure a precise volts per octave signal. Moreover, precision resistors could be substituted as voltage dropping elements without changing the function of the circuitry.

FIG. 8 depicts a voltage-controlled filter which is of quite simple construction and performs quite adequately in comparison to much more complex voltage-controlled filters in prior art synthesizer systems. An audio signal containing multiple harmonic components is fed through the network of resistors R173 to 177 and capacitor C33 to the positive input of operational amplifier OA2. Capacitor C33 and resistor R176 form a high pass filter which attenuates low frequency components of input audio signal.

Capacitors C28 to C31 together with matched field effect transistors FET 1 to FET 4 comprise four cascaded RC phase shift networks in a negative feedback loop of operational amplifier OA2. The effective source-drain resistance of FET 1 to FET 4 is controlled by the voltage on a control signal input to each of their gate electrodes and alters the frequency response of each network. A some signal frequency depending upon the gate voltage, the four phase shift networks will produce a 180° phase shift in the signal transmitted therethrough. This frequency will be denoted the resonant frequency. Consequently, at the resonant frequency, the negative feedback, in effect, is positive feedback and this peaks the gain of operational amplifier OA2 as shown in FIG. 9. Operational amplifier OA1 is operated in a unity gain mode and prevents the phase shift networks from loading the input to operational amplifier OA2. For signal frequencies substantially lower than the resonant frequency, the cascaded phase shift networks highly attenuate the signal so there is very little negative feedback for those frequencies. However, due to the effect of negative feedback via resistor R147 and positive feedback via resistors R148 and R149 and capacitor C34, a net positive feedback occurs for low frequency signals to provide fixed amplifier gain at those frequencies as shown in FIG. 9. For signal frequencies substantially above the resonant frequency the cascaded phase shift networks provide essentially no effective phase shift or attenuation, and thus for those frequencies negative feedback far outweighs any positive feedback and the gain of amplifier OA2 rapidly drops toward zero above the resonant frequency as shown in FIG. 9.

Thus, by controlling the DC control signal envelope into the gates of FET 1 through FET 4, the resonant frequency of the filter can be dynamically varied. Transistor T51 and its associated circuit components comprise an AC amplifier for the output signal from the voltage-controlled filter.

The following table sets forth a typical set of component values and types which can be employed in the circuitry of FIGS. 3 through 8. RESISTORS Numbers Ohms Numbers Ohms ______________________________________ 7, 20 100 25 200 24 270 5, 27, 28 1K 14, 26 1.5K 37 5K 8, 32, 15, 42, 43, 44, 18 15K 31, 32 10K 9, 21, 10, 22 22K 39 20K 4, 23 47K 35, 41 50K 36 82K 1, 2, 6, 17, 19, 29, 30, 45 100K 38 150K 40 170K 13 220K 12 330K 16 470K 3, 33, 34 1M 11 22M CAPACITORS Numbers Microfarads ______________________________________ 3,4 .0033 1 .015 2 25 All Diodes ITT 2045 Transistors NPN Sprague 2N4954 PNP Sprague 32S6438 IC Divider 171 to 174 (FIG. 5) Nucleonics Products SSF 5002 ______________________________________

It should be apparent that numerous modifications in the above-described embodiments of this invention could be made without departing from the scope of this invention as claimed in the following claims.