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Claims  |
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What is claimed is:
1. An electronic organ comprising
a. a reference voltage source,
b. at least one group of tablets for selectively passing the current from
said reference source,
c. at least one group of memory means for separating the current which has
passed through each of said tablets into a plurality of currents each with
a predetermined different magnitude with respect to each other,
d. at least one group of first means for mixing selected currents from said
memory means,
e. means for generating top octave waves,
f. a plurality of groups of means for dividing stepwise the frequency of
the waves of each interval of said top octave waves,
g. a DC voltage source,
h. at least one keyboard consisting of keys for selectively passing the
voltage from said DC voltage source,
i. a plurality of indirect keying means each for modulating the current
from each of said mixing means by the frequency-divided waves and
selectively passing the modulated waves in response to the operation of
each of said keys of said keyboard, and
j. at least one means for converting the waves of said indirect keying
means into the musical sounds.
2. An electronic organ as set forth in claim 1 further comprising at least
one group of second mixing circuits inserted between said indirect keying
means and said means for converting the waves from said indirect keying
means into the musical sounds for mixing the modulated waves from said
indirect keying means for each octave.
3. An electronic organ as set forth in claim 2 further comprising at least
one group of variable filter means inserted between said second mixing
circuits and said means for converting the waves from said indirect keying
means into the musical sounds for transmitting only the desired higher
harmonics of the mixed modulated waves in response to the selection of
said tablets; and at least one filter control circuit for controlling said
variable filter means in response to the selection of said tablets.
4. An electronic organ as set forth in claim 2 wherein each of said second
mixing circuits comprises an operational amplifier.
5. An electronic organ as set forth in claim 3 further comprising at least
one special effects circuit means inserted between said group of said
variable filter means and said means for converting the waves into the
musical sounds.
6. An electronic organ as set forth in claim 3 wherein each of said
variable filter means has a different frequency characteristics.
7. An electronic organ as set forth in claim 1 further comprising tablet
number detecting means for controlling said reference voltage source in
response to the number of selected tablets.
8. An electronic organ as set forth in claim 7 wherein said tablet number
detecting means comprises a circuit consisting of npn transistors
connected in parallel, the voltages from each selected tablet being
applied as the control input to said npn transistors; and an operational
amplifier to which is applied the output of said parallel circuit as the
input.
9. An electronic organ as set forth in claim 1 further comprising key
number detecting means for controlling said reference voltage in response
to the number of pressed keys.
10. An electronic organ as set forth in claim 9 wherein said key number
detecting means comprises a parallel circuit of MOS-FETs to which are
impressed the voltages from key switches as the control input
respectively; and an operational amplifier connected in series to said
parallel circuit.
11. An electronic organ as set forth in claim 9 wherein said reference
voltage source comprises a transistorized differential amplifier, a power
transistor to which is applied the output of said differential amplifier
as the control input; and resistors for applying the bias voltages to said
transistors.
12. An electronic organ as set forth in claim 1 further comprising an
envelop control device inserted between a connection of said keys and said
indirect keying means and the ground.
13. An electronic organ as set forth in claim 1 wherein each of said memory
means comprises a group of series circuits each consisting of a resistor
and a diode, said series circuits being connected in parallel to said
tablets, said resistors having a different value of resistance.
14. An electronic organ as set forth in claim 1 wherein each of said first
mixing means comprises an operational amplifier.
15. An electronic organ as set forth in claim 1 wherein each of said
indirect keying means comprises two FET transistors connected in series,
the base of one of said two FET transistors being connected to one
terminal of said frequency-divided waves, the base of the other FET
transistor being connected to each of said keys.
16. An electronic organ as set forth in claim 1 wherein each of said
frequency dividing means comprises two series of frequency dividers, the
first stage of one of said two series being a 1/3 frequency divider, the
first stage of the other series being a 1/4 frequency divider, and the
remaining stages being 1/2 frequency dividers.
17. An electronic organ comprising
a. a reference voltage source,
b. at least one group of tablets for selectively passing the current from
said reference source,
c. at least one group of memory means for separating the current which has
passed through each of said tablets into a plurality of currents each with
a predetermined different magnitude,
d. at least one group of first means for mixing selected currents from said
memory means,
e. means for generating top octave waves,
f. a plurality of groups of means for dividing stepwise the frequency of
the waves of each interval of said top octave waves,
g. a DC voltage source,
h. at least one keyboard consisting of keys for selectively passing the
voltage from said DC voltage source,
i. a plurality of indirect keying means each for modulating the current
from each of said mixing means by the frequency-divided waves and
selectively passing the modulated waves in response to the operation of
each of said keys, and
j. at least one means for converting the waves from said indirect keying
means into the musical sounds;
k. at least one group of second mixing circuits inserted between said
indirect keying means and said means for converting the waves from said
indirect keying means into the musical sounds for mixing the modulated
waves from said indirect keying means for each octave,
l. at least one group of variable filter means inserted between said second
mixing circuits and said means for converting the waves from said indirect
keying means into the musical sounds for transmitting only the desired
higher harmonics of the mixed modulated waves in response to the selection
of said tablets; and at least one filter control circuit for controlling
said variable filter means in response to the selection of said tablets,
m. at least one effect circuit means inserted between said group of said
variable filter means and said means for converting the waves into the
musical sounds,
n. tablet number detecting means for controlling said reference voltage
source in response to the number of selected tablets,
o. key number detecting means for controlling said reference voltage in
response to the number of pressed keys; and
p. an envelop control device inserted between a connection of said keys and
said indirect keying means and the ground.
18. An electronic organ as set forth in claim 17 further comprising
a plurality of said tablet groups,
a plurality of said memory groups,
a plurality of said first mixing means groups,
a plurality of said second mixing means groups,
a plurality of said variable filter means groups, and
a plurality of said effect circuit groups, the numbers of each of said
groups being equal to the number of said keyboards,
a plurality of frequency dividing means groups equal in number to the
intervals of said top octave waves,
a plurality of said indirect keying circuit groups, and
a plurality of said envelop control circuits, the numbers of each of said
two groups being equal to that of said keys. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an electronic organ.
In generators of sound in the conventional electronic organs, the
combination of oscillators and frequency dividers is used to produce the
square waves with a fundamental frequency corresponding a key pressed and
square waves with a frequency of an integral multiple of the fundamental
frequency. These square waves are combined or synthesized in a desired
ratio, and the combined or synthesized output is made to pass through a
tone filter so that undesired frequency spectrum may be removed. Moreover,
in general, the direct keying system is used for intermittently
interrupting the square wave trains with the above harmonic series.
However, a large number of filters are required in order to determine a
desired ratio in which the harmonic components are combined and to produce
various timbres, so that the conventional electronic organs are complex in
construction and expensive. In order to overcome this problem, there has
been proposed a system in which timbres are combined into groups each
consisting of a few or several or tens timbres and these timbre groups are
made to pass through low-pass filters. However, this system has a defect
that the timbres and volume are considerably different at high and low
tones. Furthermore, it is difficult to simplify the construction so that
the associated circuits may not be fabricated in the form of LSI.
SUMMARY OF THE INVENTION
The present invention has for its object to provide an electronic organ
which uses a generator of sound of the type synthesizing the repetitively
occuring waves such as square waves, makes it possible to be fabricated in
the form of LSI (large scale integration), and is simple in construction.
The fundamental construction of the present invention comprises memory
means for memorizing in an analog manner the synthesizing ratio of
repetitively occuring waves such as square waves for synthesizing
qualities of sound in response to signals from a means for selecting
qualities of sound (to be referred to as tablets hereinafter); a mixing
circuit for adding the synthesizing ratio for each repetitively occuring
wave when the means for selecting qualities of sound are selected;
frequency dividers for dividing the oscillation frequencies of top octave
generators, an indirect keying circuit for interrupting the current
corresponding to the ratio of repetitively occuring waves in response to
the repetitively occuring signal waveforms from the frequency dividers and
intermittently interrupting said current in response to on-off signals
from a keyboard; a synthesizing circuit for combining the outputs from the
indirect keying circuit into a group for each octave; a variable filter
for changing the frequency characteristics electronically in response to
the outputs from said tablets; and detectors of key number and tablet
number for detecting the number of keys pressed and the number of tablets
selected, respectively, for effecting the additive control of the
amplitudes of the output tone signals.
The present invention may attain the following features and advantages:
1. The stage for synthesizing the repetitively occuring waves such as
square waves is made up of transistors such as MOS-FETs; the means for
controlling qualities of sound for determining the ratio specific to a
selected tablet is made up of resistor arrays; and operational amplifiers
are used. Therefore, they are fabricated in the form of LSI, and the
construction is simple.
2. The repetitive occuring waves such as square waves in each order from
the keying circuit are added by the operational amplifiers so that the
additive control is facilitated.
3. The characteristic of the variable filter is electronically selected in
response to selected tablets so that a large number of qualities of sound
may be produced by a relatively small number of filters.
4. The sound quality may be improved over the conventional electronic
organs.
BREIF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block diagram illustrating the fundamental
construction of the preferred embodiments of the present invention;
FIG. 2 is a circuit diagram illustrating major components thereof;
FIG. 3 is a circuit diagram of tablet switches and a waveform memory
circuit;
FIG. 4 is a block diagram of one example of an electronically variable
filter;
FIG. 5 shows the characteristics of the variable filter shown in FIG. 4;
FIG. 6 is a circuit diagram of a detector of key number and a variable
reference voltage source;
FIG. 7 shows the output gain characteristic curve obtained by the circuit
shown in FIG. 6;
FIG. 8 is a circuit diagram of a detector of tablet number and a variable
reference voltage source;
FIG. 9 shows the output gain characteristic curve obtained by the circuit
shown in FIG. 8;
FIG. 10 is a circuit diagram of an arrangement in which both a detector of
key number and a detector of tablet number are provided and a variable
reference voltage source which is controlled in response to the outputs
from said two detectors is also provided;
FIG. 11 is a diagram illustrating the voltage at the point V.sub.B in the
circuit shown in FIG. 10;
FIG. 12 is a diagram illustrating the voltage at the point V.sub.D in the
circuit shown in FIG. 10;
FIG. 13 shows the output gain characteristic curve obtained by the circuit
shown in FIG. 10; and
FIG. 14 is a circuit diagram illustrating an example in which the synthesis
for each scale and for each octave are accomplished in two steps in a
mixing circuit 8 shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, reference numeral 1 designates generators for
producing the notes C through B in the top octave in the equally tempered
scale; 2, frequency dividers; 3, a keyboard; 4, switching circuits to be
referred to as "tablets" hereinafter in this specification for selecting
desired timbres or tone colors; and 5, memory means or bank of resistor
arrays for determining the intensity ratio for each tablet to combine the
trains of square waveforms with a frequency which is an integral multiple
of the basic or fundamental frequency, the resistor arrays 5a, 5b, . . .
5n being different in resistance ratio for the tablets 4a, 4b, . . . 4n,
respectively. Reference numeral 6 designates mixing circuits or bank of
first operational amplifiers for combining the intensity ratios of tones
selected by the selected tablets as will be described in detail
hereinafter; 7, indirect keying circuits or waveform synthesizers
consisting of MOS-FET groups for forming the basic or fundamental waveform
of the tone by intermittently interrupting the rectangular waves from the
generators 1 as well as the analog signals from the selected tablets in
response to the signals from the keyboard switches 3a, 3b, . . . 3n; 2a,
1/2 frequency dividers; 2j, 1/4 frequency dividers; and 2b-2e, 2g-2n and
2p, 1/2 frequency dividers.
Reference numeral 8 designates mixing circuits or second operational
amplifiers for combining the tone signals for each octave produced by
synthesizing the square waveforms specific to the selected tablets as will
be described in detail hereinafter; 9, variable filters in which the pass
bands are varied by electronic switches so that the outputs from the
second operational amplifiers 8 may contain only desired harmonics; 14,
special effects circuits; 10, filter control circuit for controlling the
variable filters 9 in response to the output from the selected tablets;
15, an audio amplifier; 16, a speaker; 11, a detector for detecting the
number of pressed keys or a detector of key number so as to control the
voltages to be applied to the tablets depending upon the number of pressed
keys; 12, a detector for detecting the number of selected tablets or a
detector of tablet number so as to control the voltages to be applied to
tablets depending upon the number of selected tablets; 13, a variable
reference voltage source for generating a reference voltage to be applied
to the tablets 4 in response to both outputs from the detectors 11 and 12;
and 18, a device for effecting the envelop control such as sustain effect.
Next the mode of operation will be described. The C note output from the
generator 1 is divided by the frequency dividers 2 into the rectangular
waveforms with frequencies f, 1/2f, 2f, 3f and 4f. In like manner, the
outputs C sharp through B from the generators 1 are divided by the
frequency dividers 2 into the rectangular waveforms.
In the MOS-FET group 7, the rectangular waveform tone signal with the
frequency 4f is applied to the gate of a first transistor of a transistor
pair 7a, and a DC voltage from a DC voltage source 17 is applied to the
gate of the other or second transistor when the keyboard 3 is pressed.
Therefore, the transistor pair 7a intermittently interrupts the analog
signal from the operational amplifier 6a at the frequency 4f when the note
"C.sub.2 " key switch 3a is pressed. In like manner, a transistor-pair 7b
intermittently interrupts the analog signal from the operational amplifier
6b in response to the rectangular waveform with the frequency 3f when the
C.sub.2 key is pressed. In like manner, transistor pairs 7c-7f
intermittently interrupt the outputs from the operational amplifiers
6c-6f. Even though only the output from the C.sub.2 key is shown in FIG.
2, the output from each note key in each octave is also connected to the
gate of the second transistor in the other MOS-FET group. For instance
when the "C.sub.3 " key is pressed, the output from the operational
amplifier 6a is intermittently interrupted in response to the output from
the frequency divider 2d; that is, the rectangular waveforms of the next
higher octave. In like manner, the output from the operational amplifier
6b is intermittently interrupted in response to the output from the
frequency divider 2m.
Referring to FIG. 3, the tablet group 4 includes a plurality of tablets 4a,
4b, . . . 4n, and when the tablet 4a is closed, the output voltage V.sub.o
from the variable reference voltage source 13 (See FIGS. 1 and 2) is
applied to the resistor array 5a. The resistance ratio among the resistor
arrays 5a, 5b, . . . 5n is so selected that the mixing ratio of the
rectangular waveforms with the frequencies 4f, 3f, 2f, 3/2f, f and f/2 may
be determined in response to the tone quality or timbre selected by the
tablets 4a, 4b, . . . 4n. For instance, in the resistor array 5a.
Ra.sub.1 : Ra.sub.2 = 2 : 1 (1)
Therefore when only the tablet 4a is closed,
##EQU1##
where the voltage drops across the diodes Da.sub.1 and Da.sub.2 inserted
for the purpose of preventing the reverse current flow are disregarded.
Therefore the ratio between the output voltages Va and Vb from the first
operational amplifiers 6a and 6b is
Va : Vb = 1 : 2 (4)
if Rfa = Rfb.
Therefore, the step wave in which the rectangular waveforms with the
frequencies 4f and 3f are mixed in the ratio of 1 to 2 appears at the
input terminal C.sub.2 of the second operational amplifier 8a
corresponding to the C.sub.2 note key (See FIG. 2).
When three tablets 4a, 4b and 4c are pressed simultaneously, the output
voltage from the operational amplifier 6a is given by
##EQU2##
From Eq. (5) it is seen that the outputs from the tablets 4a, 4b and 4c
are added. Same is true for the operational amplifiers 6b - 6f. That is,
##EQU3##
where Rfa = Rfb = . . . = Rff.
The ON resistance Ron of each FET of the transistor pairs 7a - 7f is the
operational resistance for combining the rectangular waveforms whose
fundamental spectrum is a harmonic. The output from, for instance, the
operational amplifier 8a V8a is given by
##EQU4##
where only the C.sub.2 note key is pressed.
As described above, the synthesis ratio is determined by the resistance
ratios of the resistor arrays 5a, 5b, . . . 5n for the tablets 4a, 4b, . .
. 4n from Eqs. (5) - (11), and the rectangular waveform whose fundamental
spectrum is a harmonic is synthesized as the sum of the voltages, and
appears at the output of the operational amplifier 8 as the step wave
making up the single note signal selected by the keyboard. The addition is
carried out by the operational amplifier 8a when the C sharp.sub.2 ,
D.sub.2, . . . B.sub.2 note keys are pressed simultaneously.
In summary, the operational amplifier 8a accomplishes the addition of the
tone signal made up of the rectangular waveforms of one octave from
C.sub.2 to B.sub.2. In this case, the ON resistance Ron of FET is the
operational resistance for the addition of the analog signals of notes in
each octave. In like manner, the operational amplifier 8b accomplishes the
addition for one octave from C.sub.3 to B.sub.3. Same is true for other
operational amplifiers 8.
The output tone signals from the operational amplifiers 8 are made to pass
through the filters 9a, 9b, . . . in the variable filter group 9 so that
the undesired harmonic components are removed. Thus the tone signals
contain only the desired harmonic components.
The electronic organs are designed based upon the pipe organs so that tens
of tone qualities or timbres may be produced. However, they may be
generally divided into five kinds based upon the harmonic components.
Therefore according to the instant embodiment, five filter characteristics
as shown at (A) - (E) in FIG. 5 are provided, and are switched
electronically in response to the selection of the tablets.
One example of the variable filter is shown in FIG. 4. In FIG. 4, 101
denotes a low-pass filter (LPF1) with the 18 dB/oct. characteristic as
shown in FIG. 5(A); 102, a high-pass filter (HPF1) whose characteristic is
opposite to that of the low-pass filter LPF1 101 in the low and high
frequency range, and to which is negatively fed back by the output from
LPF1 101; 103, a low-pass filter LPF2 with the characteristic of 12
dB/oct. as shown in FIG. 5(B); 104, a high-pass filter HPF2 whose
characteristic is opposite to that of LPF2 103 in the high and low
frequency ranges and which is negatively fed back by the output from LPF2
103; 105, a low-pass filter LPF3 with the characteristic of 6 dB/oct. as
shown in FIG. 5(C); and 106, 107, 108 and 109, analog gates for switching
the outputs from the filters. Numeral 110 denotes a matrix circuit one of
the outputs a, b, c, d and e of which rises to 1 level in response to the
selection of the tablets 4a, 4b, . . . 4n. Therefore, in response to the
input signals S.sub.1 , S.sub.2, S.sub.3 to the matrix circuit 110, one of
the filter characteristics shown in FIG. 5(A), (B), (C), (D) and (E) is
electronically selected. The input signals S.sub.1, S.sub.2, S.sub.3
represent the 4.2.1 coded signal which in turn represents the selected
tablet. For each octave the variable filters 9a, 9b, . . . have their
cutoff frequencies varied so that the filter characteristics for tablets
4a, 4b, . . . 4n may be provided for all octaves.
The outputs from the filter group 9 are combined into the output for the
upper keys and the output for the lower keys, which are applied through
the special effects circuits 14U and 14L, respectively, to the audio
amplifier 15 so that they are converted into sounds by the speaker.
Next the circuits 11, 12 and 13 will be described. FIG. 6 is a circuit
diagram of the detector 11 and the variable reference voltage source 13.
SW.sub.1, SW.sub.2, . . . SW.sub.n are key switches; Q.sub.1, Q.sub.2, . .
. Q.sub.n, are MOS-FETs whose ON resistance is equal; Ro.sub.1 is a
dividing resistance; Rs.sub.1, a series resistance; Rf.sub.1, a feedback
resistor; OP.sub.1, an operational amplifier. Let ON resistance of
MOS-FETs Q.sub.1, Q.sub.2, . . . Q.sub.n be Ron and assume that the number
of n key switches are pressed. Then the voltage at the point A is
##EQU5##
Rs.sub.1 is sufficiently grater than Ron and Ro.sub.1. Therefore the
output V.sub.1 of the operational amplifier OP.sub.1 is given by
##EQU6##
The greater the number of keys, the lower V.sub.1 becomes. The variable
reference voltage source consisting of transistors Tr.sub.1 to Tr.sub.3
and resistors R.sub.21 to R.sub.24 is so arranged as to exhibit the linear
input-output characteristic curve. Then the output voltage Vo.sub.1 from
the variable reference voltage source is given by
##EQU7##
The output Vo.sub.1 exhibits the characteristic similar to that the output
V.sub.1. In equation (14), a.sub.1 is a gain. Eq. (14) is indicated by the
Vo characteristic curve in FIG. 7. In this graph, Ron/Ro.sub.1 is equal to
2, and output voltage is 1 when n is 1.
Assume that the voltage applied to the tablet group 4 is constant because
it is not controlled by the detector and that the amplification degree of
the audio amplifier 15 be linear up to a sufficiently high signal level.
Then the output tone signal level is increased in proportion to the number
of keys pressed, but when the detector is used, the output gain
characteristic as shown in FIG. 7 may be obtained. It is more natural for
tone quality or timbre that the output shows the tendency of saturation as
shown in FIG. 7 as the number of keys pressed is increased rather than
that the output is increased. The detector of tablet number is similar in
construction and mode of operation to the detector of key number, but the
detector of tablet number detects the number of tablets selected instead
of the keys pressed.
FIG. 8 is a diagram of the detector of tablet number 12 and the variable
reference source 13. T.sub.1, T.sub.2, . . . T.sub.n are tablet switches;
Q.sub.11, Q.sub.12, . . . Q.sub.1n are NPN transistors with a small
saturation resistance; Ro.sub.2, a voltage dividing resistor; Rs.sub.2, a
series resistor; Rf.sub.2, a feedback resistor; and OP.sub.2, an
operational amplifier. R.sub.1, R.sub.2, . . . R.sub.n have the same value
or different values. When Rt = R.sub.1 = R.sub.2 = . . . = R.sub.n and m
tablets are selected, the voltage at point B is given by
##EQU8##
if Rs.sub.1 is sufficiently higher than Rt and Ro.sub.2. The output of the
operational amplifier OP.sub.2 is given by
##EQU9##
The larger the number of tablets selected, the greater m becomes while the
lower V.sub.2 becomes.
When the variable reference voltage source consisting of transistors
Tr.sub.11 to Tr.sub.13 and resistors R.sub.11 to R.sub.14 is so arranged
as to exhibit the linear input-output characteristic, the output voltage
Vo.sub.2 is given by
##EQU10##
This is shown in FIG. 9, and a.sub.2 is a gain. The characteristic curve
Vo.sub.2 /Vo.sub.2 (when only one tablet is selected) shown in FIG. 9 is
obtained and Rt/Ro.sub.2 = 1.2.
When the number of keys pressed is constant and the detector of tablet
number with the above construction is used, the output tone signal
characteristic curve as shown in FIG. 9 is obtained. It is more natural
for tone color or timbre that as the number of tablets selected is
increased, the output exhibits the saturation tendency as shown in FIG. 9
rather than it increases. So far R.sub.1 = R.sub.2 = . . . = R.sub.n, but
the resistors may have different values; that is, R.sub.1 .noteq. R.sub.2
.noteq. . . . .noteq.R.sub.n. Thus the fine control on the addition
between tablets may be attained.
Next the mode of controlling the tone signal when both the number of keys
pressed and the number of tablets selected are detected will be described.
Referring to FIG. 10, K.sub.1, K.sub.2, . . . are key switches; Q.sub.1,
Q.sub.2, . . . are MOS-FETs whose ON resistance is equal; Ro.sub.3 is a
voltage dividing resistor; Rs.sub.3 is a series resistor; Rf.sub.3 is a
feedback resistor; and OP.sub.3 is an operational amplifier; T.sub.1,
T.sub.2, . . . are tablet switches; Q.sub.11, Q.sub.12, . . . are
transistors or switching elements; R.sub.1, R.sub.2, . . . are resistors
whose value are equal or different; R.sub.31 is a voltage division
resistor; and Tr.sub.33, Tr.sub.34, Tr.sub.35, R.sub.32, R.sub.33,
R.sub.34, R.sub.35 and D.sub.1 make up a voltage source.
Next the mode of operation will be described. First we consider the state
where MOS-FETs Q.sub.1, Q.sub.2, . . . are not saturated; that is, V.sub.B
< V.sub.DD - V.sub.T, where V.sub.T = threshold voltage, and the body
effect is disregarded. When n keys are pressed, the number of MOS-FETs
which are turned on is also n. Let the current flowing through one MOS-FET
be I.sub.D, then the following relations are held between V.sub.A and
V.sub.B :
I.sub.D = -.beta.[V.sub.DD - V.sub.T)V.sub.B - 1/2 V.sub.B.sup.2 ](15)
V.sub.A - V.sub.B = n.I.sub.D.Ro.sub.3 (16)
where
##EQU11##
where .epsilon..sub.ox = permittivity of oxide,
t.sub.ox = thickness of oxide over channel,
.mu. = average surface mobility in channel,
W = width of channel, and
L = length of channel.
It was assumed R.sub.s >> Ro.sub.3, and the current flowing through R.sub.5
is disregarded.
When V.sub.A and V.sub.DD are maintained constant, the relation between the
number of keys pressed n and V.sub.B is obtained as shown in FIG. 11 from
Eqs. (15) and (16). The DC voltage V.sub.B is reversed in polarity and
amplified by the operational amplifier OP.sub.3 so that output V.sub.C is
obtained. The relation between n and V.sub.C is similar to that of V.sub.B
described above.
When m tablets are selected and the saturation resistance VCE (SAT) between
the collector and emitter of transistor Q.sub.11 is disregarded.
##EQU12##
if Rt = R.sub.1 = R.sub.2 = . . . Therefore when V.sub.C is maintained
constant, the relation between m and V.sub.D is obtained as shown in FIG.
12. V.sub.D is amplified by the variable reference voltage source
consisting of Tr.sub.33 to Tr.sub.35, R.sub.32 to R.sub.35 and D.sub.1,
and the output voltage Vo is given by
##EQU13##
where V.sub.D1 is a forward voltage drop across diode D.sub.1. The output
voltage Vo is connected to Vo shown in FIG. 3.
The output voltage is increased V.sub.D1 in order to compensate the voltage
drops across diodes Da.sub.1, Da.sub.2, . . . shown in FIG. 2. When the
diodes with substantially similar characteristics are used as Da.sub.1,
Da.sub.2, . . . the voltage variation including the variation due to the
temperature variation may be absorbed.
When the voltage applied to the tablet group 4 is not controlled by the
detectors of key number and tablet number and is maintained at a constant
level, and if the audio amplifier 15 exhibits the linear amplification
degree up to a sufficiently high signal level, the output tone signal
level increases in proportion to the number of keys pressed and to the
number of tablets selected. But when the detectors of the type described
above are used, vo is gradually decreased as shown in FIG. 13. Thus by the
detection of the number of keys pressed and the number of tablets
selected, the output tone signal with the tendency of being saturated in a
predetermined manner may be produced.
The saturation characteristic may bse arbitarily varied by changing the
voltage division resistors Ro.sub.3 and R.sub.31.
* * * * *
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