Programmable dynamic filter

A programmable dynamic filter for use with a time-shared electronic organ or the like includes a filter circuit responsive to a logical state of a control signal for tracking notes generated by the instrument. Means are provided for programming the filter independently of generated notes by selectively adjusting the pulse width of the control signal to achieve desired musical effects.

BACKGROUND OF THE INVENTION 
The present invention relates to programmable circuits for electronic 
musical instruments. More particularly, the invention relates to 
programmable dynamic filters for use with time-shared electronic organs 
having frequency response characteristics which are controllable 
independently of the audio frequency of the musical note being played. 
Conventionally, electronic musical instruments such as organs and 
synthesizers employ tone generators of two general types. Synthesizers 
have generally utilized a voltage-controlled oscillator, the control 
voltage applied thereto being a function of the note to be sounded. These 
systems are monophonic, being adapted to sound only a single note at a 
time. Organs have normally utilized either a separate oscillator for each 
note which is to be generated or a single oscillator whose output is 
passed through suitable dividers to obtain signals at each frequency which 
the instrument will be required to sound. Separate keyer circuits, 
envelope generators and filters are employed to suitably shape the tone 
signals, and many such circuits are required to produce a variety of 
different tones and voices. These circuits are each dedicated to 
respective audio tone signals of particular frequencies in order that 
their timing characteristics may be "tuned" to those frequencies. As a 
result, hundreds of circuits are required in a prior art organ. This 
multiplicity of circuitry significantly affects the size, weight, 
complexity, power consumption and cost of the instrument. 
Recently, time-shared systems have been proposed which greatly reduce the 
number of circuits which are required to implement an electronic organ. 
One such system is disclosed in co-pending application Ser. No. 835,832, 
filed Sept. 22, 1977 in the name of Richard S. Swain et al, entitled "Tone 
Generating System for Electronic Musical Instrument" and assigned to the 
assignee of the present invention. A variety of programmable circuits for 
use with the Swain et al system are disclosed in co-pending application 
Ser. No. 835,695, filed Sept. 22, 1977 in the name of Glenn Gross, 
entitled "Programmable Circuits for Electronic Musical Instrument" and 
also assigned to the assignee of the present invention. 
In the time-shared electronic organ disclosed in the foregoing co-pending 
applications, a limited number, preferably ten or twelve, of musical 
note-sounding channels are provided. Each channel, which typically 
consists of a priority note generator, an envelope generator and driver, a 
programmable keyer and a programmable filter, can be assigned as needed to 
sound any note in the entire musical range of the instrument by the use of 
time-shared techniques. Moreover, to further reduce circuit multiplicity, 
the frequency dependent circuits of each channel, such as the filters, are 
programmable to facilitate the use of a single filter for processing all 
the musical tones which may be produced by a particular note generator. In 
other words, the frequency response of a channel filter is preferably 
tailored according to the frequency of the note being generated in order 
to maintain proper musical characteristics. Thus, for example, the filter 
would exhibit a relatively high frequency range for the higher pitch 
musical notes produced by the tone generator and a lower frequency range 
for the lower pitch notes. This is accomplished by generating an encoded 
signal identifying the octave or half-octave in which a generated tone 
signal is contained and programming the filter in accordance therewith. 
The filter is thereby operated to exhibit an appropriately different 
response depending upon the frequency of the generated tone signal, this 
technique being commonly referred to as "tracking." Each different 
frequency response is characterized by a respective frequency range 
normally referred to as the tracking interval of the filter. 
However, in order to achieve certain musical effects, it may be desirable 
to program the channel filter for modulating the filter's frequency 
response or tracking interval beyond or in addition to the response 
achieved from slavishly tracking or following the tone being played. 
Stated otherwise, the channel filter should be programmable for initially 
tracking the tone being played and should further be independently 
controllable whereby this initially tracking interval may be adjusted or 
modulated to produce various desired effects. 
Various techniques for modulating a filter beyond its initial tracking 
interval corresponding to a tone being sounded are known in the art. In 
one such system, an exponential converter operates a voltage-controlled 
filter, the filter having an input for receiving a generated tone signal. 
The exponential converter has two inputs, one input being connected for 
receiving a DC signal corresponding to the selected note and the other 
receiving an independently derived modulating signal. The output of the 
converter is therefore an analog signal operating the voltage-controlled 
filter in response to both the note being played and the independently 
derived modulating signal. Systems of this type, being analog in nature, 
are not readily suited for inclusion in a time-shared digital electronic 
organ of the variety disclosed in the previously mentioned co-pending 
applications. 
U.S. Pat. No. 3,974,461 to Luce discloses another prior art filter which 
may be operated for modulating its tracking interval beyond an initial 
tracking interval corresponding to a note being played. In the Luce 
system, a filter is provided consisting of a series of gates interposed 
between a plurality of individual low-pass filter sections. The frequency 
response or tracking interval of the aggregate filter is therefore 
determined by the conduction states of the gates. Control of the gates is 
affected by the output of a monostable multivibrator which develops output 
pulses of fixed duration at a repetition rate determined by a 
voltage-controlled oscillator connected to its input. The 
voltage-controlled oscillator is, in turn, operated in response to an 
analog input signal derived from a summing circuit which may combine a 
keyboard related analog signal as well as an independently generated 
modulating signal. It will be appreciated that the repetition rate of the 
pulses produced by the monostable multivibrator, which is ultimately 
dependent upon the analog signal developed by the summing circuit, 
controls the frequency response and therefore the tracking interval of the 
filter. This signal, which is partly digital in nature and partly analog, 
is also not readily suited for incorporation in a completely digital 
time-shared instrument. Furthermore, quite contrary to the teachings of 
this reference, the elimination of voltage-controlled ocillators is one of 
the primary purposes of time-shared systems. 
SUMMARY OF THE INVENTION 
It is a primary object of the invention to provide a new and improved 
programmable filter for use with a time-shared electronic organ or the 
like. 
It is more specific object of the invention to provide a programmable 
filter for use with a time-shared electronic organ whose initial tracking 
interval, corresponding to the frequency of a note being sounded, is 
independently adjustable to achieve desired musical effects. 
In accordance with these and other useful objects, an improved programmable 
filter is provided for use with a time-shared electronic organ or the 
like. The organ includes means for generating a digital encoded control 
signal defining the portion of the musical scale containing a generated 
tone signal. The programmable filter includes a plurality of analog 
switches connected in association with a filter circuit such that the 
filter is responsive to a logical state of the encoded control signal for 
tracking notes sounded by the organ. The filter further includes a circuit 
coupled between the instrument and the analog switches which is responsive 
to a modulating signal for selectively varying the pulse width of the 
pulses comprising the control signal. By suitably varying the pulse width 
of the control signal the intial tracking interval of the filter, which is 
dependent upon the frequency and pulse width of the unmodified control 
signal, may be modulated or adjusted independently of the tone signal 
being sounded. 
The pulse width modification circuit may comprise a voltage-controlled 
monostable multivibrator responsive to the control signal and an 
independently developed modulating signal. In this embodiment, control 
signal pulses trigger the monostable multivibrator while the voltage of 
the modulating signal determines its unstable state duration. 
Alternatively, a bi-stable device such as a flip-flop circuit may be used 
as a pulse width modification device. In this case, the control signal 
pulses repetitively set the bi-stable device while a logic circuit 
repetitively resets the device under the control of a selectable digital 
code representing the modulating signal. 
In either embodiment, the pulse width of the control signal pulses may be 
effectively controlled for modulating the programmable filter beyond its 
initial tracking interval corresponding to the note presently being 
sounded.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, FIG. 1 shows in general terms a fragment of 
an electronic musical instrument incorporating the present invention. The 
omitted portions of the musical instrument are either conventional in 
nature or are disclosed in detail in the above-cited co-pending 
applications. The musical instrument is preferably of the type which has a 
keyboard, such as a synthesizer or organ. Each of the keys on the 
keyboard, when operated by the musician, closes a switch resulting in the 
circuit of FIG. 1 generating and processing an electronic tone signal, the 
fundamental frequency of which is equal to the pitch of the selected 
musical note. 
The circuit which generates the electronic tone signal is a priority note 
generator 15, a specialized digital circuit which is preferably realized 
in the form of an integrated circuit. In brief terms, the priority note 
generator is a circuit in which a high frequency clock pulse train is 
divided down to any desired musical frequency by means of a digitally 
controlled programmable divider to produce a musical tone signal. Further 
details of the priority generator and its operation are disclosed in the 
co-pending applications cited above. The priority note generator has a 
number of output lines. The tone signal is developed on a line 17 and 
coupled from priority note generator 15 to a keyer 19. A drive signal is 
developed on a line 18 and coupled to a pair of envelope generators 20a 
and 20b. Envelope generator 20a, which is coupled to keyer 19 by a line 
21, produces an envelope signal simulating the attack and decay 
characteristics of an acoustical tone. The envelope signal produced by 
generator 20a is combined with the tone signal on line 17 by keyer 19 and 
applied to a programmable filter circuit 24. Programmable filter 24 is 
controlled by a serial bit code developed on a line 25 by priority note 
generator 15, the code being dependent upon the frequency of the tone 
signal being generated. In this manner, the frequency response 
characterizing filter 24 is continuously adjusted according to the code 
developed on line 25 and therefore according to the note selected for 
production by priority note generator 15. Thus, for example, when a high 
frequency note is selected, priority note generator 15 develops a code on 
line 25 causing programmable filter 24 to exhibit a relatively high 
cut-off frequency. On the other hand, for lower frequency notes, the code 
developed on line 25 results in the formulation of correspondingly lower 
cut-off frequencies. 
As so far described, the circuit of FIG. 1 is entirely disclosed in the two 
co-pending applications cited above. The present invention focuses on ways 
in which the flexibility of programmable filter 24 is increased for 
providing musical effects not strictly dictated by the control code 
developed on line 25. As will be explained in further detail hereinafter, 
the foregoing is, in part, accomplished by means of, for example, the 
connection of envelope generators 20a and 20b and/or an external generator 
29 via lines 23, 26, 28 and 31 and switch 30 to programmable filter 24. 
One embodiment of the invention is illustrated in detail in FIG. 2. In 
accordance with the previously discussed co-pending applications, priority 
note generator 15 develops on output line 25 a serial code defining the 
octave or half-octave of the musical scale containing the note being 
simultaneously generated. Either the frequency or duty cycle of the 
control signal developed on line 25 can be suitably adjusted for 
performing this identification task. In the embodiment illustrated in FIG. 
2, priority note generator 15 includes a high frequency clock 40 supplying 
a clock signal to the clock input of a programmable divider 42 over a line 
44. Programmable divider 42, in response to an octave control code, is 
operable for developing a variable frequency control signal on line 25. 
Thus, the octave control code programs divider 42 for exhibiting a 
division factor dependent upon the octave or half-octave of the musical 
scale containing the tone being sounded. As a result, the frequency of the 
control signal on line 25 is directly related to the octave or half-octave 
of the musical scale containing the selected note. It will be appreciated 
that decreasing the division factor characterizing divider 42 results in a 
higher frequency control signal on line 25 while increasing the division 
factor proportionately decreases the frequency of the control signal. 
In the previously mentioned co-pending applications, the control signal 
developed on line 25 is directly applied to a pair of analog switches 46 
and 48 forming part of a filter 50. Filter 50 further includes a musical 
tone signal receiving input terminal 52 coupled to the output terminal of 
keyer 19 (FIG. 1). The tone signal is coupled through analog switch 46 to 
one end of a resistor 54 and then through the second analog switch 48 to a 
second resistor 56. The other end of resistor 56 is connected to an 
amplifier 58 which drives an output terminal 60. A feedback capacitor 62 
is connected from the output terminal 60 to the junction of resistor 54 
and switch 48 and a capacitor 64 is connected in shunt with the input 
terminal of amplifier 58. 
Resistors 54 and 56 determine the charging rate of capacitors 62 and 64. 
When analog switches 46 and 48 are continuously conductive, the filter has 
one characteristic. When switches 46 and 48 are operated in response to 
various lower frequency control signals, however, the filter has various 
different characteristics, because of the change in the average current 
through resistors 54 and 56. In this manner, i.e. by controlling the 
characteristics of the filter according to a frequency encoded control 
signal which is dependent upon the octave or half-octave of the musical 
scale containing a selected note, the filter is made to track the tones 
being sounded by the instrument. 
According to the present invention, a pulse width adjusting device is 
interposed between programmable divider 42 and analog switches 46 and 48 
for the purpose of controlling the characteristics of filter 50 
independently of the tone signals being sounded. Thus, for example, in 
FIG. 2 a voltage-controlled monostable multivibrator 66 has an input 
connected for receiving the control signal developed on line 25 and an 
output connected to analog switches 46 and 48 by a line 68. A modulating 
signal is developed by a modulating signal generator 70 and coupled 
through an attenuator 72 to the control input of monostable multivibrator 
66. 
Monostable multivibrator 66, which may be a 555 type timing circuit 
available from a number of manufacturers, is triggered into its unstable 
state by the pulses comprising the control signal on line 25 for a time 
duration dependent upon the voltage developed at the output of attenuator 
72. Therefore, the width of the pulses comprising the control signal may 
be selectively varied by generating suitable modulating signals and 
attentuating them by a desired amount. 
In this regard, it will be appreciated that modulating signal generator 70 
may comprise either external generator 29 or, alternatively, envelope 
generators 20a or 20b. Thus, the modulating signal supplied to monostable 
multivibrator 66 may take any of a number of forms and may comprise, for 
example, a DC signal or an AC signal developed by external generator 29 
which may be manually operable by means of a suitable potentiometer or the 
like or which may comprise various other forms of signal generation 
apparatus for providing any desired modulating signal. The modulating 
signal may also be derived from envelope generator 20a or envelope 
generator 20b by operation of switch 30 whereby it would simulate the 
attack and decay characteristics of the respective generators. 
Operation of the circuit illustrated in FIG. 2 can be conveniently 
explained with reference to the waveforms shown in FIGS. 3 and 4. Waveform 
A of FIG. 3 represents a control signal developed on line 25 having a 
frequency f.sub.1. The frequency f.sub.1 directly associates the selected 
note with the octave or half-octave of the musical scale containing the 
note. Should a selected note be contained within another octave or 
half-octave, the frequency of waveform A would change accordingly. For 
example, waveform C represents a control signal on line 25 having a 
frequency 2f.sub.1 and would correspond to a selected note contained 
within a higher octave or half-octave of the musical scale. 
Now, as previously discussed, the frequency response of filter circuit 50 
is governed by the conduction times of switches 46 and 48. During a period 
T of the signal represented by waveform A, the conduction time of switches 
46 and 48 corresponds to the width d of pulse 74. Referring to FIG. 4, 
this would correspond to a filter frequency response or range having a 
cut-off frequency at A. The frequency range defined by point A may 
alternatively be referred to as the initial tracking interval of the 
filter. If a higher frequency note is selected a higher frequency control 
signal is developed on line 25, such as the signal having a frequency 
2f.sub.1 as represented by waveform C. Since the signal represented by 
waveform C causes switches 46 and 48 to conduct twice as long as the 
signal represented by waveform A, the frequency response or initial 
tracking interval of the filter is increased and characterized by a 
cut-off frequency at point C. In this manner, the frequency response of 
filter 50 continuously tracks the control signal developed on line 25 and 
thereby the frequency of the notes generated by priority note generator 
15. 
The presented invention allows for increased flexibility in the generation 
of musical sounds by enabling the tracking interval of filter 50 to be 
adjusted independently of the frequency of generated tone signals. For 
example, waveform B represents an output of monostable multivibrator 66 in 
response to the control signal represented by waveform A and a DC 
modulating signal supplied from attenuator 72. It will be observed that 
the pulse width of the signal represented by waveform B is substantially 
increased relative to the control signal's original pulse width resulting 
in longer conduction times of switches 46 and 48. Consequently, the 
initial tracking interval of the filter is increased from point A to point 
B. The increased bandwidth filter response will, of course, pass a larger 
number of harmonics of the tone signal creating a different musical effect 
than would be achieved solely in response to the control signal developed 
on line 25. The extent of the modulation of the initial tracking interval 
of the filter is directly related to the increased pulse width of the 
signal represented by waveform B. This pulse width may be increased by 
reducing the attenuation introduced by attenuator 72 or by increasing the 
DC signal developed by modulating signal generator 70. It will in addition 
be recognized that the pulse width characterizing waveform B can be made 
smaller than the pulse width characterizing waveform A by suitably setting 
generator 70 and attenuator 72, in which case the filter response 
bandwidth or tracking interval is decreased compared to that which would 
result in response to waveform A. Also, modulating signal generator 70 
could supply an alternating signal or any other wave shape to continuously 
cause the pulse width of the signal represented by waveform B to vary 
whereby the initial tracking interval defined by point A would similarly 
vary. 
Waveform D of FIG. 3 represents the output of monostable multivibrator 66 
in response to a control signal on line 25 corresponding to waveform C and 
a DC modulating signal identical to that used to generate the signal 
represented by waveform B. It will be noted that the conduction time of 
switches 46 and 48 in response to waveform D is twice that in response to 
waveform B. Therefore, the frequency range of filter 50 in response to the 
signal represented by waveform D has a cut-off frequency at point D which, 
in terms of frequency, represents an expanded tracking interval equal to C 
(corresponding to the initial tracking interval at frequency 2f.sub.1) 
multiplied by the ratio B/A. In other words, the expanded tracking 
interval is a linear function of the initial tracking interval for a 
constant modulating signal. 
In the foregoing embodiment, the invention was shown as controlling the 
characteristics of a low-pass filter. However, it will be appreciated that 
filters exhibiting other characteristics, e.g. band-pass and high-pass 
filters, may likewise be controlled. 
FIG. 5 illustrates another embodiment of the invention. In this embodiment, 
a bi-stable device such as a flip-flop 76 is connected between 
programmable divider 42 and switches 46 and 48, the set input of the 
flip-flop being supplied by line 25 and its Q output being coupled to 
switches 46 and 48. The clock pulses developed by clock 40 of priority 
note generator 15 are coupled by a line 78 to the clock input of a 
multistage binary counter 80 whose reset terminal is supplied with control 
signal pulses from line 25. The output of counter 80 supplies one input of 
a comparator circuit 82, a second input of comparator 82 being supplied 
with the output of a multiplier circuit 84. The output of comparator 82 
is, in turn, connected to the reset terminal of flip-flop 76. Finally, 
multiplier 84 is supplied with inputs representing a modulating signal 
code and a modulation depth code. The modulating signal code supplied to 
multiplier 84 corresponds to the output of modulating signal generator 70 
in FIG. 2 while the modulation depth code corresponds to the effect 
introduced by attenuator 72. The result of multiplying the two codes in 
multiplier 84 is the production of a code at the second input of 
comparator 82 representing a modulation function for modulating the 
tracking interval of filter 50. 
In operation, a desired modulation function, represented by a modulating 
signal code and a modulation depth code, is supplied to comparator 82 from 
multiplier 84. After being reset by a control signal pulse on line 25, 
counter 80 proceeds to count clock pulses supplied on line 78. The control 
signal pulse resetting counter 80 simultaneously sets flip-flop 76 so that 
the signal on line 68 supplying switch 46 and 48 is logically high. After 
some period of time, the count developed at the output of counter 80 
coincides with the modulation function code and this equality condition is 
detected by comparator 82 which resets flip-flop 76 causing the output on 
line 68 to go low. Thusly, a pulse is developed on line 68 in response to 
a control signal pulse on line 25 and having a duration dependent upon the 
modulating signal code and modulation depth code supplied to multiplier 
84. As in the case of FIG. 2, the width of the pulse developed on line 68 
determines the extent to which the initial tracking interval is extended. 
Moreover, by suitably developing the modulating signal code and modulation 
depth code various modulation effects such as previously discussed may be 
achieved. 
A method for developing either the modulating signal code or the modulation 
depth code is illustrated in FIG. 6. A clock 86, typically operated at a 
frequency significantly less than the frequency of the clock signal 
produced by clock 40, supplies a multistage counter 88 which in turn 
addresses a ROM 90. ROM 90 may be programmed to develop any desired 
sequence of codes at its output 92 for supplying multiplier 84 in response 
to addressing signals from counter 88. Alternatively, if a constant code, 
either a modulating signal code or a modulation depth code, is desired, a 
suitably preset register may be used to supply the code to multiplier 84. 
Yet further, a variable signal derived by operation of a potentiometer or 
the like may be employed to supply an analog-digital converter, which, in 
turn, would supply multiplier 84 in accordance with the manual or 
automatic adjustments of the potentiometer. 
While particular embodiments of the present invention have been shown and 
described, it will be obvious to those skilled in the art that various 
changes and modifications may be made without departing from the invention 
in its broader aspects. The aim of the appended claims, therefore, is to 
cover all such changes and modifications as fall within the spirit and 
scope of the invention.