Electronic musical instrument

An electronic musical instrument is provided with a temporal variation circuit of SCF parameters for temporally varying the filter characteristic of a switched capacitor filter circuit, a control circuit for digitally controlling the temporal variation circuit and a touch response circuit for detecting, by scanning, touch response data in performance, whereby temporal variations of a musical waveform signal are digitally controlled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electronic musical instrument adapted 
for digitally changing a musical waveform with time and in accordance with 
a touch response through utilization of a switched capacitor filter 
circuit for controlling a harmonic component of a source waveform signal 
in accordance with a desired timbre. 
2. Description of the Prior Art 
Conventionally, a musical tone generation system for electronic musical 
instruments is roughly divided into an analog sound source system and a 
digital sound source system, which respectively have merits and demerits. 
The digital sound source system permits synthesization of a musical 
waveform element by digital calculations, and hence is capable of 
producing timbre over a wide range, but since it posseses the defects of 
an enormous circuit scale and limitations imposed on the synthesization of 
timbre by the amount of calculation and the time therefor, it is employed 
only in some high-grade models. On the other hand, the analog sound source 
system comprises a source waveform generator for generating a source 
waveform signal corresponding to a note frequency and containing harmonic 
components in abundance, a filter circuit for controlling the harmonic 
components of the source waveform signal in accordance with a timbre 
desired to produce and an envelope generator for generating a desired 
envelope. These circuits have undergone various improvements as well-known 
VCO (Voltage Controlled Oscillator), VCF (Voltage Controlled Filter) and 
VCA (Voltage Controlled Amplifier) which employ voltage as a common 
control parameter. Further, many proposals have been made on digital 
control of electronic musical instruments through utilization of digital 
electronic circuit technologies typified by the microcomputer technology. 
For example, in the source waveform generator, the pitch corresponding to 
a note frequency is obtained with high accuracy by DCO (Digital Control 
Oscillator) using a "program counter". Moreover, storage and display of 
timbre setting data on the control panel, instantaneous modification of 
may musical parameters and simultaneous control of a plurality of channels 
by time-shared operations can also e achieved easily by the employment of 
the microcomputer technology. 
However, many problems have been pointed out in connection with the 
digitization of an electronic musical instrument of the conventional 
analog sound source system. For instance, the VCF (Voltage Controlled 
Filter) could have been constructed at a low cost since it is necessary 
only to supply voltage of a volume specified on the control panel, but the 
supply of data via the microcomputer involves an A/D conversion and a D/A 
conversion, and hence calls for a very complex circuit arrangement. 
Further, in order to digitize control of an EG (Envelope Generator) for 
generating a control voltage which is supplied to the VCF to obtain 
temporal variations of timbre, it is necessary to supply the voltage after 
D/A converting it for each of such parameters as A (Attack), D (Decay), S 
(Sustain) and R (Release). This requires a large-scale circuit arrangement 
and analog fine control (offset control). Besides, in view of the 
characters of circuit parts forming the VCF, it is very difficult to 
employ an analog LSI in which a large-scale circuit is formed on one chip, 
and the manufacturing costs also present a problem. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an electronic 
musical instrument of the analog sound source system which can be 
fabricated as an LSI and whose filter portion is digitized through the use 
of an SCF (Switched Capacitor Filter) circuit which is a digitally 
controllable analog signal filter. 
Briefly stated, the electronic musical instrument of the present invention 
is provided with a temporal variation circuit of SCF parameters for 
temporally changing the filter characteristic of the switched capacitor 
filter, a control circuit for digitally controlling the temporal variation 
circuit and a touch response circuit for detecting, by scanning, touch 
response data in performance, whereby temporal variations in a musical 
waveform signal are digitally controlled.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the drawings, embodiments of the present invention will 
hereinafter be described in detail. 
FIG. 1 illustrates in block form the general arrangement of the electronic 
musical instrument of the present invention. In FIG. 1, reference numeral 
1 indicates a tone tablet; 2 designates a keyboard; 3 identifies a 
pressed-key detect and generator assignment circuit; 4 denotes a source 
waveform generator; 5 represents a tone control filter circuit; and 6 
shows an envelope generator. 
In response to timbre data and performance data entered from the keyboard 1 
and the tone tablet 2 the pressed-key detect and generator assignment 
circuit 3 applies control signals to the associated circuits. The source 
waveform generator 4 responds to the performance data and the timbre data 
from the pressed-key detect and generator assignment circuit 3 to generate 
a source waveform signal corresponding to a note frequency. The output of 
the circuit 3 is provided to the tone control filter circuit 5 via a 
simple low-pass filter for cutting off a turn-back noise resulting from an 
operation of a switched capacitor filter. As a result of this, a desired 
timbre configuration and timbre variation are set in the tone control 
filter circuit 5. The envelope generator 5 sets amplitude modulation data 
such as the rise and fall and an envelope characteristic of each musical 
sound in accordance with the performance data from the presesd-key detect 
and generator assignment circuit 3. The analog signal output from the 
envelope generator 6 is provided to a sound system 7 including an "Effect" 
circuit, an amplifier and a speaker. 
FIG. 2 illustrates in block form a specific example of the arrangement 
including a switched capacitor filter circuit and control circuits 
associated therewith which constitute the tone control filter circuit 5 
shown in FIG. 1. Reference numeral 10 indicates a switched capacitor 
filter circuit; 11 designates a clock generator for generating a clock 
signal for setting the cut-off frequency of the switched capacitor filter 
circuit 10; 12 identifies a Q circuit for setting Q character data of the 
switched capacitor filter circuit 10; 13 denotes a control circuit for 
temporally changing parameters of the clock generator 11 and the Q 
circuit; and 14 represents a fixed filter circuit for cutting off sampling 
noise which occurs in the switched capacitor filter circuit 10. 
When supplied with rise and fall data on a musical signal from the 
pressed-key detect and generator assignment circuit 3, the control circuit 
13 supplies the clock generator 11 and the Q circuit 12 with digital data 
corresponding to the temporal variations of the musical signal in a 
sequential order. The clock signal avaiable which from the clock generator 
11 is to set the cut-off frequency of the switched capacitor filter 
circuit 10, and corresponds to a VCF envelope signal in the conventional 
VCF type electronic musical instrument. In the present invention, however, 
by batch-processing the pressed-key detect and generator assignment 
circuit 3 and the control circuit 13 by a CPU on a time-shared basis, a 
desired filter operation can be set under the control of a program. 
Consequently, according to the present invention, a timbre operation with 
a high degree of freedom can be achieved with a simpler arrangement than 
in the case of the conventional VCF type electronic musical instrument 
which requires an exclusive VCF envelope signal generator. The Q circuit 
14 yields a control signal for selecting analog parameters which 
constitute the switched capacitor filter. For example, by selectively 
switching a plurality of resistors and circuit connecitons, the Q 
character data of the filter, which are of importance to the timbre 
operation, are selectively set. Accoridng to the present invention, it is 
possible to implement the tone control filter circuit 5 as a monolithic 
LSI after forming the clock genrator 11, the Q circuit 12 and the control 
circuit 13 by hardware. This is a great advantage over the prior art VCF 
type electronic musical instrument needs a large number of parts and hence 
is expensive. 
FIGS. 3A and B show circuit diagrams explanatory of the principles of 
operation of a switched capacitor circuit forming the switched capacitor 
filter circuit 10 in FIG. 2. In FIG. 3(A), reference character Cf 
indicates a capacitor which assumes a major role in the circuit operation, 
and SW1 and SW2 designate transmission gate switches formed by MOS 
transistors, respectively. The switch SW1 is supplied with an input clock 
signal CLK, whereas the switch SW2 is driven, in the opposite phase, by a 
clock signal into which the input clock signal is inverted by an inverter 
circuit Inv. The amount of charge .DELTA.Q which is transferred from the 
input side to the output side in one period of the input clock signal CLK 
can be expressed as follows: 
EQU .DELTA.Q=Cf(Vin-Vout) . . . (1) 
where Vin and Vout are potentials at the input and output sides, 
respectively. Consequently, a mean current I which flows from the input 
side to the output side during one period of the input clock signal CLK 
becomes as follows: 
EQU I=.DELTA.Q/T=Cf(Vin-Vout)/T . . . (2) 
On the other hand, current I which flows from the input side to the output 
side via a resistor R in FIG. 3(B) is as follows: 
EQU I=(Vin-Vout)/R . . . (3) 
For the sufficiently short period T, if the switched capacitor circuit of 
FIG. 3(A) is expressed as the equivalent resistor R of FIG. 3(B), it 
follows from expressions (2) and (3) that 
EQU Cf(Vin-Vout)/T=(Vin-Vout)/R.thrfore. R=T/Cf . . . (4) 
Generally, a parameter which determines the charactristics of a CR active 
filter circuit, such as a cut-off frequency and so on, is a time constant 
which is expressed by the product of C and R: 
EQU 1/CR . . . (5) 
Here, if the switched capacitor circuit of FIG. 3(A) is applied, as the 
equivalent resistor R of FIG. 3(B), to expression (5), then it follows 
that 
EQU 1/(C.multidot.(T/Cf))=Cf/(C.multidot.T)=F.multidot.(Cf/C) . . . (6) 
where F is the clock frequency of the input clock signal CLK. The time 
constant depends upon the input clock frequency and the ratio between the 
electrostatic capacities of two capacitors used in the switched capacitor 
circuit and the acitve filter circuit. In the fabrication of a monolithic 
LSI, it is very difficult to set absolute values of the electrostatic 
capacities of individual capacitors within the accuracy of an analog 
filter. But since the ratio of electrostatic capacity between two 
capacitors, that is, the relative accuracy, as shown by expression (6), 
can easily be kept to about 1%, practical filter characteristics can be 
obtained with the switched capacitor filter circuit. Further, according to 
expression (6), the time constant can be determined by controlling such a 
digital quantity as the input clock frequency. The electrostatic 
capacities of the capacitors can be fixed constant only by setting their 
relative ratio in such small capacities that can be adopted on the 
monolithic LSI. By employing the switched capacitor filter circuit which 
is used as a component of the CR active filter circuit, as described 
above, an analog filter of high accuracy, which has been very difficult to 
obtain in the past, can be implemented as a circuit which is possible of 
digital control and fabrication as an LSI. 
FIG. 4 illustrates a specific example of the arrangement of control 
cirucits associated with the switched capacitor filter circuit depicted in 
FIG. 2. In FIG. 4, reference numeral 10 indicates a switched capacitor 
filter circuit; 14 designates a fixed filter circuit for cutting off 
sampling noise generated in the switched capacitor filter circuit 10; 20 
identifies a first master clock generator; 11 denotes a variable frequency 
divide circuit; 22 represents a cut-off characteristic memory; 23 shows an 
address counter; 24 referes to a second master clock generator; 25 
signifies a first data select circuit; 26 indicates a tone select circuit; 
27 designates a second data select circuit; 28 identifies a Q character 
memory; and 29 denotes a Q select circuit. 
A description will be given, with reference to FIGS. 5A-5C, of the 
operation of the circuit arrangement shown in FIG. 4. The pressed-key 
detect and generator assignment circuit 3 supplies the tone select circuit 
26 with timbre setting data including temporal variations. The tone select 
circuit 26 provides timbre setting selection data to the first data select 
circuit 25, by which is sepcified a data block of the cut-off 
characteristic memory 22. The cut-off characteristic memory 22 has an 
arrangement such, for example, as shown in FIG. 5(A). The storage area of 
the memory 22 is divided by high-order addresses from the first data 
select circuit 25 into timbre blocks, each of which has stored therein an 
amount of data corresponding to common low-order addresses. In accordance 
with a temporal variation speed parameter which is set by the pressed-key 
detect and generator assignment circuit 3, the first master clock 
generator 20 supplies the address counter 23 with a fixed master clock 
signal which forms the basis of temporal variations of timbre. The address 
counter 23 is initialized by a tone generation start signal (KEY-ON 
signal) from the pressed-key detect and generator assignment circuit 3, 
and generates an address signal which is incremented by the clock signal 
from the first master clock generator 20 and provides the address signal 
as the low-order address to the cut-off characteristic memory 22. The 
cut-off characteristic memory 22 has stored therein data, for instance, as 
shown in FIG. 5(B). That is, the memory 22 has prestored therein at the 
common low-order addresses cut-off characteristic data corresponding to 
individual temporal variations of each timbre, as indicated by D1, D2, . . 
. in FIG. 5(B). The memory 22 can be formed by a ROM, and if necessary, it 
is also possible to employ a RAM and to store therein data by calculating 
it with a CPU at any time. Data accessed by the high- and low-order 
addresses in the cut-off characteristic memory 22 is supplied to the 
variable frequency divide circuit 21. The variable frequency divide 
circuit 21 generates a clock signal for setting the cut-off frequency of 
the switched capacitor filter circuit 10. The variable frequency divide 
circuit 21 can be constituted by a well-known circuit such as a 
programmable counter circuit, a rate multiplier circuit or the like. In 
this case, however, as will be seen from FIG. 3, it is desirable that the 
clock signal by the switched capacitor filter circuit have a duty of 50%, 
and it is required to apply it to the frequency divide circuit, as 
required. In the second master clock generator 24 which provides a master 
clock signal to the frequency divide circuit 21, it is effective to set 
the clock frequency as high as possible to satisfy the requirements that 
the clock signal for setting the cut-off frequency be set sufficiently 
high in frequcncy for avoiding the turn-back noise generation and that the 
frequency range of the clock signal which is selectively produced by the 
frequency divide circuit 21 be set sufficiently wide. FIG. 5(C) shows an 
example of the output clock signal of the variable frequency divide 
circuit 21. As will be seen form FIG. 5(C), the clock signal for setting 
the cut-off frequency of the switched capacitor filter circuit 10 varies 
with the lapse of time, implementing the timbre operation corresponding to 
the operation of the voltage controlled filter in the conventional 
electronic musical instrument. In the case of the prior art analog VCF 
system, it is possible to obtain only timbre variations within the range 
of the circuit characteristic of the envelope generator which generates 
the temporal variation parameter. In contrast thereto, according to the 
present invention, it is possible to arbitrarily enlarge and reduce the 
time axis by the first master clock generator 20, to set a desired 
temporal variation pattern by the cut-off characterisic memory 22 and to 
produce, with ease, such a periodic timbre variation as a wow-wow effect 
according to the access or clear method of the address counter 23. 
Therefore, the present invention is capable of producing richer musical 
sounds than does the conventional electronic musical instrument. 
FIG. 6 illustrates another example of the specific arrangement of the 
control circuits associated with the switched capacitor filter circuit 
shown in FIG. 2. In FIG. 6, reference numeral 10 indicates a switched 
capacitor filter circuit; 14 designates a fixed filter circuit for cutting 
off sampling noise which is generated in the switched capacitor filter 
circuit 10; 30 identifies a third master clock generator; 31 denotes a 
frequency divide circuit; 32 represents a third data select circuit; 33 
shows a select signal counter; 34 refers to a fourth master clock 
generator; and 35 signifies a control timing generator. 
A description will be given, with reference to FIGS. 7A-C, of the operation 
of the embodiment shown in FIG. 6. The pressed-key detect and generator 
assignment circuit 3 supplies the control timing generator 35 with timbre 
setting data on the speed of the temporal variation. The fourth master 
clock generator 34, which is supplied with a necessary control signal from 
the control timing generator 35, yields a master clock signal for the 
select signal coutner 33. On the other hand, the third master clock 
generator 30 generates a master clock signal of a sufficiently high 
frequency which corresponding to the clock signal for setting the cut-off 
frequency of the switched capacitor filter circuit 10. The master clock 
signal from the fourth master clock generator 34 is provided to the 
frequency divide circuit 31. The frequency divide circuit 31 is formed by, 
for example, eight stages of cascade-connected binary counters, and 
outputs 01, 02, . . . and 08 of the respective stages each have an output 
frequency equal to one-half that of the preceding stage. See FIG. 7A. The 
outputs 01, 02, . . . and 08 of the respective stages of the frequency 
divide circuit 31 are applied to the third data select circuit 32. The 
third data select circuit 32 is a multiplexer circuit which selects one of 
the eight inputs by, for instance, three-bit control address. That is, the 
third data select circuit 32 uses the output signal from the select signal 
counter 33 as the control address to select one clock signal from the 
outputs 01 to 08 of the respective stages of the frequency divide circuit 
31. FIG. 7(B) shows the operation of the select signal counter 33. The 
tone generation start (KEY-ON) data from the pressed-key detect and 
generator assignment circuit 3 is provided, as a clear count start signal, 
by the control timing generator 35 to the select signal counter 33, which 
yields output signals corresponding to addresses for sequentially 
selecting the outputs 01 to 08 of the respective stages of the frequency 
divide circuit 31. FIG. 7(C) shows the clock signal that is applied, by 
the select address from the select signal counter 33, from the third data 
select circuit 32 to the switched capacitor filter circuit 10. After the 
start of tone generation (KEY-ON) the clock frequency varies in the manner 
of geometric progression at equal time intervals, producing temporal 
variations with an exponential characteristic as a whole. This is the 
filter characteristic that is very effective for synthesizing the timbre 
of a piano, harpsichord or like musical insturment which contains harmonic 
components in abundance at the moment of its tone generation. This greatly 
contributes to the fabrication of the active filter circuit as an LSI 
which could not have been realized as a timbre operation circuit of a 
simple circuit arrangement and of digital control. 
FIG. 8 illustrates still another example of the arrangement of the switched 
capacitor filter circuit and the associated control circuits which are 
provided in the tone control filter circuit 5 shown in FIG. 1. In FIG. 8, 
reference numeral 10 indicates a switched capacitor filter circuit; 40 
designates a clock generator for generating a clock signal for setting the 
cut-off frequency of the switched capacitor filter circuit 10; 41 
identifies a Q circuit for setting the Q characteristic of the switched 
capacitor filter circuit 10; 43 denotes a temporal variation control 
circuit for temporally changing parameters of the clock generator 40 and 
the Q circuit 41; 42 represents a touch response control circuit for 
controlling the parameters of the clock generator 40 and the Q circuit 41 
and the operation of the temporal variation control circuit 43 in 
accordance with a touch response; and 14 shows a fixed filter circuit for 
cutting off sampling noise which is produced in the switched capacitor 
filter circuit 10. 
A description will be given, with reference to FIGS. 9A-C, of the operation 
of the embodiment depicted in FIG. 8. When supplied with rise and fall 
data of a musical sound signal from the pressed-key detect and generator 
assignment circuit 3, the temporal variation control circuit 43 
successively supplies the clock generator 40 and the Q circuit 41 with 
digital data corresponding to the temporal variations of the musical sound 
signal. The clock signal which is produced by the clock generator 40 is 
intended to set the cut-off frequency of the switched capacitor filter 
circuit 10 and corresponds to a VCF envelope signal in the conventional 
VCF type electronic musical instrument. The Q circuit 41 yields a control 
signal for selecting an analog parameter which constitutes the switched 
capacitor filter; for example, by selective switching of a plurality of 
resistors and circuit connections, the Q character data important for the 
timbre operation is selectively set. On the other hand, the pressed-key 
detect and generator assignment circuit 3 provides, as touch response 
data, the strength of a touch on the keyboard to the touch response 
control circuit 42 for each tone generator. The touch response control 
circuit 42 applies a volume control signal of the musical sound to the 
envelope generator 6, as required, and at the same time, controls a touch 
response operation of timbre in the tone control filter circuit 5. FIG. 
9(A) shows an example in which the touch response control circuit 42 
responds to the clock signal from the clock generator 40. The curve (a) 
shows temporal variations of the clock frequency for a soft touch on the 
keyboard and the curve (b) temporal variations of the clock frequency for 
a hard touch on the keyboard. The difference in the variation 
characteristic between the clock frequencies corresponds to the cut-off 
frequency of the switched capacitor filter circuit 10, so that for the 
timbre of the musical sound the clock frequency undergoes temporal 
variations of a characteristic containing harmonics. That is, for the soft 
touch in the case of the curve (a), not only the volume which is added by 
the envelope generator 6 is small but also the timbre undergoes 
striking-sound-like temporal variations a relatively smoothly. On the 
other for the hard touch in the case of the curve (b), not only the volume 
which is added by the envelope generator 6 is large but also the timbre 
contains harmonics over a wide range and undergoes striking-sound-like 
temporal variations. This well corresponds to the characteritic of a 
natural musical instrument such as a piano, vibraphone or the like, and is 
very effective timbre operation function for producing natural musical 
sounds in the electronic musical instrument. FIG. 9(B) shows an example in 
which the touch response control circuit 42 responds to the Q factor which 
is set in the Q circuit 41. The curve (c) shows temporal variations of the 
Q factor for a soft touch on the keyboard and the curve (d) temporal 
variations of the Q factor for a hard touch on the keyboard. The 
difference between the variation characteristics of the Q factor is 
reflected as the Q factor of the switched capacitor filter circuit 10, and 
for the timbre of the musical sound, the Q factor undergoes temporal 
variations of a characteristic containing harmonics. That is, for the soft 
touch in the case of the curve (c), the peak of the Q factor is low and 
the timbre undergoes smooth temporal variations, but for the hard touch in 
the case of the curve (d), the peak of the Q factor is high and the timbre 
has a formant characteristic peculiar thereto and undergoes temporal 
variations. This is very effective as a touch response expression which 
well corresponds to the characteristic of the timbre of such a natural 
musical instrument as saxophone, a trumpet or the like. FIG. 9(C) shows an 
example in which the touch response control circuit 42 responds to the 
temporal variation parameter in the temporal variation control circuit 43, 
in connection with the cut-off frequency of the switched capacitor filter 
circuit 10. The curve (e) shows temporal variations of the parameter for a 
soft touch on the keyboard and the curve (f) temporal variations of the 
parameter for a hard touch on the keyboard. The difference between the 
variation characteristics of the paramerter is reflected as a difference 
between the modes of variation in the cut-off frequency of the switched 
capacitor filter circuit 10, and for the timbre of the musical sound, the 
cut-off frequency produces resonance and reverberation owing to the speed 
of the temporal variations of the characteristic containing harmonics. 
That is, for the soft touch in the case of the curve (e), the cut-off 
frequency undergoes relatively fast temporal variations, but for the hard 
touch in the case of the curve (f), it sharply rises but gradually falls, 
producing such an effect as if harmonics resonate with strings other than 
that struck hard. This is very effective as a touch response expression 
which well corresponds to the characteristic of timbre of a quitar, piano 
or like natural musical instrument. 
As described above, according to the present invention, in the electronic 
musical instrument which employs a switched capacitor filter circuit for 
controlling harmonic components of a source waveform signal in accordance 
with a desired timbre, the filter portion of the electronic musical 
instrument of the analog sound source system which can be fabricated as an 
LSI is adapted for digital control, thereby providing a musical sound 
generating system which varies the musical waveform with time and in 
accordance with a touch response by a simple arrangement. Accordingly, the 
present invention offers an electronic musical instrument of high 
musicality, and hence greatly contributes to the creation of good music. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel convepts of the present 
invention.