Title:
CONTROL CIRCUIT FOR SUSTAIN KEYER CIRCUIT IN ELECTRONIC MUSICAL INSTRUMENT
United States Patent 3665091
Abstract:
An electronic musical instrument has electronic keyers each including a time constant circuit containing a capacitor to give a sustain envelope to a switched tone signal. The improvement comprises: a switching element connected to the electronic keyer, said switching element being a field effect transistor and having a control electrode and another grounded electrode; and means connected to said control electrode for generating a relatively high frequency rectangular wave whose duty factor is variable, wherein by continuously varying the duty factor of the output signal derived from said rectangular wave generating means to control the control electrode of said switching element, a desired sustain time as well as the optimum auditory decay envelope characteristic are obtained.
US Patent References:
SWITCHING DEVICE FOR ELECTRONIC MUSICAL INSTRUMENT WITH SUSTAIN EFFECT
Teranishi - October 1970 - 3535972
High-speed transistor pulse repeater circuit
Kan - January 1968 - 3363116
Apparatus for demodulating an fm carrier
Ott - October 1968 - 3407358
Variable time constant learning means
Harris et al. - December 1968 - 3414735
SWITCHING DEVICE FOR ELECTRONIC MUSICAL INSTRUMENT WITH SUSTAIN EFFECT
Teranishi - October 1970 - 3535972
High-speed transistor pulse repeater circuit
Kan - January 1968 - 3363116
Apparatus for demodulating an fm carrier
Ott - October 1968 - 3407358
Variable time constant learning means
Harris et al. - December 1968 - 3414735
Application Number:
05/027028
Publication Date:
05/23/1972
Filing Date:
04/09/1970
View Patent Images:
Export Citation:
Assignee:
Nippon Gakki Seizo Kabushiki Kaisha (Hamamatsu-shi, Shizuoka-key, JA)
Primary Class:
Other Classes:
984/322, 84/702
International Classes:
G10H1/057; G10H1/02
Field of Search:
84/1.01,1.13,1.26,DIG.7,DIG.8,DIG.23 307/246,264,268,304,251
Other References:
jacob Millman & Herbert Taub; Pulse, Digital, and Switching Waveforms; Copyright 1965 by McGraw-Hill, Inc. pp. 389-394..
Primary Examiner:
Myers, Lewis H.
Assistant Examiner:
Witkowski, Stanley J.
Claims:
I claim
1. In an electronic musical instrument including sustain keyers each of which has a key-operated switch operable to produce as audible tone signals and a capacitor for sustain-keying, the improvement in a sustain time control circuit, for and associated with each said sustain keyer, comprising:
2. A control circuit according to claim 1, in which said rectangular wave generating means comprises a triangular wave generating circuit, a Schmitt trigger circuit adapted to receive an output signal of a repetition frequency from said triangular wave generating circuit and to generate a rectangular wave, and means for varying a D.C. input potential to said Schmitt trigger circuit.
3. A control circuit according to claim 2, in which said means for varying the D.C. input potential comprises a variable resistor connected to said Schmitt trigger circuit for varying a D.C. input potential of the Schmitt trigger circuit.
4. An arrangement as in claim 1 wherein said switching element is a field-effect transistor.
5. A sustain keyer circuit comprising:
1. In an electronic musical instrument including sustain keyers each of which has a key-operated switch operable to produce as audible tone signals and a capacitor for sustain-keying, the improvement in a sustain time control circuit, for and associated with each said sustain keyer, comprising:
2. A control circuit according to claim 1, in which said rectangular wave generating means comprises a triangular wave generating circuit, a Schmitt trigger circuit adapted to receive an output signal of a repetition frequency from said triangular wave generating circuit and to generate a rectangular wave, and means for varying a D.C. input potential to said Schmitt trigger circuit.
3. A control circuit according to claim 2, in which said means for varying the D.C. input potential comprises a variable resistor connected to said Schmitt trigger circuit for varying a D.C. input potential of the Schmitt trigger circuit.
4. An arrangement as in claim 1 wherein said switching element is a field-effect transistor.
5. A sustain keyer circuit comprising:
Description:
BACKGROUND OF THE INVENTION
The present invention is concerned with a sustain keyer circuit for use in electronic musical instruments, and more particularly, it relates to an improvement in sustain time control circuits for use in electronic musical instruments.
Among the sustain keyer circuits which I have developed of late and which are capable of providing for a sustain effect, there is the arrangement and which is the subject of copending U.S. application Ser. No. 27,027 filed April 9, 1970) which comprises first and second two field effect transistors (FET's), Q 1 and Q 2 , which are connected in series to each other from both DC and AC viewpoints, said first FET Q 1 being arranged so that its source electrode is grounded and that a tone signal is adapted to be applied to its gate electrode via an input terminal T 1 and further that its drain electrode is connected to the source electrode of the second FET Q 2 , whereas the drain electrode of this second FET Q 2 is connected, on the one hand, to a voltage source -V d via a load resistance element R and, on the other hand, to an output terminal T 2 via a capacitor C, whereby forming an electronic switch, there being interposed a serial circuit consisting of a resistance element R o and a key-operated switch S o between said voltage source -V d and the gate of the second FET Q 2 which constitutes the keying terminal K of the electronic switch to thereby switch a keying voltage to be applied to the gate electrode of the second FET Q 2 , there further being connected a capacitor C between the gate electrode of the second FET Q 2 and the ground point from AC viewpoint (herein it is -V d ) to form a keying circuit so that said tone signal may be derived at the output terminal T 2 upon keying of said key-operated switch, while, on the other hand, the gate electrode of said second FET Q 2 is connected to the drain electrode of a third FET Q 3 whose source electrode is grounded and whose gate electrode is connected to a slidable contact r a of a variable resistor (potentiometer) R a which is interposed between the ground and a variable D.C. source, i.e., the voltage source -V d .
Referring now to the drawings, both the sustain keyer circuit SA given in the block defined by a-dot-and-a-dash line in FIG. 1 and said third FET Q 3 shown in FIG. 1 are to be installed in, for example, a console of an electronic musical instrument in a number corresponding to the large number, for example, 90 -100, of tone generators installed in accordance with the number of the keys of the electronic musical instrument. On the other hand, there is employed only one variable resistor R a which is assigned to adjust the gate potential of the third FET Q 3 to determine the value of resistance between the drain electrode and the source electrode thereof and which has its slidable contact connected in common to the large number of gate electrodes of FET Q 3 of the type described and intended for controlling the sustain time and used as variable resistance elements of a correspondingly large number, so that, by the operation of said variable resistor R a , the sustain time (i.e., the decay time) of the large number of output signals after the keying-off of the corresponding number of key-operated switches S o may be adjusted simultaneously to the desired time through the large number of FET Q 3 's.
In the aforesaid arrangement, it is to be noted that the drain-source resistance of the FET Q 3 will increase as, for example, the position of the slidable contact r a of the variable resistor R a is brought closer to the ground side thereof, and that, therefore, the transient time of charging of the capacitor C o , or in other words the sustain time, in the electronic musical instrument, will be as long as the duration of time from t o to t 2 as shown by the chain curve in FIG. 5. Contrariwise, as the aforesaid slidable contact r a of the variable resistor R a is positioned closer to the voltage source -V d side, the resistance value between the drain-source of the FET Q 3 will decrease so that the sustain time will become shortened to the duration of time from t o to t 1 as shown in FIG. 5. Thus, it will become possible to relatively easily control the sustain time of an electronic musical instrument. In view of the fact, however, that, in the aforesaid arrangement of the prior art, the FET Q 3 has a non-linear voltage versus current characteristic, the decay characteristic of the tone signal within the aforesaid sustain time will assume an upwardly curved pattern as shown by the envelope curve in logarithmic (dB) scale in FIG. 5. The tone decay having the foregoing decay characteristic in an electronic musical instrument is not desirable because of the un-naturalness of the decaying pattern.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a sustain time control circuit capable of arbitrarily controlling the sustain time in an electronic musical instrument and having an improved output signal decay characteristic of a sustain keyer circuit.
Another object of the present invention is to provide a sustain control circuit for use in an electronic musical instrument, wherein there is provided a switching element having an internal resistance such as a field effect transistor which, together with a capacitor, constitutes a time constant circuit and also having a control electrode and being controlled by a high frequency rectangular wave to thereby obtain a desired sustain effect.
A further object of the present invention is to provide a sustain time control circuit in an electronic musical instrument which is such that an arrangement capable of continuously varying the duty factor of the high frequency rectangular wave for controlling a sustain effect can be obtained easily and at a low cost.
Other objects, features and attendant advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing the circuit of my prior invention which is described in more detail in U.S. application Ser. No. 27,027filed April 9, 1970;
FIG. 2 is a circuit diagram containing a sustain time control circuit embodying the present invention;
FIGS. 3a, 3b and 4 are diagrams illustrating, respectively, the operation of the sustain time control circuit embodying the present invention; and
FIG. 5 is a diagram illustrating the operation of the circuit shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2, there is shown an embodiment of the present invention, in which arrangement is provided so that the output signal produced in a triangular wave form having a predetermined repetition frequency, such as 40 kHz, which is higher than the audible frequency and which is obtained from a triangular wave generator G is applied to the input side of the known Schmitt trigger circuit F (given within the block defined by the chain line in FIG. 2) which is comprised of elements such as transistors Q 4 and Q 5 and resistors, via a DC blocking capacitor C 1 and a diode D in this order, and that the rectangular wave output terminal T o of said Schmitt trigger circuit F is connected to the gate electrode of the FET Q 3 having other two electrodes connected to the ground and the gate electrode of the FET Q 2 - which constitutes the keying terminal K - of the sustain keyer circuit SA' (given within the block defined by a-dot-and-a-dash line in FIG. 2) which is constructed in exactly the same fashion with my earlier sustain keyer circuit SA (given within the block defined by the chain line in FIG. 1), so that the rectangular wave output signal of the aforesaid predetermined repetition frequency which is produced in said Schmitt trigger circuit F is applied to the gate electrode of the FET Q 3 . By use of the aforesaid circuit arrangement, the latter is operative in such a way that, by changing the DC potential of the point d of connection between the aforesaid capacitor C 1 and the diode D via a variable resistor RB which is interposed between the voltage source -V c and the ground, the duty factor (meaning the ratio of a pulse width to a pulse repetition period of the rectangular wave signal produced at the aforesaid output terminal T o is varied continuously to thereby be able to continuously change the conducting-to-nonconducting time ratio of said FET Q 3 (in other words, the time ratio between the momentary conducting time and the momentary non-conducting time of this FET Q 3 ) from 0 to ∞. It is to be noted that both the sustain keyer circuit SA' which is given in the block defined by a-dot-and-a-dash line in FIG. 2 and the aforesaid FET Q 3 are provided in, for example, the console of an electronic musical instrument each in number corresponding to the multiplicit number of the tone generators installed in accordance with the number of the keys of the electronic music instrument. It should be noted also that such elements as the triangular wave generator G, the variable resistor RB and the Schmitt trigger circuit F are provided one each in such a way that the combination of these elements is used in common to said multiplicit number of FET Q 3 to continuously effect simultaneous change of the conductive time of said multiplicity of FET Q 3 by changing the duty factor of the rectangular wave signal generated at one output terminal T o of said Schmitt trigger circuit F.
The sustain time control circuit according to the present invention which for use in an electronic musical instrument is of the foregoing arrangement.
Description will hereunder be made on the function of this circuitry. The key-operated switch S o which is arranged to be adapted to be closed in association with the depression applied to the selected keys of the keyboard of an electronic musical instrument is keyed by the player or the user of the instrument. The values of the resistance elements R and R o and the value of the voltage source -V d are preliminarily set so that both the FET Q 1 and the FET Q 2 are simultaneously rendered "cut-off" or "on" in accordance with the opened or closed state of the switch S o . Along with this, arrangement is provided so that the transient characteristic in the course of the beginning of conduction of the FET Q 1 and the FET Q 2 is determined by the time constant which, in turn, is determined by the values assumed, whenever the key-operated switch S o is closed, by the capacitor C o and by the resistance element R o which are interposed between the gate circuit of the FET Q 2 to thereby determine the build-up characteristic of the tone signal derived from the output terminal T 2 of the sustain keyer circuit SA', and that, on the other hand, whenever the key-operated switch S o is opened, the transient characteristic in the course of the ending of conduction of both the FET Q 1 and the FET Q 2 is determined by the time constant which, in turn is determined by both of the value of the capacitor C o and the value of resistance produced between the drain electrode and the source electrode of the FET Q 3 to thereby obtain a desired decay characteristic of said tone signal (the so-called sustain time), and further that a tone source signal having a predetermined frequency and waveform derived from a known tone generator such as a flip-flop circuit (not shown) which is installed separately is applied always to the input terminal T 1 .
Let us now assume that the circuitry is in the state such that the DC level of an output signal form the triangular wave generator G is set at a predetermined value by means of the variable resistor RB and that the duty factor of the rectangular wave produced at the output of the Schimitt trigger circuit F is set at a predetermined value to have the conducting to non-conducting time ratio of the FET Q 3 set at a predetermined value of the order of, for example, 0.5. When the key-operated switch S o is opened in such a state of the circuitry, the junction between the drain electrode of the FET Q 3 and the gate electrode of the FET Q 2 , i.e., the keying terminal K, is at the ground potential. Therefore, no negative voltage for rendering the FET Q 2 conductive is applied to the gate electrode of this EFT Q 2 , so that this latter FET Q 2 is held in its "cut-off" state. At the same time, no negative voltage is applied either to the drain electrode of the FET Q 1 which is connected to the source electrode of the FET Q 2 , so that this FET Q 1 is also held in its "cut-0ff" state. Thus, both of the two FET's, Q 1 and Q 2 , are caused to remain in their "cut-off" state. Accordingly, the tone signal which is always applied to the gate electrode of the FET Q 1 does not appear at the drain side of the FET Q 2 .
During the foregoing operation of the circuitry, it will be understood that the capacitor C o which is connected to the gate circuit of the FET Q 2 is in the state of being charged up with the voltage of the power source -V d by the repetition of the momentary conducting which is effected between the drain electrode and the source electrode of the FET Q 3 , with the keying terminal K having a voltage potential substantially equal to the ground potential zero.
On the other hand, when the key-operated switch S o is closed, the charge on the capacitor C o which has been in the state of being charged up in the mode of operation described above is discharged through the resistance element R o which, in turn, is connected in series with said key-operated switch S o , and along with this, the potentials of both the gate electrode of the FET Q 2 and the drain electrode of the FET Q 3 approach the negative potential of the power source -V d . As a result, the FET Q 2 becomes conductive first and, almost simultaneously therewith, the FET Q 1 will also become conductive, and accordingly, the tone signal which is applied to the input of the FET Q 1 will be detected at the output terminal T 2 via the drain electrode of this FET Q 1 and the source electrode and the drain electrode of the FET Q 2 , successively in this order. The tone signal thus detected may be amplified as desired to be converted to an audible sound wave to be given out from such an appliance as a loud speaker.
Next, when the key-operated switch S o is opened again at the time indicated by t o in FIG. 4, the capacitor C o which has been in the state of being discharged in the mode of operation described above begins to be charged up with the voltage of the power source -V d through the average (apparent) resistance between the source and the drain of the FET Q 3 which is rapidly switched to be momentarily conducting and momentarily non-conducting, and along with this, the gate potential of the FET Q 2 as well as the drain potential of the FET Q 3 approach the ground potential. Also, when the potentials of these two electrodes constituting a keying terminal K take a value in excess of (meaning more positive than) a predetermined potential at the time of either t 1 or t 2 given in FIG. 4, the FET Q 2 and the FET Q 1 are both rendered non-conductive, respectively. Thus, these two FET's, Q 2 and Q 1 , are cut off in the same manner as described in connection with the opened state of the key-operated switch S o . As a result, the tone signal which is applied to the input terminal T 1 of the FET Q 1 becomes shut off by these FET's, Q 2 and Q 1 , at the end of a lapse of the sustain time (t 1 or t 2 ) which is determined by the time constant which, in turn, is determined by the capacity of the capacitor C o and the apparent resistance resulting from the conducting to non-conducting time ratio between the drain and the source electrodes of the FET Q 3 , and thus, no tone signal becomes to be detected at the output terminal T 2 of the sustain keyer circuit SA'.
The aforesaid sustain time (t 1 or t 2 ) or the decay time of an electronic musical instrument may be controlled by varying the DC level of the triangular wave signal applied to the input side of the Schmitt trigger circuit F, by altering the resistance value of such an element as the variable resistor RB to thereby alter the duty factor of the rectangular wave output signal derived at the output of the said Schmitt trigger circuit F, whereby continuously varying both the momentary duration a of cut-off time and the momentary duration b of conducting time of the FET Q 3 in the manner as shown in FIGS. 3a and 3b, continuously from the pattern shown in FIG. 3a to the pattern shown in FIG. 3b. For example, in the state of the duty factor shown in FIG. 3a, the momentary conducting period of time b is considerably smaller as compared with the momentary cut-off period of time a, so that the length of charge-up time of the capacitor C o following the opening of the aforesaid key-operated switch S o (meaning after the time t o in FIG. 4) will accordingly become greater. Hence, the sustain time or the decay time of the tone signal which is detected at the output terminal T 2 under the aforesaid condition will be as long as the period of time from t o to t 2 as indicated by the straight chain line in FIG. 4. Contrariwise, in case where the momentary conducting period of time b of the FET Q 3 is substantially great as compared with its momentary cut-off period of time a as shown in FIG. 3b, the length of charge-up time of the capacitor C o following the opening of the key-operated switch S o (meaning after the time t o in FIG. 4) is reduced, so that the sustain time or the decay time of the aforesaid tone signal will become as short as the period of time from t o to t 1 as shown by the straight solid line in FIG. 4. In this way, the sustain time of an electronic musical instrument can be controlled. However, in the present invention, in particular, the circuitry is arranged in such a way that the gate potential of the FET Q 3 is controlled at a very high repetition rate, such as 40 kHz so that the FET Q 3 can alternately become fully conductive and fully non-conductive. As a result, the FET Q 3 can effect the switching between its cut-off state and its conductive state at a high speed, regardless of the nonlinear operation characteristic of this FET Q 3 . Thus, the FET Q 3 is always held in either the fully conductive or the fully non-conductive state, and the average (apparent) internal resistance between the drain and the source electrodes of this FET Q 3 will be caused by vary by the varying duty factor. Therefore, it is possible to consider that the voltage to current characteristic is of a value equivalent to that of a variable resistor having a linear voltage to current characteristic. Thus, the decay characteristic of the tone signal within the aforesaid sustain time can be obtained as one which is quite close to a complete rectilinear line as shown by the envelope straight lines in logarithmic (dB) scale in FIG. 4, so that the optimum auditory exponential decay pattern can be obtained.
It is to be noted that, by the employment of only one each of the rectangular wave signal generating means -- such as the triangular wave generator G, the Schmitt trigger circuit F, the diode D and the variable resistor RB, which are capable of varying the duty factor -- it is possible to effect the switching of a number of FET Q 3 assigned for controlling the sustain time. Thus, the circuitry of the present invention is of many advantages that it can be produced easily and at a low cost, which is highly useful industrially.
Description has been made on the instance in which the capacitor C o is connected between the gate electrode of the FET Q 2 and the power source -V d . It should be understood, however, that this capacitor C o may, as a matter of course, be inserted between the gate electrode of the FET Q 2 and the ground. In such an instance, it should be noted that, although the variation of the gate potential of the FET Q 3 resulting from the keying action of the key-operated switch C o is done in exactly the same way as that described above, the relation of charging and discharging of the capacitor C o per se will be the reverse of that described in connection with the preceding example.
Description has been directed also to an embodiment in which FET Q 3 is used as the switching element for controlling the sustain time. It should be understood easily, however, that this switching element may employ elements of other types, such as a three-poled diode, a drift transistor, an alloy diffused transistor or a micro-alloy diffused transistor.
The present invention is concerned with a sustain keyer circuit for use in electronic musical instruments, and more particularly, it relates to an improvement in sustain time control circuits for use in electronic musical instruments.
Among the sustain keyer circuits which I have developed of late and which are capable of providing for a sustain effect, there is the arrangement and which is the subject of copending U.S. application Ser. No. 27,027 filed April 9, 1970) which comprises first and second two field effect transistors (FET's), Q 1 and Q 2 , which are connected in series to each other from both DC and AC viewpoints, said first FET Q 1 being arranged so that its source electrode is grounded and that a tone signal is adapted to be applied to its gate electrode via an input terminal T 1 and further that its drain electrode is connected to the source electrode of the second FET Q 2 , whereas the drain electrode of this second FET Q 2 is connected, on the one hand, to a voltage source -V d via a load resistance element R and, on the other hand, to an output terminal T 2 via a capacitor C, whereby forming an electronic switch, there being interposed a serial circuit consisting of a resistance element R o and a key-operated switch S o between said voltage source -V d and the gate of the second FET Q 2 which constitutes the keying terminal K of the electronic switch to thereby switch a keying voltage to be applied to the gate electrode of the second FET Q 2 , there further being connected a capacitor C between the gate electrode of the second FET Q 2 and the ground point from AC viewpoint (herein it is -V d ) to form a keying circuit so that said tone signal may be derived at the output terminal T 2 upon keying of said key-operated switch, while, on the other hand, the gate electrode of said second FET Q 2 is connected to the drain electrode of a third FET Q 3 whose source electrode is grounded and whose gate electrode is connected to a slidable contact r a of a variable resistor (potentiometer) R a which is interposed between the ground and a variable D.C. source, i.e., the voltage source -V d .
Referring now to the drawings, both the sustain keyer circuit SA given in the block defined by a-dot-and-a-dash line in FIG. 1 and said third FET Q 3 shown in FIG. 1 are to be installed in, for example, a console of an electronic musical instrument in a number corresponding to the large number, for example, 90 -100, of tone generators installed in accordance with the number of the keys of the electronic musical instrument. On the other hand, there is employed only one variable resistor R a which is assigned to adjust the gate potential of the third FET Q 3 to determine the value of resistance between the drain electrode and the source electrode thereof and which has its slidable contact connected in common to the large number of gate electrodes of FET Q 3 of the type described and intended for controlling the sustain time and used as variable resistance elements of a correspondingly large number, so that, by the operation of said variable resistor R a , the sustain time (i.e., the decay time) of the large number of output signals after the keying-off of the corresponding number of key-operated switches S o may be adjusted simultaneously to the desired time through the large number of FET Q 3 's.
In the aforesaid arrangement, it is to be noted that the drain-source resistance of the FET Q 3 will increase as, for example, the position of the slidable contact r a of the variable resistor R a is brought closer to the ground side thereof, and that, therefore, the transient time of charging of the capacitor C o , or in other words the sustain time, in the electronic musical instrument, will be as long as the duration of time from t o to t 2 as shown by the chain curve in FIG. 5. Contrariwise, as the aforesaid slidable contact r a of the variable resistor R a is positioned closer to the voltage source -V d side, the resistance value between the drain-source of the FET Q 3 will decrease so that the sustain time will become shortened to the duration of time from t o to t 1 as shown in FIG. 5. Thus, it will become possible to relatively easily control the sustain time of an electronic musical instrument. In view of the fact, however, that, in the aforesaid arrangement of the prior art, the FET Q 3 has a non-linear voltage versus current characteristic, the decay characteristic of the tone signal within the aforesaid sustain time will assume an upwardly curved pattern as shown by the envelope curve in logarithmic (dB) scale in FIG. 5. The tone decay having the foregoing decay characteristic in an electronic musical instrument is not desirable because of the un-naturalness of the decaying pattern.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a sustain time control circuit capable of arbitrarily controlling the sustain time in an electronic musical instrument and having an improved output signal decay characteristic of a sustain keyer circuit.
Another object of the present invention is to provide a sustain control circuit for use in an electronic musical instrument, wherein there is provided a switching element having an internal resistance such as a field effect transistor which, together with a capacitor, constitutes a time constant circuit and also having a control electrode and being controlled by a high frequency rectangular wave to thereby obtain a desired sustain effect.
A further object of the present invention is to provide a sustain time control circuit in an electronic musical instrument which is such that an arrangement capable of continuously varying the duty factor of the high frequency rectangular wave for controlling a sustain effect can be obtained easily and at a low cost.
Other objects, features and attendant advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing the circuit of my prior invention which is described in more detail in U.S. application Ser. No. 27,027filed April 9, 1970;
FIG. 2 is a circuit diagram containing a sustain time control circuit embodying the present invention;
FIGS. 3a, 3b and 4 are diagrams illustrating, respectively, the operation of the sustain time control circuit embodying the present invention; and
FIG. 5 is a diagram illustrating the operation of the circuit shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2, there is shown an embodiment of the present invention, in which arrangement is provided so that the output signal produced in a triangular wave form having a predetermined repetition frequency, such as 40 kHz, which is higher than the audible frequency and which is obtained from a triangular wave generator G is applied to the input side of the known Schmitt trigger circuit F (given within the block defined by the chain line in FIG. 2) which is comprised of elements such as transistors Q 4 and Q 5 and resistors, via a DC blocking capacitor C 1 and a diode D in this order, and that the rectangular wave output terminal T o of said Schmitt trigger circuit F is connected to the gate electrode of the FET Q 3 having other two electrodes connected to the ground and the gate electrode of the FET Q 2 - which constitutes the keying terminal K - of the sustain keyer circuit SA' (given within the block defined by a-dot-and-a-dash line in FIG. 2) which is constructed in exactly the same fashion with my earlier sustain keyer circuit SA (given within the block defined by the chain line in FIG. 1), so that the rectangular wave output signal of the aforesaid predetermined repetition frequency which is produced in said Schmitt trigger circuit F is applied to the gate electrode of the FET Q 3 . By use of the aforesaid circuit arrangement, the latter is operative in such a way that, by changing the DC potential of the point d of connection between the aforesaid capacitor C 1 and the diode D via a variable resistor RB which is interposed between the voltage source -V c and the ground, the duty factor (meaning the ratio of a pulse width to a pulse repetition period of the rectangular wave signal produced at the aforesaid output terminal T o is varied continuously to thereby be able to continuously change the conducting-to-nonconducting time ratio of said FET Q 3 (in other words, the time ratio between the momentary conducting time and the momentary non-conducting time of this FET Q 3 ) from 0 to ∞. It is to be noted that both the sustain keyer circuit SA' which is given in the block defined by a-dot-and-a-dash line in FIG. 2 and the aforesaid FET Q 3 are provided in, for example, the console of an electronic musical instrument each in number corresponding to the multiplicit number of the tone generators installed in accordance with the number of the keys of the electronic music instrument. It should be noted also that such elements as the triangular wave generator G, the variable resistor RB and the Schmitt trigger circuit F are provided one each in such a way that the combination of these elements is used in common to said multiplicit number of FET Q 3 to continuously effect simultaneous change of the conductive time of said multiplicity of FET Q 3 by changing the duty factor of the rectangular wave signal generated at one output terminal T o of said Schmitt trigger circuit F.
The sustain time control circuit according to the present invention which for use in an electronic musical instrument is of the foregoing arrangement.
Description will hereunder be made on the function of this circuitry. The key-operated switch S o which is arranged to be adapted to be closed in association with the depression applied to the selected keys of the keyboard of an electronic musical instrument is keyed by the player or the user of the instrument. The values of the resistance elements R and R o and the value of the voltage source -V d are preliminarily set so that both the FET Q 1 and the FET Q 2 are simultaneously rendered "cut-off" or "on" in accordance with the opened or closed state of the switch S o . Along with this, arrangement is provided so that the transient characteristic in the course of the beginning of conduction of the FET Q 1 and the FET Q 2 is determined by the time constant which, in turn, is determined by the values assumed, whenever the key-operated switch S o is closed, by the capacitor C o and by the resistance element R o which are interposed between the gate circuit of the FET Q 2 to thereby determine the build-up characteristic of the tone signal derived from the output terminal T 2 of the sustain keyer circuit SA', and that, on the other hand, whenever the key-operated switch S o is opened, the transient characteristic in the course of the ending of conduction of both the FET Q 1 and the FET Q 2 is determined by the time constant which, in turn is determined by both of the value of the capacitor C o and the value of resistance produced between the drain electrode and the source electrode of the FET Q 3 to thereby obtain a desired decay characteristic of said tone signal (the so-called sustain time), and further that a tone source signal having a predetermined frequency and waveform derived from a known tone generator such as a flip-flop circuit (not shown) which is installed separately is applied always to the input terminal T 1 .
Let us now assume that the circuitry is in the state such that the DC level of an output signal form the triangular wave generator G is set at a predetermined value by means of the variable resistor RB and that the duty factor of the rectangular wave produced at the output of the Schimitt trigger circuit F is set at a predetermined value to have the conducting to non-conducting time ratio of the FET Q 3 set at a predetermined value of the order of, for example, 0.5. When the key-operated switch S o is opened in such a state of the circuitry, the junction between the drain electrode of the FET Q 3 and the gate electrode of the FET Q 2 , i.e., the keying terminal K, is at the ground potential. Therefore, no negative voltage for rendering the FET Q 2 conductive is applied to the gate electrode of this EFT Q 2 , so that this latter FET Q 2 is held in its "cut-off" state. At the same time, no negative voltage is applied either to the drain electrode of the FET Q 1 which is connected to the source electrode of the FET Q 2 , so that this FET Q 1 is also held in its "cut-0ff" state. Thus, both of the two FET's, Q 1 and Q 2 , are caused to remain in their "cut-off" state. Accordingly, the tone signal which is always applied to the gate electrode of the FET Q 1 does not appear at the drain side of the FET Q 2 .
During the foregoing operation of the circuitry, it will be understood that the capacitor C o which is connected to the gate circuit of the FET Q 2 is in the state of being charged up with the voltage of the power source -V d by the repetition of the momentary conducting which is effected between the drain electrode and the source electrode of the FET Q 3 , with the keying terminal K having a voltage potential substantially equal to the ground potential zero.
On the other hand, when the key-operated switch S o is closed, the charge on the capacitor C o which has been in the state of being charged up in the mode of operation described above is discharged through the resistance element R o which, in turn, is connected in series with said key-operated switch S o , and along with this, the potentials of both the gate electrode of the FET Q 2 and the drain electrode of the FET Q 3 approach the negative potential of the power source -V d . As a result, the FET Q 2 becomes conductive first and, almost simultaneously therewith, the FET Q 1 will also become conductive, and accordingly, the tone signal which is applied to the input of the FET Q 1 will be detected at the output terminal T 2 via the drain electrode of this FET Q 1 and the source electrode and the drain electrode of the FET Q 2 , successively in this order. The tone signal thus detected may be amplified as desired to be converted to an audible sound wave to be given out from such an appliance as a loud speaker.
Next, when the key-operated switch S o is opened again at the time indicated by t o in FIG. 4, the capacitor C o which has been in the state of being discharged in the mode of operation described above begins to be charged up with the voltage of the power source -V d through the average (apparent) resistance between the source and the drain of the FET Q 3 which is rapidly switched to be momentarily conducting and momentarily non-conducting, and along with this, the gate potential of the FET Q 2 as well as the drain potential of the FET Q 3 approach the ground potential. Also, when the potentials of these two electrodes constituting a keying terminal K take a value in excess of (meaning more positive than) a predetermined potential at the time of either t 1 or t 2 given in FIG. 4, the FET Q 2 and the FET Q 1 are both rendered non-conductive, respectively. Thus, these two FET's, Q 2 and Q 1 , are cut off in the same manner as described in connection with the opened state of the key-operated switch S o . As a result, the tone signal which is applied to the input terminal T 1 of the FET Q 1 becomes shut off by these FET's, Q 2 and Q 1 , at the end of a lapse of the sustain time (t 1 or t 2 ) which is determined by the time constant which, in turn, is determined by the capacity of the capacitor C o and the apparent resistance resulting from the conducting to non-conducting time ratio between the drain and the source electrodes of the FET Q 3 , and thus, no tone signal becomes to be detected at the output terminal T 2 of the sustain keyer circuit SA'.
The aforesaid sustain time (t 1 or t 2 ) or the decay time of an electronic musical instrument may be controlled by varying the DC level of the triangular wave signal applied to the input side of the Schmitt trigger circuit F, by altering the resistance value of such an element as the variable resistor RB to thereby alter the duty factor of the rectangular wave output signal derived at the output of the said Schmitt trigger circuit F, whereby continuously varying both the momentary duration a of cut-off time and the momentary duration b of conducting time of the FET Q 3 in the manner as shown in FIGS. 3a and 3b, continuously from the pattern shown in FIG. 3a to the pattern shown in FIG. 3b. For example, in the state of the duty factor shown in FIG. 3a, the momentary conducting period of time b is considerably smaller as compared with the momentary cut-off period of time a, so that the length of charge-up time of the capacitor C o following the opening of the aforesaid key-operated switch S o (meaning after the time t o in FIG. 4) will accordingly become greater. Hence, the sustain time or the decay time of the tone signal which is detected at the output terminal T 2 under the aforesaid condition will be as long as the period of time from t o to t 2 as indicated by the straight chain line in FIG. 4. Contrariwise, in case where the momentary conducting period of time b of the FET Q 3 is substantially great as compared with its momentary cut-off period of time a as shown in FIG. 3b, the length of charge-up time of the capacitor C o following the opening of the key-operated switch S o (meaning after the time t o in FIG. 4) is reduced, so that the sustain time or the decay time of the aforesaid tone signal will become as short as the period of time from t o to t 1 as shown by the straight solid line in FIG. 4. In this way, the sustain time of an electronic musical instrument can be controlled. However, in the present invention, in particular, the circuitry is arranged in such a way that the gate potential of the FET Q 3 is controlled at a very high repetition rate, such as 40 kHz so that the FET Q 3 can alternately become fully conductive and fully non-conductive. As a result, the FET Q 3 can effect the switching between its cut-off state and its conductive state at a high speed, regardless of the nonlinear operation characteristic of this FET Q 3 . Thus, the FET Q 3 is always held in either the fully conductive or the fully non-conductive state, and the average (apparent) internal resistance between the drain and the source electrodes of this FET Q 3 will be caused by vary by the varying duty factor. Therefore, it is possible to consider that the voltage to current characteristic is of a value equivalent to that of a variable resistor having a linear voltage to current characteristic. Thus, the decay characteristic of the tone signal within the aforesaid sustain time can be obtained as one which is quite close to a complete rectilinear line as shown by the envelope straight lines in logarithmic (dB) scale in FIG. 4, so that the optimum auditory exponential decay pattern can be obtained.
It is to be noted that, by the employment of only one each of the rectangular wave signal generating means -- such as the triangular wave generator G, the Schmitt trigger circuit F, the diode D and the variable resistor RB, which are capable of varying the duty factor -- it is possible to effect the switching of a number of FET Q 3 assigned for controlling the sustain time. Thus, the circuitry of the present invention is of many advantages that it can be produced easily and at a low cost, which is highly useful industrially.
Description has been made on the instance in which the capacitor C o is connected between the gate electrode of the FET Q 2 and the power source -V d . It should be understood, however, that this capacitor C o may, as a matter of course, be inserted between the gate electrode of the FET Q 2 and the ground. In such an instance, it should be noted that, although the variation of the gate potential of the FET Q 3 resulting from the keying action of the key-operated switch C o is done in exactly the same way as that described above, the relation of charging and discharging of the capacitor C o per se will be the reverse of that described in connection with the preceding example.
Description has been directed also to an embodiment in which FET Q 3 is used as the switching element for controlling the sustain time. It should be understood easily, however, that this switching element may employ elements of other types, such as a three-poled diode, a drift transistor, an alloy diffused transistor or a micro-alloy diffused transistor.