Envelope Signal Forming Circuit

Abstract

A conventional envelope signal forming circuit for producing percussive envelope signals in an electronic musical instrument is improved by the provision of a circuitry for setting a control voltage which is utilized to control a charge-discharge capacitor therein so as to reduce the overlapping of a plurality of envelope signals in one aspect of this invention and to allow the envelope signal to have a controllable sustained portion in another aspect thereof.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to an envelope signal forming circuit in an electronic musical instrument and more particularly, it relates to the control of the waveform of the envelope signal produced from an envelope signal forming circuit to be supplied to a tone keyer circuit.

Heretofore, an envelope signal forming circuit as shown in, for instance, FIG. 1 has been employed in connection with the tone keyer of a musical instrument, in order to produce percussive tones having an abrupt build-up and a subsequent gradual decay.

In this circuit (FIG. 1), one terminal of a charge-discharge capacitor C.sub.1 is connected through a reverse-current blocking element D.sub.1 such as a diode to a first terminal of a charge-discharge capacitor C.sub.2, while the other terminal of the capacitor C.sub.1 is connected to the positive terminal of the power source E through a charging resistor R.sub.1 and a key switch K which is one of the keys in a keyboard.

Upon closure of the key switch K, charging the capacitors C.sub.1 and C.sub.2 is started through a circuit loop consisting of the power source E, the key switch K, the resistor R.sub.1, the capacitor C.sub.1, the diode D.sub.1, the capacitor C.sub.2, and the ground line. In this operation, a voltage developed at the output terminal T.sub.1 connected to the first terminal of the capacitor C.sub.2 is as shown in FIG. 2(A).

That is, the voltage abruptly rises with the time constant determined by the values of the capacitors C.sub.1 and C.sub.2 and the resistor R.sub.1 at the instant t.sub.1 at which the key switch K is closed. Soon, the capacitors C.sub.2 is charged up. Thereafter, charging the capacitor C.sub.1 is still continued through a charge resistor R.sub.2 and the ground line, but the current for charging the capacitor C.sub.1 gradually decreases with the time constant mainly determined by the values of the capacitor C.sub.1 and the resistor R.sub.2 after the instant t.sub.1. Therefore, a voltage at a circuit point X where the capacitor C.sub.1 and the resistor R.sub.2 are connected together decreases toward the ground level, and therefore the capacitor C.sub.2 is discharged through a discharge resistor R.sub.3.

Thus, the voltage at the output terminal T.sub.1 decreases with the time constant determined mainly by the values of the capacitor C.sub.2 and the resistor R.sub.3 for the period of from the instant t.sub.1 to the instant t.sub.2 and reaches zero volt at the instant t.sub.2.

After the instant t.sub.2, the key switch K is opened. As a result, the capacitor C.sub.1 is discharged through the resistors R.sub.1 and R.sub.4 and a diode D.sub.2, but the voltage at the output terminal T.sub.1 remains unchanged and therefore the waveform of this voltage becomes a straight line as shown in FIG. 2(A) in which the X-axis represents the time while the Y-axis the voltage in dB.

As is shown in FIG. 3, the envelope signal forming circuit 1 having the organization described above is actuated by the key depression in the key board 2 in correspondence to the ON-OFF operation of the key switch provided for each of the keys, and introduces the output voltage of the output terminal T.sub.1, as an envelope deciding signal EN, to a tone keyer 3, whereby the output signal of a tone generator 4 is fed to a tone coloring circuit 5 through the gating operation of the tone keyer 3. The tone coloring circuit 5 receives the keyed tone signals having respective envelopes determined by the envelope signal forming circuits which are respectively provided for all the keys, thereby producing a composite signal. This composite signal is applied through an amplifier 6 to a loudspeaker 7.

In this connection, the case where, for instance, two keys are successively depressed at the instants t.sub.1 and t.sub.1a whereby two percussive tone signals are produced will be considered. In this case, the waveforms F and Fa of the two percussive tone signals are sometimes overlapped with each other as is shown by a region h hatched with slant lines in FIG. 2(B), thus resulting in the overlapping of two produced tones. Therefore, the composite tone of the two tones thus overlapped is unpleasant and unclear to an auditor and is not crispy.

Furthermore, a conventional tone keying circuit arrangement for producing precussive tones and sustained tones and composite tones of these tones in an electronic musical instrument, as is shown in FIG. 6, comprises a tone keyer 3A for sustained tones (hereinafter referred to as "a sustained tone keyer 3A"), a tone keyer 3B for percussive tones (hereinafter referred to as "a percussive tone keyer 3B"), and an envelope signal forming circuit 1. These tone keyers 3A and 3B and the envelope signal forming circuit 1 are provided for each of the keys in a keyboard 2, and the operation of the percussive tone keyer 3B is controlled by the envelope forming circuit 1.

The conventional tone keying circuit arrangement further comprises a tone generator 4 connected to the inputs of the tone keyers 3A and 3B, and a tone coloring circuit 5 connected to the outputs of the tone keyers 3A and 3B respectively through variable resistors (tone level controls) 8 and 9 which respectively, control the signal levels to be supplied to the tone coloring circuit 5.

In this conventional tone keying circuit arrangement, the sustained tone keyer 3B produces a sustained tone signal S.sub.1 which is sustained for the period of from the instant t.sub.1 at which a key is depressed by a finger or the like to the instant t.sub.2 at which the key thus depressed is released as is shown in FIG. 7(A), while the percussive tone keyer 3B produces a percussive tone signal S.sub.2 which rises first at the instant t.sub.1 and then decays in response to the output of the envelope signal forming circuit 1 as is shown in FIG. 7(B). These outputs S.sub.1 and S.sub.2 are applied, as the output of the tone keying circuit arrangement, through the respective variable resistors 8 and 9 to the tone coloring circuit 5.

In the circuit of FIG. 6, if the variable resistor 9 is set at its maximum attenuation position while the variable resistor 8 is set at its minimum attenuation position, the sustained tone signal S.sub.1 as shown in FIG. 7(A) can be obtained. In contrast, if the variable resistor 9 is set at its minimum attenuation position while the variable resistor 8 is set at its maximum attenuation position, the percussive tone signal S.sub.2 as shown in FIG. 7(B) can be obtained.

In addition, if these variable resistors 8 and 9 are selectively set, different tone signals having different envelopes as shown in FIGS. 7(C) and 7(D) can be obtained.

In this connection, a circuit such as shown in FIG. 8 has been employed as the sustained tone keyer 3A, and a circuit such as shown in FIG. 9 has been employed as the percussive tone keyer 3B together with the envelope signal forming circuit 1.

Thus, it is necessary in the conventional tone keying circuit arrangement to separately provide two tone keyers 3A and 3B for each of the keys. Therefore, the number of circuits and parts in the conventional tone keying circuit arrangement is inevitably great.

SUMMARY OF THE INVENTION

Accordingly, a first object of this invention is to eliminate the above-described difficulties accompanying the conventional envelope signal forming circuit.

A second object of the invention is to provide an envelope signal forming circuit in which a percussive waveform having an abruptly rising portion and a gently decaying portion is obtained, and in the case when the decaying portions of percussive waveforms succeedingly produced are overlapped, the overlapping of the waveforms can be sufficiently reduced.

A third object of the invention is to provide an envelope signal forming circuit which produces an envelope signal output having a controllable waveform such as a percussive waveform, a sustained waveform or a waveform consisting of these waveforms.

A fourth object of the invention is to improve an envelope signal forming circuit to reduce the number of circuits and parts required for a tone keying circuit arrangement.

The foregoing objects and other objects of the present invention will become more apparent from the following detailed description and the appended claims when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a conventional envelope signal forming circuit is an electronic musical instrument;

FIGS. 2(A) and 2(B) are graphical representations illustrating envelope signals obtained by the conventional envelope signal forming circuit shown in FIG. 1;

FIG. 3 is a block diagram showing an electronic musical instrument to which the present invention is applied;

FIG. 4 is a circuit diagram illustrating one embodiment of the invention.

FIGS. 5(A) and 5(B) are graphical representations illustrating envelope signals produced from the circuit shown in FIG. 4;

FIG. 6 is a block diagram of a conventional tone keying circuit arrangement in an electronic musical instrument;

FIGS. 7(A), 7(B), 7(C) and 7(D) are graphical representations illustrating keyed tone signal envelopes produced from the conventional tone keying circuit arrangement shown in FIG. 6;

FIG. 8 is a circuit diagram of a tone keyer for sustained tones in the conventional tone keying circuit arrangement shown in FIG. 6;

FIG. 9 is a circuit diagram of a tone keyer for purcussive tones together with an envelope signal forming circuit in the conventional tone keying circuit arrangement shown in FIG. 6;

FIG. 10 is a circuit diagram illustrating another embodiment of the invention; and

FIGS. 11(A), 11(B), 11(C) and 11(D) are graphical representations illustrating envelope signals produced by the circuit shown in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 4, there is shown a circuit diagram illustrating one embodiment of the envelope signal forming circuit of this invention, which is similar to the circuit diagram of FIG. 1 except that a decay control circuit H is added thereto.

This decay control circuit H comprises a control voltage setting device VR which is, for instance, a variable resistor connected in parallel to the power source E. The sliding piece, or the wiper, of the variable resistor VR is connected to a discharge resistance element which is, for instance, a resistor R.sub.5 connected to a unidirectional element which is, for instance a diode D.sub.3. One terminal (which is an anode in FIG. 4) is connected to the non-ground end of the capacitor C.sub.2. The resistance of the resistor R.sub.5 is selected to be much smaller than that of a resistor R.sub.3 connected in parallel to the capacitor C.sub.2.

The operation of the envelope signal forming circuit shown in FIG. 4 is as follows:

When the key switch K is closed at the instant t.sub.1, the capacitors C.sub.1 and C.sub.2 are charged through the loop which was described with reference to FIG. 1. Soon, the capacitor C.sub.2 is fully charged up. Therefore, the voltage of the output terminal T.sub.1 abruptly rises at the instant t.sub.1 as is shown in FIG. 5(A). Thereafter, the capacitor C.sub.2 is discharged in such a manner that the discharge condition of the capacitor C.sub.2 is determined from the relationship between the voltage at the non-grounded end of the capacitor C.sub.2, that is, the voltage at the anode side of the diode D.sub.3 and the voltage at the cathode side of the diode D.sub.3, that is, the setting voltage of the control voltage setting device VR.

1. In the case where the wiper of the variable resistor VR is positioned at the non-grounded end P.sub.1 thereof or is positioned near the non-grounded end P.sub.1 whereby the setting voltage is higher than the charge voltage of the capacitor C.sub.2, the envelope signal forming circuit will operate as follows:

In this case, the diode D.sub.3 is in OFF state, or is non-conductive. Accordingly the capacitor C.sub.2 is discharged only through the resistor R.sub.3. As a result, the voltage at the output terminal T.sub.1 decreases with the time constant determined mainly from the values of the capacitor C.sub.2 and the resistor R.sub.3, as is shown by a curve I.sub.0 in FIG. 5(A).

2. In the case where the wiper of the variable resistor VR is positioned at the grounded end P.sub.2 thereof whereby the wiper is applied with the ground voltage, the capacitor C.sub.2 which has been charged is now discharged through the diode D.sub.3 and the resistor R.sub.5. Accordingly, the voltage at the output terminal T.sub.1 decreases with the time constant determined mainly from the values of the capacitor C.sub.2 and the resistor R.sub.5 as is shown by a curve b in FIG. 5(A). In this operation, since the resistance of the resistor R.sub.5 is much smaller than that of the resistor R.sub.3 as was described before, the gradient of the curve b is great.

Soon, the charged voltage of the capacitor C.sub.2 decreases, as a result of which the diode D.sub.3 becomes non-conductive. Thereafter, the capacitor is therefore discharged through the resistor R.sub.3. Accordingly, the voltage at the output terminal T.sub.1 decreases at a gradient which is substantially equal to that of the curve I.sub.0 as is shown by a curve I.sub.1 in FIG. 5(A).

3. In the case where the wiper of the variable resistor VR is positioned at a point between the non-grounded end P.sub.1 and the grounded end P.sub.2 thereof, the envelope signal forming circuit operates in the same manner as in the case of paragraph (2) above except that, when the diode D.sub.3 turns non-conductive, the remaining voltage in the capacitor C.sub.2 is higher than that in the case of paragraph (2) in correspondence to the position of the wiper. Accordingly, the voltage at the output terminal T.sub.1 decreases first along the curve b and then along a curve I.sub.2 or a curve I.sub.3.

The voltages thus developed at the output terminal T.sub.1 are utilized as the envelope signal EN.

As was described above the capacitor C.sub.2 is discharged through the resistor R.sub.5 of the decay control circuit H, depending on the voltage of the capacitor C.sub.2. This resistor R.sub.5 is much smaller in resistance than the resistor R.sub.3 of sufficiently high resistance which is connected in parallel to the capacitor C.sub.2. Therefore, the decaying portion of the waveform of the envelope signal is a controllable waveform consisting of a curve having a sufficiently great gradient and a curve having a relatively small gradient.

The formation of the controllable waveform, as is apparent from FIG. 5(B), contributes to the controlling of the overlapping of, for instance, two keyed tone signal produced at short time intervals. That is, the overlapping of the waveforms can be controlled to be reduced as required, thus remarkably improving unclear and non-crispy tones which may be created by the overlapping of the tones.

Thus, merely by the provision of the decay control circuit H simple in composition, the decaying waveform which is substantially the same, in the rising portion of the waveform, its peak value and the decaying portion having a relatively gentle gradient, as the decaying waveform obtained by the conventional envelope signal forming circuit and which accordingly has a percussive tone characteristic substantially equal to that of the decaying waveform obtained by the conventional keying arrangement can be obtained.

The decay control circuit H shown in FIG. 4 is not limitative. That is, all that is necessary for the composition of this circuit H is that when the charged voltage in the capacitor C.sub.2 becomes lower than the setting voltage, the discharge paths of the capacitor C.sub.2 are interchanged.

Hereinafter, another example of the envelope forming circuit provided according to another aspect of the invention will be described, being embodied in the tone keying circuit arrangement shown in FIG. 6.

This example, or the envelope forming circuit 1, as is shown in FIG. 10, comprises two charge-discharge capacitors C.sub.1 and C.sub.2. One terminal of the capacitor C.sub.1 is connected to one terminal of the capacitor C.sub.2 through a reverse-current blocking element D.sub.1 such as a diode. The other terminal of the capacitor C.sub.1 is connected to the positive terminal of the power source E through a charge resistor R.sub.1 and the key switch K, while the other terminal of the capacitor C.sub.2 is grounded. A charge resistor R.sub.2 for charging the capacitor C.sub.1 is connected in parallel to a series circuit of the diode D.sub.1 and the capacitor C.sub.2. A series circuit of a discharge resistor R.sub.4 having a relatively high resistance and a diode D.sub.2 is connected in parallel to a series circuit of the capacitor C.sub.1 and the resistor R.sub.1. Furthermore, a discharge resistor R.sub.3 which is much higher in resistance than the resistor R.sub.2 is connected in parallel to the capacitor C.sub.2 to which an output terminal T.sub.1 is connected.

It can be readily understand that a circuit composed of the circuit elements described above is known in the art, when compared with that of FIG. 1. To the circuit thus composed a resistance varying circuit RU comprising, for instance, a field-effect transistor FET is added, according to the invention, as is shown in FIG. 10.

The gate of the transistor FET is connected to the control voltage terminal of a control voltage setting device VR such as a variable resistor connected across the power source E. If necessary, the gate of the transistor FET may be connected through a resistor R.sub.5 to the control voltage terminal, or the wiper of the variable resistor VR.

When the wiper of the variable resistor VR is positioned at the grounded end P.sub.2 of the variable resistor, the ground voltage is applied to the gate of the transistor FET, and accordingly the transistor FET becomes non-conductive, that is, the transistor FET shows its maximum resistance.

On the other hand, when the wiper is positioned at the other end P.sub.1 of the variable resistor VR, the maximum voltage of the power source is applied to the gate of the transistor FET, and accordingly the transistor FET becomes conductive, that is, the transistor FET shows its minimum resistance.

Thus, if the wiper is moved to position at a point between the grounded end P.sub.2 of the variable resistor VR and the other end P.sub.1, the transistor FET will show a resistance between its maximum and minimum resistances in correspondence to the moving quantity of the wiper.

In FIG. 10, reference characters Ga and Gb designate terminals to which the corresponding portions of the other envelope signal forming circuit respectively provided for the other keys in the keyboard are connected.

1. In the case where the wiper of the variable resistor VR is positioned at the grounded end P.sub.2 thereof, the operation of the envelope signal forming circuit shown in FIG. 10 is as follows:

In this case, the transistor FET is non-conductive as was described above. Therefore, the capacitors C.sub.1 and C.sub.2 are charged through a loop consisting of the power source E, the key switch K, the resistor R.sub.1, the capacitor C.sub.1, the diode D.sub.1, the capacitor C.sub.2, and the ground line.

In this charge operation, a time constant is sufficiently small, being determined by the values of the capacitors C.sub.1 and C.sub.2 and the resistor R.sub.1. Accordingly, a voltage at the output terminal T.sub.1 abruptly rises at the instant t.sub.1 at which the key switch K is closed, as shown in FIG. 11(A). Soon, the capacitor C.sub.2 is fully charged up, but charging the capacitor C.sub.1 is continued through the resistor R.sub.1. Therefore, a voltage at the connecting point X of the capacitor C.sub.1 and the resistor R.sub.2 decreases to the ground potential according to the decrease of the charge in the capacitor C.sub.1, and whereupon the capacitor C.sub.2 is discharged through the resistor R.sub.3. Accordingly, the waveform of a voltage at the output terminal T.sub.1 becomes a decaying waveform V.sub.1 which decays at a gradient determined mainly by the values of the capacitor C.sub.2 and the resistor R.sub.3 as is shown in FIG. 11(A).

Thereafter, the key switch K is opened at the instant t.sub.2 and simultaneously the capacitor C.sub.1 starts to discharge through the resistors R.sub.1 and R.sub.4 and the diode D.sub.2. However, at the instant t.sub.2 the voltage at the output terminal T.sub.1 remains unchanged because the capacitor C.sub.2 has already been discharged and the diode D.sub.1 blocks a negative going change.

2. In the case where the wiper of the variable resistor VR is positioned at the non-grounded end P.sub.1, the envelope signal forming circuit operates as follows:

In this case, the transistor FET is conductive. When the key switch K is closed at the instant t.sub.1, the capacitors C.sub.1 and C.sub.2 are charged through the loop in the same manner as in the case described above, and soon the capacitor C.sub.2 is fully charged up. Then, a potential at the connecting point X is maintained at a voltage determined by the voltage dividing ratio of the resistor R.sub.1 to the resistor R.sub.2, that is, a voltage approximately equal to the voltage of the power source in this case, since the capacitor C.sub.1 is shunted by the conductive transistor FET. Accordingly, the potential at the output terminal T.sub.1 is maintained unchanged until the time instant t.sub.2 at which the key switch K is opened.

Upon opening of the key switch K, the capacitor C.sub.2 starts to discharge with a time constant determined mainly by the values of the capacitor C.sub.2 and the resistor R.sub.3.

Thus, the voltage at the output terminal T.sub.1 has a sustained waveform V.sub.2 which is similar to a keying waveform as is shown in FIG. 11(D).

3. In the case where the wiper of the variable resistor VR is positioned at a point between the both ends P.sub.1 and P.sub.2 of the variable resistor VR, the envelope signal forming circuit operates as follows:

In this case, the capacitor C.sub.1 is bypassed by the resistance of the transistor FET. Therefore, after the capacitor C.sub.2 has been fully charged up through the loop by closing the key switch K, a potential at the connecting point X is maintained at a voltage determined by the voltage dividing ratio of the sum of the resistances of the resistor R.sub.1 and the transistor FET to the resistance of the resistor R.sub.3. Accordingly, the capacitor C.sub.2 is discharged by as much as the difference voltage between the voltage at the connecting point X and the voltage of the capacitor C.sub.2.

Thus, the voltage at the output terminal T.sub.1 has a waveform V.sub.3 consisting of a decaying waveform portion v.sub.1 which abruptly rises at the instant t.sub.1 and then decays and a sustained waveform portion v.sub.2 which succeeds to the decaying portion v.sub.1, as is shown in FIGS. 11(B) and 11(C).

The level of the sustained waveform v.sub.2 is determined by the resistance of the transistor FET, or by the position of the wiper of the variable resistor VR. Accordingly, when the wiper is positioned near the grounded-end P.sub.2 of the variable resistor VR, the level of the sustained waveform v.sub.2 is relatively low as is shown in FIG. 11(B), while when the wiper is positioned near the non-grounded end P.sub.1 of the variable resistor VR, the sustaining level is relatively high as is shown in FIG. 11(C). In this way, the level of the sustained waveform portion v.sub.2 can be controlled by means of the control voltage setting device, that is, the variable resistor VR.

Thus, according to the invention, an envelope signal having a controllable waveform such as a decaying waveform, a sustained waveform or a waveform consisting of these waveforms can be obtained, and the control of the waveforms can be continuously carried out through a simple circuit, that is, the resistance varying circuit RU connected in parallel to the capacitor C.sub.1 of the envelope signal forming circuit 1.

Accordingly, when the present invention is applied to a tone keying circuit arrangement in an electronic musical instrument, only one switching circuit can control all of the keys of one keyboards. Therefore, the number of circuits and parts in the tone keying circuit arrangement can be reduced to be much smaller than that of the conventional tone keying circuit arrangement. For instance, a conventional tone keying circuit arrangement having 61 keyboards has 122 tone keyers, but this number of tone keyers can be reduced to 61 according to the invention.

Claims

I claim:

1. In an improved envelope signal forming circuit having a first charge-discharge capacitor, a diode, a second charge-discharge capacitor connected through said diode to said first charge-discharge capacitor, a first charge resistor connected in parallel to a series circuit consisting of said diode and said second charge-discharge capacitor, and a second discharge resistor connected in parallel to said second charge-discharge capacitor for producing as an envelope signal the voltage of said second charge-discharge capacitor, the improvement which comprises:

a field-effect transistor to control the envelope signal having a decaying waveform, said field-effect transistor having its drain and source electrodes directly connected across said first charge-discharge capacitor; and

a control voltage setting device comprising a variable resistor and a power source connected in parallel thereto, the wiper arm of said variable resistor being connected to the gate electrode of said field-effect transistor, the resistence of said field-effect transistor being controlled by the voltage set by said variable resistor to control the envelope signal.