US3260784A

US3260784A - Electronic musical instrument having one or more keyboards

Info

Publication number: US3260784A
Application number: US409331A
Authority: US
Inventors: Wolfgang J Wehrmann
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-11-07
Filing date: 1964-11-05
Publication date: 1966-07-12
Anticipated expiration: 1983-07-12
Also published as: AT240144B; DE1447437A1; DE1447437B2; DE1447437C3

Description

IN VEN TOR W. J- WEHRMANN Filed Nov. 5, 1964 if lf25\ 26 l rsf?, W 181 I( C. S. E

July 12, 1966 ELECTRONIC MUSICAL INSTRUMENT HAVING'ONE OR MORE KEYBOARDS S.F V.C.

United States Patent O 3,260,784 ELECTRONIC MUSICAL INSTRUMENT HAVING ONE OR MORE KEYBOARDS Wolfgang J. Wehrmann, 3 Jungherrnsteig, Vienna, Austria Filed Nov. 5, 1964, Ser. No. 409,331 Claims priority, application Austria, Nov. 7, 1963, A 8,893/63 11 Claims. (Cl. 84-1.01)

The present invention relates to an electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner that upon depression of a plurality of keys of one manual the lowest of the depressed keys de-fines one musical voice and the highest of the depressed keys defines the other musical voice.

A known method for producing simultaneously two tones on a keyboard-operated instrument is to use a serial mechanism of contacts so that only the lowest and the highest key of a plurality of depressed keys can work. In an instrument of this type for each voice a number of serial changeover contacts is needed equal to the number of keys in the keyboard. At an overall range of several octaves the contact banks become very bulky and considerable practical difficulties are encountered in carrying this method into practice.

According to another well known concept two serial oscillator networks are being used, parts of which are lbeing short-circuited by key contacts, one oscillator network for generating the lowest tone consisting of serially connected capacitors, the other oscillator network for generating the highest tone consisting of serially connected inductances or resistors. A serious disadvantage of this method is that large capacitors and eventually also large inductances have to be tuned.

An object of this invention is to achieve the effect of duophonic playing in a new and very profitable manner with respect to prior art, so that for both musical voices of one common manual only relatively low impedances of one kind, e.g., small inductances for both voices, have to be tuned.

Another object of the invention is to provide a tone generating circuit by which an exact musical transposition of all tones corresponding to the keys of one manual may be obtained by variation of only a few frequency determinant elements.

With these objects in view the invention provides an electronic musical instrument comprising in combination per playing manual at least three oscillators, two of said oscillators being 'tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low pass or band pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low pass or band pass filter.

3,260,784 Patented July 12, 1966 ICC The respective constant frequency may be the same for both voices, in this case there are only three oscillators for each keyboard needed, otherwise four. By deriving one constant frequency from the other by dividing, multiplying and mixing there are also only three oscillators needed but more modulators and low pass or band pass filters. The duophonic playing may be extended by -multiplying the circuits for duophonic playing, and all switchable frequency determinant elements are switched simultaneously by the keys of one keyboard. By extending the duophonic playing you can simulate an effect like on the organ by records.

Further objects, features and advantages of the invention will be more apparent fromthe following description taken in connection with the accompanying drawing, illustrating a circuit diagram of a preferred embodiment of the present invention in which transistors are being used as active elements for oscillators, modulators, or mixers, and amplifiers.

The drawing shows

oscillators

10, 11 and 12, modulators or

mixers

27 and 28 with respective low-

pass filters

29 and 30, and a swelling unit 43. Oscillator 12 generating aconstant frequency consists of the transistor 1, an oscillating circuit with the coil 2 and the capacitors 3, 4 and 5. The stability of working of the transistor is obtained, as usually, by an emitter resistance 6 and a base-voltagedivider consisting of the resistors 7 and 8. For A.C. the capacitor 9 between the base and the ground is la short circuit. The oscillating voltage is obtained at the emitter. The circuit diagrams for the

oscillators

10 and 11 for the variable frequencies are quite similar. The variable frequencies are switched by contacts actuated by the keys. Some of the contacts (13 up to 18) are depicted. Each key switches two contacts. ln the circuit diagram you can only see the contact pairs (13, 16), (14, 17) and (15, 18). The inductances of the oscillating circuits of the

oscillators

10 and 11 consist of

serial coils

19, 20, 21, 22 and 23, 24, 25, 26 which are connected by the associated contacts with the negative pole of the battery, if you press the respective keys. The circuit diagram shows the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillators are turned to their respective highest frequencies by the lowest and the highest depressed key. The frequencies of the

oscillators

10 and 11 or 11 and 12, respectively, are mixed in the

modulators

27 and 28, respectively, and at the output of the low-pass filters ,29 and 30, respectively, the differential frequency originates from the mixture product. The frequency of the oscillator 10 is always lower, the frequency of the Oscillator 11 always higher than the frequency of the oscillator 12, therefore the lowest differential frequency, which corresponds to the pressed keys, originates at the output of the low-pass filter 29 and the highest differential frequency, which corresponds to the pressed keys, originates at the output of the low-pass filter 30. The ,frequencies of the

oscillators

10, 11 and 12 may be varied by the capacitors 31, 32 and 5 in the same ratio, so that any transposition of the mixture frequencies can be obtained. The

inductances

20, 21, 22, 24, 25, 26 are so small, that they can be coils with ferromagnetic cores, therefore they are small in size, cheap and very easy and exactly tunable. The base voltage divider of the transistor of each tunable oscillator has an adjustable resistor by which frequency variation of said tunable oscillators due to kvariations of the supply voltage may be conformed to the frequency variation of the fixed oscillator due to said variation of the supply voltage, so that dependence of the differential mixing frequencies on the supply voltage is minimized. The

modulators

27 and 28 work quite analogously, therefore only modulator ifi 27 will be described. The modulator or mixer consists of two cascaded transistors, one transistor (35) with grounded emitter, the other one (36) with grounded collector, the collector of the transistor 3S being D.C.- coupled to the emitter of the transistor 36. The base of the transistor 35 is coupled with the fixed oscillator 12 over the relatively low resistor 3S, and the base of the transistor 36 is coupled with the tunable oscillator over the relatively high resistor 37, the ratio between said relatively high resistor 37 and -said relatively low resistor 38 being from five to twenty. The differential mixing frequency is picked up from the connecting point 39 of the collector of said first-named transistor and the emitter of said second-named transistor. The following low-pass filter 29 avoids unwished frequencies. The circuit diagram shows also an embodiment of the swelling unit. The oscillator ttl produces a constant frequency, which is coupled in the resonant circuit 41 over a resistor 47. The resonant circuit 41 has a .coil 46. The inductance of this coil can be varied by shifting its ferrite core. The devices 41 (resonant circuit), 42, (rectifying circuit) and 44 (gate circuit) can exist for each key and voice or for all keys common but for each voice separate. The ferrite core 46 can be coupled therefore with one or all keys of a keyboard and is shifted by pressing a key. By pressing a key the resonant circuit 41 gets in or out of tune to the frequency of the oscillator 40, so that the A.C. voltage amplitude at the gate circuit 42 is varied. In the same manner the D.C. voltage at the resistor 48 and the filter capacitor 4.9 changes. This D.C. voltage originates at the point 5t) of the gate circuit 44 and the potential of the point 50 becomes so negative, that the diode 5l becomes conductive. If the polarity of the diode 51 is reversed the D.C. voltage gradient at the point 5f! must change, too. The current of the differential frequency at the output of the low-pass filter 29 fiows into the base of the transistor 53, `can pass the conductive diode 51, the capacitor 52, and reaches the output 54 of the gate circuit 44. The device 43 for swelling has a gate circuit (44 or 45, respectively) for each voice, and the first voice, which comes from the low-pass filter 29 passes the gate 44, the second voice from the filter passes the gate 45.

The circuit diagram shows that each voice is audible or can be stored over an own sound and volume control (55, 57, 56, 58). It is also possible to use common sound and volume control for both voices.

What is claimed is:

1. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines on musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable Oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest l depressed key, and sound forming and producing means connected to the output of said low pass or band pass lter.

Z. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator :being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable o-scillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching serially connected inductances in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter.

3. An electronic musical instrument having one or `more keyboards which permits |a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the |lowest of the depressed keys defines ione musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at `least three oscillators, tvwo of said oscillators being tunable in steps by means of said keys, two modulators olr mixers each lfed by one tunable and one fixed `oscillator land followed by a low-pass or bland-pass filtrer to` obtiain the differential mixing frequencies, thevariable frequency of one tunable oscillator being lower thlan the constant frequency of the respective fixed oscillator land the variable frequency tof the other tunable oscillator being higher than the constant frequency off the respective fixed oscillator, the frequencies lof the tunable oscillators being vaxried by switching serially connected capactors in both oscillators, the switching `contacts of said keys being so connected i-n circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the -lowest depressed key, the tunable oscillator, the frequency of which is higher than the fnequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound foinming and producing means connected to the output of said low-pass yor band-pass filter.

4. An electronic musical instrument having one or more keyboards which permits ra duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the llowest of the depressed keys defines one musical voice and the -highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at lleast three oscillators, two of said oscillators being tuniable in `steps by means of said keys, two modulators or mixers each fed by one tunable 'and Ione fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being Ilower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher thlan the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression o'f a plurality of keys the tu-nable oscillator, the frequency Iof which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is 'higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter, the frequency determinant elements of the kind which are not switchable by the keys in the tunable oscillators may be varied in all oscillators by the saine factor for making any transposition of the differential mixing frequencies.

5. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines lone musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two o-f said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and lone fixed oscillator and followed by a low-pass or band-pass filter to Iobtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the oonstant frequency of the respective fixed oscillator and variable frequency of the Iother tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied `by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, thek tunable oscillator, the frequency of which is higher than the frequencyr of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, yand. sound forming and producing means connected to the output of said low-.pass yor bandspass filter, each modulator or mixer consisting of two cascaded transistors, one transistor with grounded emitter, the other yone with grounded collector, the collector of said first-named transistor bein-g D.C.coupled to the emitter of said secondnamed transistor, the blase of said first-named transistor being coupled with the fixed oscillator over a relatively low resistance, and the base of said second-named transistor being coupled with the tunable oscillator over a relatively high resistance, the ratio between said relativeL ly high resistance Iand said relatively low |iesistance being from live to twenty, said differential mixing frequency 4being picked up from the connecting point of the collector of said first-named transistor and the emitter of said second-named transistor.

6. An electronic musical instrument having one or more keyboards which permits .a duo-phonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual .at least three oscillators, two of said oscillators being tunable in steps by `means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed bya low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of whichis higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter, and a swelling unit including per each musical voice at least one gate and one constant frequency auxiliary oscillator, per manual at least one resonant circuit, which may be brought in or out of tune with respect to the frequency of the auxiliary oscillator by actuation of said keys, and at least one rectifying circuit for rectifying the voltage of said resonant circuit, the rectified voltage controlling and opening said 4gate gradually upon depression of oney of said keys.

7. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tun: able oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter, and a swelling unit including per each musical voice at least one gate and one constant frequency auxiliary oscillator, per each key one resonant circuit, which may be brought in or out of tune with respect to the frequency of the auxiliary oscillator by actuation of said key, and at least one rectifying circuit per each voice for rectifying the voltage of said resonant circuit, the rectified voltage controlling and opening said gate gradually upon depression of said key.

8. An electronic musical instrument having one or more keyboards which permits aduophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical `voice and the highest of the depressed keys defines the other musical Voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps lby means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching simi-lar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respect-ive highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter, and a swelling unit including per each musical voice at least one gate and one constant frequency auxiliary oscillator, and one or more resonant circuits consisting of a fixed capacitor and a coil with a ferromagnetic core slidable therein and mechanically connected with said key or said keys, respectively, and `at least one rectifying circuit for rectifying the voltage of said resonant circuit, the rectified voltage controlling and opening said gate gradually upon depression of one of said keys.

9. An electronic musical instrument comprising in combination per playing manual at least three transistorized oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscilator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and sound forming and producing means connected to the output of said low-pass or band-pass filter, the base voltage divider of the transistor of each tunable oscillator having an adjustable resistor by which frequency variation of said tunable oscillators due to the variation of the supply voltage may be conformed to the frequency variation of the fixed oscillator due to said variation of the supply voltage, so that dependence of the differential mixing frequencies on the supply voltage is minimized.

10. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and individual sound forming and producing means connected to the output of said low-pass or band-pass filter for each voice.

11. An electronic musical instrument having one or more keyboards which permits a duophonic playing on each playing manual in such a manner, that upon depression of a plurality of keys of one manual the lowest of the depressed keys defines one musical voice and the highest of the depressed keys defines the other musical voice, comprising in combination per playing manual at least three oscillators, two of said oscillators being tunable in steps by means of said keys, two modulators or mixers each fed by one tunable and one fixed oscillator and followed by a low-pass or band-pass filter to obtain the differential mixing frequencies, the variable frequency of one tunable oscillator being lower than the constant frequency of the respective fixed oscillator 4and the variable frequency of the other tunable oscillator being higher than the constant frequency of the respective fixed oscillator, the frequencies of the tunable oscillators being varied by switching similar serially connected frequency determinant elements in both oscillators, the switching contacts of said keys being so connected in circuit that upon depression of a plurality of keys the tunable oscillator, the frequency of which is lower than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the lowest depressed key, the tunable oscillator, the frequency of which is higher than the frequency of the fixed oscillator, is being tuned to its respective highest frequency by the highest depressed key, and common sound forming and producing means connected to the output of said low-pass or band-pass filter for all voices of a manual.

No references cited.

ARTHUR GAUSS, Primary Examiner,

I. C. EDELL, Assistant Examiner.

Claims

1. AN ELECTRONIC MUSICAL INSTRUMENT HAVING ONE OR MORE KEYBOARDS WHICH PERMITS A DUOPHONIC PLAYING ON EACH PLAYING MANUAL IN SUCH A MANNER, THAT UPON DEPRESSION OF A PLURALITY OF KEYS OF ONE MANUAL THE LOWEST OF THE DEPRESSED KEYS DEFINES ON MUSICAL VOICE AND THE HIGHEST OF THE DEPRESSED KEYS DEFINES THE OTHER MUSICAL VOICE, COMPRISING IN COMBINATION PER PLAYING MANUAL AT LEAST THREE OSCLLATORS, TWO OF SAID OSCILLATORS BEING TURNABLE IN STEPS BY MEANS OF SAID KEYS, TWO MODULATORS OR MIXERS EACH FED BY ONE TURNABLE AND ONE FIXED OSCILLATOR AND FOLLOWED BY A LOW-PASS OR BAND-PASS FILTER TO OBTAIN THE DIFFERENTIAL MIXING FREQUENCIES, THE VARIABLE FREQUENCY OF ONE TUNABLE OSCILLATOR BEING LOWER THAN THE CONSTANT FREQUENCY OF THE RESPECTIVE FIXED OSCILLATOR AND THE VARIABLE FREQUENCY OF THE OTHER TUNABLE OSCILLATOR BEING HIGHER THAN THE CONSTANT FREQUENCY OF THE RESPECTIVE FIXED OSCILLATOR, THE FREQUENCIES OF THE TUNABLE OSCILLATORS BEING VARIED BY SWITCHING SIMILAR SERIALLY CONNECTED FREQUENCY DETERMINANT ELEMENTS IN BOTH OSCILLATORS, THE SWITCHING CONTACTS OF SAID KEYS BEING SO CONNECTED IN CIRCUIT THAT UPON DEPRESSION OF A PLURALITY OF KEYS THE TUNABLE OSCILLATOR, THE FREQUENCY OF WHICH IS LOWER THAN THE FREQUENCY OF THE FIXED OSCILLATOR, IS BEING TUNED TO