Sample hold arrangement for a key signal in an electronic musical instrument

A sample hold arrangement for a key signal in an electronic musical instrument in which a keyboard circuit generates a voltage corresponding to a depressed key. The keyboard circuit is connected at its output terminal to an input terminal of a comparator. An output terminal of the comparator is connected to a memory capacitor and a buffer circuit through two gates connected in a series with one another. An output terminal of the buffer circuit is connected, in turn, to a second input terminal of the comparator, and one of the two gates is connected with its control electrode to a detection circuit. A circuit closing signal is generated by the detection circuit when the potentials of the two input terminals of the comparator become substantially equal. The other one of the two gates is connected with its control electrode to an output terminal of a keying signal generator which generates a keying signal of the keyboard circuit.

BACKGROUND OF THE INVENTION 
This invention relates to a sample hold apparatus for a key signal in an 
electronic musical instrument. 
An apparatus of this kind as already known in the art is shown in FIG. 1. A 
keyboard circuit 1, which generates a voltage corresponding to a depressed 
key, is connected at its output terminal 1a to an input terminal of a 
comparator 2. An output terminal of the comparator 2 is connected to a 
memory condenser 4 and a buffer circuit 5 through a gate 3. An output 
terminal of the buffer circuit 5 is connected to another input terminal of 
the comparator 2. The keyboard circuit 1 is so constructed that a 
plurality of series resistances 8, 8 . . . are connected in series to an 
electric power source terminal 6 through a constant-current circuit 7, and 
a plurality of keyswitches 10,10 . . . which are closed by depression of 
respective keys and are connected to respective connecting points of the 
resistances 8,8 . . . These key-switches 10,10 . . . are connected 
together at their movable contacts and are connected in common to the 
output terminal 1a. Key-switches 11,11 . . . for generating respective 
keying signals are located on one side of the key-switches 10,10 . . . . 
These key-switches 11, 11 . . . are connected together at their stationary 
contacts and connected in common to an electric power source terminal 12. 
They are also connected together at their movable contacts and connected 
in common to a control electrode 3a of the gate 3. The output terminal of 
the buffer circuit 5 is also connected to a voltage-controlled oscillator 
13 (hereinafter called "VCO 13"). An output terminal thereof is connected 
to a speaker 17 through a voltage-controlled filter 14 (hereinafter called 
"VCF 14"), a voltage-controlled amplifier 15 (hereinafter called "VCA 15") 
and an amplifier 16. 
Additionally, a single common output terminal 11a of the foregoing 
key-switches 11, 11 . . . is connected to control electrodes of the VCF 14 
and the VCA 15 through an envelope signal generating circuit 18 
(hereinafter called "ADSR 18"). 
Thus, if a key is depressed, a voltage corresponding to the depressed key 
is generated at the output terminal 1a of the keyboard circuit 1 and a 
keying signal is obtained at the common output terminal 11a of the 
key-switches 11, 11 . . . . The individual key switches 10 and 11 are 
ganged. As a result, the gate 3 is opened and the memory condenser 4 is 
charged so that the two input terminals of the comparator 2 may become 
equal in potential, and the VCO 13 oscillates with a frequency 
corresponding to an output voltage of the buffer circuit 5. Meanwhile, the 
foregoing keying signal drives the ADSR 18 so that an output signal 
thereof may control the VCF 14 and the VCA 15, and as a result a musical 
tone signal having an envelope is obtained from the speaker 17. 
The ADSR 18, as is well known, generates a voltage waveform A (an envelope 
signal) as shown in FIG. 2. As will be clear from this waveform A, it has 
a release time beginning at the moment when the key is released (key off), 
and thus the musical tone becomes a natural attentuated one. In view of 
this fact, it is thought necessary that the memory condenser 4 is kept at 
a properly or charged potential even after the key is released. 
Accordingly, to achieve this, the key-switches 10, 10 . . . and 11,11 . . 
. must be set so that when the key depression is released, the 
key-switches 11,11 . . . are opened earlier than the key-switches 10,10 . 
. . . Additionally, in this case, the time difference between the 
key-switches 10,10 . . . and the key-switches 11,11 . . . must be as small 
as possible, for instance, less than the ratio in which the numerator is 1 
and the denominator is several tenths of a sec. Such a setting, however, 
is extremely difficult, and it often happens that the time difference 
becomes too large or becomes zero or the key-switches 10,10 . . . are 
opened earlier. As a result, therefore, the musical tone is deformed. 
It is therefore an object of the present invention to provide an 
arrangement which is free of the disadvantages described above. 
Another object of the present invention is to provide an arrangement of the 
foregoing character, which is simple in construction and may be 
economically maintained in service. 
A further object of the present invention is to provide an arrangement, as 
described, which has a substantially long operating life. 
SUMMARY OF THE INVENTION 
The objects of the present invention are achieved by providing a keyboard 
circuit which generates a voltage corresponding to a depressed key and is 
connected at its output terminal to an input terminal of a comparator. An 
output terminal of the comparator is connected to a memory condenser and a 
buffer circuit through two gates connected in series one to another. An 
output terminal of the buffer circuit is connected to another input 
terminal of the comparator, and one of the two gates is connected at its 
control electrode to a detection circuit which generates a circuit closing 
signal when the two input terminals of the foregoing comparator become 
substantially equal in potential. The other one of the two gates is 
connected at its control electrode to an output terminal of a keying 
signal generator which generates a keying signal by an output signal of 
the keyboard circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 3 shows one embodiment of the present 
invention in which reference numeral 20 denotes a keyboard circuit which 
is so constructed that when a key is depressed, a voltage corresponding to 
that key is generated, and this keyboard circuit 20 comprises: multiple 
resistances 23,23 . . . connected in a series through a constant current 
circuit 22 to one polarity, that is, a positive polarity 21 of a power 
source; plural key-switches 24,24 . . . connected to respective connecting 
points of respective resistances 23,23 . . . ; and these key-switches 
24,24 . . . are connected together at one side contacts thereof and are 
connected in common through a constant-current circuit 25 to another 
polarity, that is a negative polarity 26 of the power source. An output 
terminal 20a is directed out from one extreme of the series key-switches 
24,24 . . . . This output terminal 20a is connected to a buffer circuit 
27. An output terminal of the buffer circuit 27 is connected to an input 
terminal 28a of a comparator 28, and an output terminal 28c thereof is 
connected to a charging memory condenser 31 and a buffer circuit 32 
through a first gate 29 and a second gate 30. An output terminal 32a of 
the buffer circuit 32 is connected to another input terminal 28b of the 
comparator 28. The two input terminals 28a,28b of the comparator 28 are 
connected to a control electrode 29a of the first gate 29 through a 
detection circuit 33 and a diode 37. The latter is so arranged that a 
circuit closing signal is generated when the two input signals to the 
input terminals 28a,28b become nearly equal. The output terminal of the 
buffer circuit 27 is, furthermore, connected to a keying signal generator 
34, and an output terminal 34a thereof is connected to a control electrode 
30a of the second gate 30 through a diode 38. 
Similarly to that shown in FIG. 1, the output terminal 32a of the buffer 
circuit 32 is also connected to the VCO 13, and the output terminal 34a of 
the keying signal generator 34 is connected to the ADSR 18. 
The buffer circuit 27, the comparator 28 and the keying signal generator 34 
each comprises an operational amplifier and the first gate 29, the second 
gate 30 and the buffer circuit 32 each comprises a FET. The detection 
circuit 33 comprises a window comparator 35 and a delay circuit 36. The 
window comparator 35 comprises two operational amplifiers 35a,35a and an 
OR circuit composed of two diodes 35b,35b connected to output terminals 
thereof. It is so arranged that -V is generated when input voltages 
V.sub.A,V.sub.B at the two operational amplifiers 35a,35a have a 
relationship of .vertline.V.sub.A - V.sub.B .vertline.&lt;.DELTA.V, and +V is 
generated when they have a relationship of .vertline.V.sub.A - V.sub.B 
.vertline.&gt;.DELTA.V. The .DELTA.V is an offset voltage given by adjustment 
of potentiometers 35c,35c and is a value approximating zero. The delay 
circuit 36 is an integration circuit comprising an OP AMP 36a, a 
resistance 36b interposed between an input terminal 36d and an output 
terminal thereof, and a resistance 36c interposed between the input 
terminal 36d and the ground. Reference numeral 39 denotes a resistance 
provided in parallel with the first gate 29. 
If, thus, a key is depressed, a voltage corresponding to the key and 
depending on the resistance 23 is obtained at the output terminal 20a and 
is applied to one input terminal of the buffer circuit 27. An output 
signal thereof is fed back to the other input terminal connected to its 
own output terminal, and a voltage at the output terminal changes from -V 
to V.sub.2 wth a slope as shown in FIG. 4(A). When this curve's(leading 
edge)passes through the zero level, an output of the keying signal 
generator 34 changes from a negative potential to a positive potential, 
and an output signal as shown in FIG. 4(B) is obtained and applied as an 
input to the second gate 30. As a result the second gate 30 is opened and 
at the same time the ADSR 18 is driven to generate an envelope signal as 
shown in FIG. 2. Meanwhile, as will be explained in detail below, the 
output terminal of the detection circuit 33 shows +V.sub.O as shown in 
FIG. 4(D) and the first gate 29 is kept open, so that an output signal of 
the comparator 28 is applied to the memory condenser or capacitor 31 
through the first gate 29 and the second gate 30. Thus, as its applied 
voltage is increased, an output of the buffer circuit 32 is increased, and 
when the potential difference V.sub.A- V.sub.B of the two input terminal 
28a,28a of the comparator 28 becomes .vertline.V.sub.A - V.sub.B 
.vertline. &lt;.DELTA.V, an output voltage of the window comparator 35 
becomes -V.sub.O as shown in FIG. 4(C). Consequently, a voltage of the 
output terminal of the delay circuit 36 decreases to reach -V.sub.O as 
shown in FIG. 4(D). When its declination passes through a point V.sub.3 on 
its way to -V.sub.O, the first gate 29 is closed, as shown in FIG. 4(E). 
Thus, until the first gate 29 is closed, and at a time instant or 
thereafter when the window comparator 35 operates, the voltages of the two 
input terminals 28a,28a of the comparator 28 become equal to one another, 
and the condenser or capacitor 31 is charged to such a level that the 
buffer circuit 32 can generate the voltage corresponding to the depressed 
key. The output voltage of the buffer circuit 32 reaches V.sub.2 while 
being changed along a slope a as shown in FIG. 4(F), and a musical tone is 
changed into that corresponding to the depressed key. 
If the depressed key is released after the lapse of a certain time, the 
input to the buffer circuit 27 becomes zero, so that the output voltage 
thereof is lowered along a line b as shown in FIG. 4(A), and the output of 
the comparator 28 and that of the keying signal generator 34 are also 
lowered, and the second gate 30 is closed as shown in FIG. 4(B). 
At the initial stage of the trailing edge b in FIG. 4(A), the input of the 
window comparator 35 becomes .vertline.V.sub.A - V.sub.B 
.vertline.&gt;.DELTA.V, and at the moment when the depressed key is released, 
its output changes to +V.sub.O as shown in FIG. 4(C). The output of the 
delay circuit 36 is increased from that moment along an inclination line c 
as shown in FIG. 4(D). When this line c passes through a point V.sub.3 ', 
as shown in FIG. 4(E), the first gate 29 is opened lagging by a time t 
behind second gate 30. 
Thus, when the key is depressed, the condenser or capacitor 31 is charged 
through the first and the second gates 29,30, and the first gate 29 is 
closed at the time of completion of charging. Therefore, the condenser or 
capacitor 31 can be maintained in its appropriate charged condition even 
after the depressed key is released and a correct musical tone can be 
generated until an envelope signal generated from the ADSR 18 ends. 
FIG. 5 shows another embodiment of the present invention. In this Figure, 
the same parts as those in FIG. 3 are designated by the same reference 
numerals. A detection circuit 40 is connected to the output terminal of 
the comparator 28, and an output terminal thereof is connected to the 
control electrode of the first gate 29. The detection circuit 40 comprises 
a window comparator 41 and a delay circuit 42. The window comparator 41 is 
different in type from that shown in FIG. 3. It is so constructed that an 
input terminal 43 thereof is connected to a negative power source terminal 
47 through the forward path of first diode 44, a first transistor 45 and a 
first resistance 46. It is also connected to a positive power source 
terminal 52 through the reverse path of second diode 48, a second 
transistor 49 and second and third resistances 50,51. A third transistor 
53 is connected at its collector to a connecting point 52' between the 
first transistor 45 and the first resistance 46, as well as the positive 
power source terminal 52 through its emitter. An output terminal 54 is 
taken from the above collector. Its base is connected to a connecting 
point between the second and the third resistances 50,51. The bases of the 
first and the second transistors 45,49 are formed to be input terminals 
55,56 for standard voltages. The standard voltages +V.sub.1,-V.sub.1 are 
applied thereto by having the voltage +V, -V at the power source terminals 
57,58 divided through resistances 59,60,61. 
The standard voltages +V.sub.1,-V.sub.1 should be determined by taking into 
consideration voltage drops between the bases and the emitters of the 
first and the second transistors 45,49 and voltage drops in the first and 
the second diodes 44,48. However, this is not required here, and the 
voltages +V.sub.1,-V.sub.1 l may be considered as the standard voltages. 
Thus, when an input voltage V.sub.IN applied to the input terminal 43 
conforms to condition +V.sub.1 &gt; V.sub.IN &gt; -V.sub.1, the first and the 
second transistors 45,49 are non-conductive and therefore the third 
transistor is also non-conductive, so that the voltage -V of the negative 
power source terminal 47 is taken out through the resistance 46 from the 
output terminal 54. (This output voltage will hereinafter be called 
"V.sub.L ".) 
If, then, it is changed to a condition of V.sub.IN &gt; -V.sub.1, the second 
transistor 49 becomes non-conductive and the first transistor 45 becomes 
conductive, so that the voltage at the output terminal 54 becomes such a 
voltage V.sub.HI that voltage drop values of the first diode 44 and the 
first transistor 45 are subtracted from the V.sub.IN. 
If, then, it is changed into a condition of V.sub.IN &gt; -V.sub.1, the first 
transistor 45 becomes non-conductive and the second transistor 49 becomes 
conductive, so that the voltage at the output terminal 54 becomes such a 
voltage V.sub.H2 that a voltage drop value of the third transistor 53 is 
subtracted from the voltage +V at the positive power source terminal 52. 
The voltages V.sub.H1,V.sub.H2 are nearly equal to one another and each 
thereof will hereinafter be called "V.sub.H ". 
The delay circuit 42 comprises a condenser or capacitor 62, a resistance 63 
and a diode 64, and operates in the following manner: When an output of 
the comparator 41 is obtained as shown in FIG. 6(A), an output of the 
delay circuit 42 becomes as shown in FIG. 6(B), so that the first gate 29 
is closed with a time delay, t. In this embodiment, the buffer circuit 27 
is omitted and a resistance 65 is interposed. 
Assume a condition that the memory condenser 31 is already charged to a 
certain potential and an output voltage Vout of the buffer circuit 32 is 
applied to the input terminal 28b of the comparator 28. Then, if any 
desired key is depressed, a voltage V.sub.x corresponding to the key is 
generated from the keyboard circuit 1 and is applied to the comparator 28. 
In this operation, there are three conditions as listed below, though they 
also are present in the embodiment of FIG. 3. 
(1) v.sub.x &gt; Vout 
(2) V.sub.x &lt; Vout 
(3) V.sub.x = Vout 
Each of these occasions will be explained as follows:-- 
(1) V.sub.x &gt; Vout 
In this condition, the V.sub.IN (+ 15V, for instance) is generated from the 
comparator 28. Between this output V.sub.IN and the standard voltages 
+V.sub.1, -V.sub.1 of the comparator 40, there is a condition V.sub.IN &gt; 
.vertline.V.sub.1 .vertline.., so that an output of the comparator 40 
becomes V.sub.H (+ 15V) as shown in FIG. 6(A). As a result, the first gate 
29 is kept open, and the second gate 30 is made conductive by a keying 
signal generated from the keying signal generator 34 simultaneously with 
depression of the selected key-switch 24. Accordingly, the output of the 
comparator 28 is charged to the memory condenser or capacitor 31. When, by 
this charge, the output Vout of the buffer circuit 32 is fed back to the 
comparator 28 and V.sub.x = Vout is obtained, the output V.sub.IN of the 
comparator 28 is brought into a condition V.sub.IN &gt;.vertline.V.sub.1 
.vertline.., and the output of comparator 40 becomes V.sub.L (-15V) as 
shown in FIG. 6(A). An output of the delay circuit 42 becomes as shown in 
FIG. 6(B), so that the first gate 29 is closed, and the condenser or 
capacitor 31 is maintained at its charged potential for keeping the 
V.sub.x = Vout. 
(2) V.sub.x &gt; Vout 
In this condition, the output of the comparator 28 becomes V.sub.IN (- 
15V), and the output of the window comparator 40 becomes V.sub.H (+ 15V) 
as shown in FIG. 6. As a result the first gate 29 becomes conductive and 
the second gate 30 becomes conductive by a keying signal. Accordingly, the 
charged potential of the condenser 31 is discharged through the first and 
the second gates 29,30. When the condition V.sub.x = Vout is established, 
in almost the same manner as in case of (1), the first gate 29 is closed 
and there remains in the memory condenser or capacitor 31, the charged 
potential to maintain the relation of V.sub.x = Vout. 
(3) V.sub.x = Vout 
This condition is identical with the case where V.sub.x = Vout in each of 
the above conditions (1), (2), and the condition remains as is. 
If, then, the depressed key is released and the key-switch 24 is opened, an 
output signal of the keying signal generator 27 disappears at that moment 
and the second gate 30 is closed. At the same time the output of the 
comparator becomes V.sub.IN (- 15V), whereby the output of the comparator 
40 becomes V.sub.H as shown in FIG. 6(A). Accordingly, when the output 
delayed by the time t through delay circuit 42 as shown in FIG. 6(B), 
reaches V.sub.2, the first gate 29 is opened. 
Thus, at the moment of closing of the key-switch 24, the memory condenser 
or capacitor 31 is rapidly charged through the first and the second gates 
29,30 so as to achieve the condition of V.sub.x = Vout. When this 
condition of V.sub.x = Vout is established, the first gate 29 is closed. 
If, then, the key-switch 24 is opened, the second gate 30 is closed at 
that instant, and the first gate 29 is opened with a delay. It is thereby 
prepared for the next operation when the key-switch 24 is closed. 
In the embodiment as shown in FIG. 3, it is necessary that the comparator 
35 has a high accuracy because .DELTA. V must be as close to zero as 
possible. Accordingly, it becomes costly because the two operational 
amplifiers of high accuracy are used. In the embodiment of FIG. 5, the 
comparator 41 may be acceptable if it can discriminate whether the output 
of the comparator 28 is between the standard voltages or not, and 
accordingly it becomes low in cost because accuracy as required for the 
foregoing comparator 35 is not required. 
Thus, according to the present invention, it is so arranged that when a key 
is depressed, a condenser is charged through first and second gates, and 
on completion of the charge thereof, the first gate is closed, so that the 
condenser can always be maintained at its appropriate charge even after 
the depressed key is released. A VOC keeps a correct frequency 
oscillation, and a correct musical tone can be generated until an envelope 
signal generated from an ADSR ends, and there do not occur the various 
problems as in the foregoing case where key-switches are provided in 
ganged form. 
Without further analysis, the foregoing will hopefully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention, and 
therefore, such adaptations should and are intended to be comprehended 
within the meaning and range of equivalents of the following claims.