US2697959A

US2697959A - Apparatus for producing complex waves at a desired frequency

Info

Publication number: US2697959A
Application number: US257820A
Authority: US
Inventors: Earle L Kent
Original assignee: Conn Ltd C G
Current assignee: CG Conn Ltd
Priority date: 1951-11-23
Filing date: 1951-11-23
Publication date: 1954-12-28
Anticipated expiration: 1971-12-28

Description

E. L. KENT APPARATUS FOR PRODUCING COMPLEX Dec. 28, 1954 2,697,959

WAVES AT A DESIRED FREQUENCY 2 Sheets-Sheet 1 Filed Nov. 23, 1951 R e m m y 3 R 2 mm M 2 D 2 R w A m D D J C M r MI- 0 Mm m mm M Fe 6 g 9 mm fi 7mm Mm WC 5 R m 3 m mm m R0 0 m mw n 4 w a 4/ f m v MI. r UT Hm 5 R C F Dec, 28, 1954 E. L. KENT 2,697,959

APPARATUS FOR PRODUCING COMPLEX WAVES AT A DESIRED FREQUENCY Filed Nov. 25, 1951 2 Sheets-Sheet 2 a; 29 L r 7 S S a q::

POWER 5 J 3 :gsumy IN V EN TOR.

AT T ORNFY w- I I i uflm I I MI I4 I ma M //zz United States Patent APPARATUS FOR PRODUCING COMPLEX WAVES AT A DESIRED FREQUENCY Earle L. Kent, Elkhart, ,lndt, assignor to gC G. Conn-Ltd Elkhart, Ind., acorporation of Indiana Application November 23, 1951, Serial No. 257,820 15 Claims. :(Cl. 841.22)

This invention relates to a method of and apparatus for producing complex waves :at a desired frequency and more specifically to the production of musical tones.

The generation of complex waves at different frequencies for musical or other purposes has heretofore generally been accomplished by synthesis; that is, by mixing in the desired relative amplitudes a plurality of sine waves of different frequencies or by using a different complex wave generator for each different frequency desired. Both types of generation are :used in electronicorgans as now .being manufactured.- far simpler and in many ways a more versatile and nexiule method of generating complex waves would be to generate a single complex wave of the desired wave form at a fixed frequency and to change its frequency as desired. One difficulty with this type of operation has been that no satisfactory frequency changer has been available which will change the fundamental frequency of the wave without a-l'ter-ingits waveform.

Mechanical frequency changers have been proposed andcan accomplish a change in frequency while maintaining wave form. However due to mechanical inertial effects such devices are not satisfactory since they can not effect sufficiently rapid changes in frequency for musical purposes. 7 v I Frequency changers as generally used in vacuum tube circuits are of two classes. One class includes multipliers and dividers which multiply or divide the fundamental frequency into integral multiples or divisors of the original wave frequency. The other class includes changers which add a single number to each frequency in the original wave. Neither of these classes of frequency changer is capable of maintaining "the original wave form and they are therefore not satisfactory for musical work.

in this connection it is to be noted that two .Waves may have similar harmonic contact from the standpoint of amplitude of the harmonics or partials "but may still be of dissimilar form because of different phase relationships between the harmonics. Even though the car can not detect phase differences the peak value :of a tone signal is controlled by the phase angle between the harmonics and in many cases unless the phase is properly controlled excessive peak levels might result with consequent overloading of the circuit or of the ear *of a listener. it is therefore desirable to maintain the signal peak values at a minimum by maintaining the optimum phase relationship of the waves making up the signal.

It is one of the objectives of the present invention to provide a frequency changing method and apparatus in which the frequency of a complex wave is changed without changing the wave form.

Another object is to provide a frequency changing method and apparatus in which a complex wave at -'a frequency higher than that desired is electrically scanned or sampled to produce a series of pulses whose amplitudes are proportional to the instantaneous amplitudes of the original wave lat progressively different phase positions. The envelope of such -.a series of pulsesis .a wave of the same :form as-the original wave .and at a frequency equal to the difference between the .frequency of the original wave and the effective scanning or sampling frequency.

It is to be observed that pulses spaced .to sample or scan the original wave may be produced by generating the pulses in the same frequency as the original wave and progressively shifting theirphase position or by gencrating the pulses at a different frequency. The effective or apparent frequency of the pulses in either case "will be different than the frequency of the original wave. in both cases the etfective pulse frequency is preferably 2,697,959 Patented Dec. 28,, 1954 ice 2 controllable to change the frequency or pitch of the output Wave.

Still another object of the invention is to provide a method of and apparatus for producing musical tones in whicha complex wave of the desired form is generated at a fixed frequency and has its frequency changed .to the desired pitch value without changing its form and is then utilized to operate a transducer such as a loud speaker.

A still further object is to provide a method .of and apparatus for producing a musical tone in which the scanning or frequency changing circuit is connected :to the transducer through keying tubes controlled by key switches.

According to one feature of the invention the frequency of the pulse generator is controlled by switches operated in sequences with the keying switches so that a wave of the desired pitch is produced and is then keyed to the transducer.

A further object is to provide a method of and appleratus for producing musical tones in which a low frequency tremolo generator is connected to the pulse generator to vary its frequency in a relatively slow cycle thereby to produce a tremolo effect.

A still further object is to provide a method of and apparatus for producing musical tones in which slurring or g'lissando effects are produced by controlling the rate of change of the frequency of the pulse generator. In a preferred construction the frequency of the pulse generator is controlled by a variable reactance device which in turn is controlled through a variable controlled voltage circuit including adjustable time delay elements to control. the rate of change of the reactance.

A still further object is to provide a method of and apparatus for producing musical tones in which the attack vand decay characteristics of the tones are .controlled in the keying circuit through parallel controlled circuits each including a variable impedance and a rectifier "with the rectifiers in the difierelnt .circu'its arranged to pass current in opposite directions.

The above and other objects and features .of the invention will be more readily apparent from the ,following description when read inconnection with accompanying drawings in which Figure 1 is a block diagram of .an apparatus for producing musical tones according to the invention;

Figures 2 and taken together, are a wiring diagram of the apparatus; 7

Figure '3 is a partial diagram showing a key circuit for multiple channel operation;

Figure 4 is a graph showing atypical complex wave and a pulse wave for scanningor sampling the same; and

Figure 5 is a graph showing the resulting wave.

As illustrated "in the block diagram Figure .1, the 8P- paratus comprising a fixed oscillator '10 which may be any desired type of generator of electrical waves and which will preferably generate complex waves at 'a fixed frequency. The frequency of such waves is higher than the frequency .of the highest desired musical tone and for musical work is preferably on the order of ;3,0 "kilocycles. The generated waves are passed through aharmonic control device ll'which may control the harmonic values in "the wave to produce a wave corresponding in shape to the desired musical tone. The output or the harmonic control device is an electric Wave at the generated frequency and Whose wave (form corresponds to the desired wave shape. This wave is supplied to a frequency changer Orscanning .device 12.

At the same time a variable frequency pulse wave is generated bya variable oscillator 13. Thefrequencyof theoscillator is controlledby a frequency control device 14 connected "to the oscillator and which is in turn controlled by the operator as desired. As shown in the-diagram the *frequency control device 14 is connected to a k'eyboard "15 through which it is controlled in turn to control the frequency of the variable oscillator 13.

The output of the variable oscillator may be used "as generated if it is -a sharp *pulse but is preferably passed through a pulse s'haper 16 wherein the pulses 'are sharpened to form a series of spaced very {short ;'sharp pulses which are supplied to the frequency changer,

The generated electrical wave as supplied to the frequency changer by the harmonic control device may take a form such as that shown at 17 in Figure 4. The pulses supplied to the frequency changer by the pulse shaper 16 are very short sharp pulses such as indicated at 18 in Figure 4. The pulses are at a different frequency than the wave and function to sample or scan the wave at progressively different phase positions so that the output of the frequency changer will comprise a series of short sharp pulses as indicated at 19 in Figure 5. The amplitude of the pulses 19 will be proportional to the instantaneous amplitude of the wave 17 at phase positions therein corresponding to the phase positions of the pulses 18. The frequency of the envelope of the pulses 19 will be equal to the difference in frequencies of the waves 17 and the pulses 18. Thus if the wave 17 has a frequency of 30 kilocycles and the pulses 18 have a frequency of 29,560 cycles the envelope of the pulses 19 will have a fundamental frequency of 440 cycles corresponding to standard A.

The pulses 19 are supplied to a formant control 21 which may be one or more resonant circuits to shape the pulse wave to correspond to desired musical sounds. The wave passed by the formant control is supplied through a keyer 22 and a power amplifier 23 to a loud speaker 24. The tone produced by the loud speaker will therefore correspond to the wave shape of the original wave 17 as modified by the formant control when it is used and will have a fundamental frequency in the example stated of 440 cycles. The block diagram of Figure 1 illustrates one complete channel but it will be understood that a complete instrument might include several such channels each producing a different desired type of tone.

In the circuit illustrated in Figure 2 the fixed oscillator is shown as a Hartley type oscillator arranged in pushpull. The oscillator includes a double tube 25 with the anodes of both sections connected through a tank circuit including an inductance 26 and a condenser 27. The opposite ends of the tank circuit are cross connected to the grids of the control sections and the cathodes are interconnected through resistors 28 the mid point of which is connected to a ground line 29. In this circuit sine waves are derived from the tank circuit through a connection 31 and pulse waves may be derived from the cathode sections. When a single cathode section is used a pulse wave similar in form to a standard string tone is produced while when waves are derived from both cathode sections in push pull the resulting wave is similar in form to a wood wind tone.

The outputs from the fixed oscillator section are supplied to the harmonic control section which, as shown, includes three

mixer tubes

32, 33 and 34. The tube 32 has its control grid connected through a blocking condenser 35 to the output lead 31 to receive the sine wave oscillations. The tube 33 has its control grid connected through a blocking condenser 36 to the cathode of the upper oscillator tube section to receive oscillations therefrom and the control grid of the tube 34 is similarly connected through a blocking condenser 37 to the cathode of the lower oscillator tube section. The anodes of the tubes 32 and 34 and the cathode of the tube 33 are connected to a common output line 38 which goes to the frequency changer or scanning section.

To control the strengths of the different signals from the mixer tubes the grids thereof are connected through

adjustable resistors

39, 41 and 42 to the ground line 29 and have condensers 43 in parallel with the resistor sections. By adjusting the resistors the amplitude of the signals passed by the several tubes can be controlled so that the difierent types of signals are supplied to the output line 38 in the desired relative strengths to produce the desired wave form.

The variable oscillator as shown at 13 in the block diagram is illustrated as comprising a vacuum tube 44 whose anode and grid are interconnected through a coil 45 and a condenser 46. The cathode of the tube is connected to ground through a resistor 47 and the grid is connected to ground through an adjustable condenser circuit 48. Pulse waves are derived from the cathode of the oscillator circuit through a line 49.

To vary the frequency of the oscillator the grid there of is connected through a variable reactance circuit to ground. This circuit as shown comprises a vacuum tube 51 whose anode is connected to the grid of the oscillator tube 44 and whose control grid is connected through a fixed resistor 52 and a variable resistor 53 to a control circuit 54. A condenser 55 is connected with the resistor 53 to form therewith a variable time delay circuit which can be adjusted to vary the rate of change of voltage on the control grid. By adjusting the resistor 53 the desired slurring or glissando effects can be produced. At the same time the voltage supplied to the control grid of the tube 51 will control its effective reactance and will thereby control the frequency of oscillation of the oscillator tube 44. Power is supplied to the oscillator 44 as well as to the oscillator 25 through a line 55 leading from a suitable power source as described more fully hereinafter. Screen grid voltage for the variable reactor tube 51 and for the

mixer tubes

32, 33 and 34 is supplied by a line 56 from a suitable power source.

The voltage pulses supplied by the oscillator 44 through the line 49 are supplied to the cathode of a blocking oscillator 57. This oscillator has its grid plate circuit supplied with voltage from a transformer 58 which is I connected to a line 59 leading to a suitable power source.

The grid of the oscillator is supplied with negative bias from a line 61 connected to the power source so that it can oscillate only when it receives a pulse from the line 49. After producing one cycle the oscillator immediately blocks itself by driving its grid more negative so that a very short pulse is produced once during each cycle of oscillating. These pulses are supplied to the control grid of a clipper section 62 which is biased in such a manner that only the peak values of the pulses will cause it to pass current. The clipped pulses are very short sharp pulses as indicated at 18 in Figure 4 and are supplied through a line 63 to the frequency changer or scanning unit.

To produce a tremolo effect a low frequency tremolo generator is provided connected to the variance reactor tube 51. This generator as shown comprises a double ended vacuum tube 64 with its sections connected in push pull and having an adjustable resistor 65 in its cathode circuit by which its frequency can be adjusted. Power is supplied to the tremolo oscillator through a line 66 and a transformer 67 through which the oscillator can be keyed to operate only when desired. Oscillations produced by the tremolo oscillator are supplied through a line 68 to the control grid of the variable reactance tube 51 and will cause the reactance thereof to vary in a rela tively slow cycle to produce a slow cyclic frequency change. This type of operation produces a very satisfactory tremolo or vibrato effect. The wave supplied by the harmonic control section to the line 38 and the pulse supplied from the clipper section 62 to the line 63 are fed to two separate control grids of a frequency changer or scanning tube 71. The tube 71 is biased normally to cut off by connection of the control grid to which the line 63 is connected to the negative bias line 61 so that the tube is normally nonconducting except when a pulse is supplied through the line 63. When a complex wave is present and pulses are being supplied from the clipper section the tube 71 will conduct each time a pulse is present and its output will be a short pulse whose amplitude is equal to the product of the amplitudes of the supplied pulse and the complex wave. Thus the tube 71 will have as its output pulses such as shown at 19 in Figure 5 whose frequency is equal to the frequency of the supplied pulses and whose amplitudes are proportional to the instantaneous amplitudes of the complex wave. The tube 71 therefore samples or scans the complex wave at the frequency of the supplied pulses and the envelope of the output pulses 19 will have the same shape as the complex wave and a frequency equal to the difference between the complex wave frequency and the pulse frequency. Since the pulse frequency can easily be changed the output of the frequency changer or scanning section can change very rapidly without interfering in any way with the original wave form.

The pulses from the tube 71' are supplied to the primary winding of a transformer 72 whose secondary winding forms the input section of a pair of keying

tubes

73 and 74. The secondary of the transformer 72 is center tapped and connected through a condenser 75 to ground and to a key circuit 76. The ends of the secondary are connected to the control grids of the

tubes

73 and 74 and ts teeh se at es tubes a e ss sestett t reats fissi ya ble si te 7? tannin evsrall returns e tilte i' ey e ti es'f 'e s'f st a secretl ae esas et ss it e 22 ssse f the S t 7 .1 th nega ive bia 1 s 1 n n or tp control attack and decay el taeter s s htse fei tt th a ist i s' 1479 Whi a e t d parallel th rct'ifiersS l and $2 toforni'i'n effect parallel circuit w ich wi pass currents in epp siteuire ttens To th t; "the tll bFSI IQ and?! conducting, 'thebia's'is sh ntled thrpugh a key switcli' 83' aridfa. fes tor 84f re'sistcr is r -ti ed a at t sti t are. ha hi h iaise el r' 9i1 Q i t e e r la i e t9 e enter its n series in the bias co'nneiitiori. l o'rie'exarnpl'e the resistor 14 y f 1. .0 thi e tesis e i n' y 2&9!) ohms'andthfe resistor 36 HiayheII hOOQohniS. When h ewiteh is P fnf e ii e 'iewil u l 12' the i tl e't 95 W 74 melts e senescence.- When the switch 83 is closed, ground potential will build p n h id hr ush use. l" a tee et in th ub s e tie e t dustms; The e t eh nse of e i ca b adju ed h eaten the r sis or t very k h r ti ue of t te et W en the switch s re as w l b i u o the tube.

a Pai at variable en al the r i e 73 an reetifi t w i h .dete e th a et silti u at bia and ther fs e t decay the'tett' eh eueh t ese eiret ts ed at ack and ay at he et arete y eeett lle wi h t e u of s n e g t filYi h at the dev ce lend its lf te et? sim e e ng u h as mi t be pe formed by a P yi device ike a tandard p ay r p ano. IO

T e 9MP. t" he e i ub s i s ied to a powe mpli r s own as c mp i n a Pa r of ube 7 and .8 connected in push pull. The output of the power ampli: st semen su l t e r a o a r s o m r 89. o Whisk n aker 2 ma e conn d. As'sh w h e'eeen et er e t an m is erm wi h r al ts 9t" ten to Wh h t e l d sneak a be e ect vely -t sted t a ust he use o d iv ng oltag is this' circuit oscillations at a high frequency and of the'desired Wave form are produced in the main oscillater 2 an a e siren the Wa f rm r po ing o the desired musical tone in the harmonic control section. These'w'aves at a high frequency are then scanned or sampled in the frequency changer section whose output s aw r O h d sire W ve form and the desir con.- led equen y Thi w re is k y d to. h o p m: plifier and. the l ud p aker thr ugh key ng s ct n in hieh attack n rel se ar easily c nt ll d- By mple adjustments of the several adjustable elements including resistors 3 (t1 and 4 the wave form can easily be controlled. Adjustment of the resistor 53, controls the slnrring and glissando effects and adjustment of the re sis'tor (55 controls the frequency of the. tremolo, which is supplied. Numerous musical effects simulating known effects can therefore easily. beproduced and by adjusting the yarious elements of the circuit novel musical effects can be obtained. It. will be noted in this connection that the frequency of the pulse generator can be controlled either continuously or through step by step keying as desired.

As illustrated the pulse generator is adapted to be keyed for which purpose the line 54 is connected to ground through any one of a series of key switches 92 which in-, troduce yarying amounts of resistance 93. into the. circuit. The switches 92. may be Operated through a keyboard such as shown at 15 in Figure 1 and arepreferably arranged to be operated in sequence with the. keying. switch 83. or with a plurality of such switches connected in parallel. For example, the keys may be arranged so that when they are depressed theyWil-l first close one of the switches 92 to select the desired frequency and will thereafter close the keying switch 83 to transmit the selected frequency to the loud spealger. If desired, the keying switches may be arranged for mechanical operation leaving the performer free to adjust the various circuit controls to produce varying musical. effects.

For multiple channel operation through a single keyboard an arrangement of switches such shownfin; Figure 3. may be employed. This figure illustrates a resisto t 93' correspcnding to the resistors 98; as in 7 43 haying a series of pointsl'the tapped. Each V is e anee t ts one s in etade hl ttuaw suiteh 9s t mite one of e e. s at bu sans .51 6 a d v esr s e dias t as k 'With this construction-syn s any one of the keys is iepr s' ed' e point o he esi ter ee r sneading herete will be semest r? t t e lewer nes of the tour output channels 93, 9 9, 1,09 and 1.03., which correspond to the line 5.4 of four different tone generating units. If more than one key is depressed simultaufioufiy, the left hand y w be eemteete o he lowe cha n :98 and. he keys progressively to the right therefrom will he con-v nected to the upper channels in PIQgressive order, In this y a p u t a d e e t eha nel can b conr ed through a n l k y e d in a simple ma ne While one embodiment of the invention has been shown and described in detail it will be understood that this is illustrative only and is not to be taken as a definition of the scope of the invention reference being had for this purpose to the appended claims,

What is claimed:

1. Apparatus for producing musical tones compnising a generator of electrical Waves of the same wave form as the desired musical tones and of a frequency higher than the frequency of the highest desired musical tone, an electrical scanning circuit connected to the generator, a pulse generator producing a series of short electrical pulses which are progressively spaced to Q01". respond to different phase positions relative to the elec: trical wave, a connection from the pulse generator to the scanning circuit to energize the scanning circuit each time a pulse is supplied thereto so that the scanning circuit Will produce a series of pulses corresponding to the instantaneous amplitudes of the wave at spaced phase positions, and a transducer connected to the scanning circuit to convert the last named series of pulses into musical tones.

2. Apparatus for producing musical tones comprising a generator of electrical Waves of the same wave form as the desired musical tones and of a frequency higher than the frequency of the highest desired musical; tong, P e generator P u a rie of short electri l. pulses at a difierent frequency than the electricalwave, a frequency changer circuit connected to hoth of the generators to be energized by said pulses thereby to. pro.- duce a series of electrical pulses corresponding fre quency to the generated pulses and in amplitude to the; instantaneous amplitude of the electrical wave, and: transducer connected to the frequency changer circuitto convert the last named series of pulses. into musical tones.

App r p u n musica o comprisin a generator of electrical waves of the. samewave form as the desired musical tones and of a; frequency higher. than the frequency of the highest desired musical; tone a pulse generator producing a series of short electrical pulses at a different frequency than the electrical. waye, a frequency changer circuit connected; to; both of the generators-to be energized by said. pulses thereby to produce a series of electrical pulses cor-responding in frequency to the generated pulses and: in amplitude to the instantaneous amplitude of the electrical; wave, a; transducer, and a keying circuit connecting the frequency changer circuit to the transducer.

4. Apparatus for producing musical tones' comprising a generator of electrical waves of the same Wave; form as the desired musical tones and of a frequency higher than the frequency of' the highest desired musical! tone a pulse generator producinga series of: shortelectrioal; pulses at a different frequency than the electr cal Wave,

tapped point a frequency changer circuit connectedto both of the;

generators to be energized by said pulses thereby totpro duce a. series; of. electrical pulses. corresponding: in fre,-. quency to the generated pulses and in amplitude to; theinstantaneous. amplitude of the electrical wave; a, circuit; n lud a r wi c onnec ed-to; thekp lse generator;

to ene iz it, a s ueen k ying i cuit; includingat to the p lseseuera e fi r qu ncy, of h gu ses. eutlea si encer i eu eo neet i en het he se lentrieai erators to produce a series of electrical pulses of the same frequency of the generated pulses and of amplitudes corresponding to the instantaneous amplitudes of the wave at different phase positions, a transducer connected to the frequency changer circuit, and control means to control the reactance of said variable reactance device.

6. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones and of a frequency higher than the frequency of the highest desired musical tone, a pulse generator producing a series of short electrical pulses, a variable reactance device connected to the pulse generator to vary the frequency of the pulses, a frequency changer circuit connected to both of the generators to produce a series of electrical pulses of the same frequency of the generated pulses and of amplitudes correspending to the instantaneous amplitudes of the wave at different phase positions, a transducer, a keying circuit including a controlling switch to connect the transducer to the frequency changer circuit, a circuit including a controlling switch to control the reactance of said variable reactance device, and key means to operate said switches in predetermined sequence.

7. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones and of a frequency higher than the frequency of the highest desired musical tone, a pulse generator producing a series of short electrical pulses, a variable reactance device connected to the pulse generator to vary the frequency of the pulses, a frequency changer circuit connected to both of the generators to produce a series of electrical pulses of the same frequency of the generated pulses and of amplitudes corresponding to the instantaneous amplitudes of the wave at different phase positions, a transducer, a keying circuit including a controlling switch to connect the transducer to the frequency changer circuit, a control circuit for the variable reactance device including a plurality of selectively operable switches to produce dilferent reactance, and keying means selectively to close the last named switches and then to close the controlling switch.

8. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones and of a frequency higher than the frequency of the highest desired musical tone, a frequency changer circuit connected to the generator to reduce the frequency of said wave without altering its wave form, a transducer, a circuit including an electron discharge tube connecting the frequency changer circuit to the transducer, a bias source connected to the tube normally biasing it to cut off, a control switch for the tube to remove the bias therefrom so that the tube will conduct, a pair of parallel circuits in the connection from the tube to the bias source, and a variable imped ance and a rectifier in each of said circuits with the rectifier set to pass current in opposite directions.

9. Apparatus for producing musical tones comprising a tone generating means to producing an electric wave corresponding to the desired musical tone, a transducer, a circuit including an electron discharge tube connecting the generating means to the transducer, a bias source, a pair of parallel circuits connecting the bias source to the tube normally to bias the tube to cutoff, each of said circuits including a variable impedance and a rectifier and the rectifiers set to pass current in different directions inv the different circuits, and a control switch to shunt the bias source to make the tube conduct.

10. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones but of a frequency higher than that of the highest desired tone, a frequency changer circuit connected to the generator, a pulse generator producing a series of short electrical pulses of a different frequency than the electrical wave, a connection from the pulse generator to the frequency changer circuit to supply pulses thereto so that the frequency changer circuit will produce a series of pulses whose amplitudes correspond to the instantaneous amplitudes of the wave at different phase positions, a control element for the pulse generator to control the pulse frequency, and a low frequency tremolo generator connected to the control element to produce a low frequency variation in the pulse frequency.

11. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones but of a frequency higher than that of the highest desired tone, a frequency changer circuit connected to the generator, a pulse generator producing a series of short electrical pulses of a d1iferent frequency than the electrical wave, a connection from the pulse generator to the frequency changer circuit to supply pulses thereto so that the frequency changer circuit will produce a series of pulses whose amplitudes correspond to the instantaneous amplitudes of the wave at different phase positions, a variable reactance device connected to the pulse generator to vary its frequency, and a low frequency tremolo generator connected to the variable reactance device to control it.

12. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones but of a frequency higher than that of the highest desired tone, a frequency changer circuit connected to the generator, a pulse generator producing a series of short electrical pulses of a different frequency than the electrical wave, a connection from the pulse generator to the frequency changer circuit to supply pulses thereto so that the frequency changer circuit will produce a series of pulses whose amplitudes correspond to the instantaneous amplitudes of the wave at different phase positions, a control element for the pulse generator to control the pulse frequency, a control circuit connected to the control element to change the effect thereof, and adjustable time delay elements in the control circuit to control the rate of change of the pulse frequency.

13. Apparatus for producing musical tones comprising a generator of electrical waves of the same wave form as the desired musical tones but of a frequency higher than that of the highest desired tone, a frequency changer circuit connected to the generator, a pulse generator producing a series of short electrical pulses of a different frequency than the electrical wave, a connection from the pulse generator to the frequency changer circuit to supply pulses thereto so that the frequency changer circuit will produce a series of pulses whose amplitudes correspond to the instantaneous amplitudes of the wave at different phase positions, a variable reactance device connected to the pulse generator to vary the pulse frequency and whose reactance varies in response to a control voltage, a control circuit connected to the variable reactance device to supply a variable control voltage thereto, and

adjustable impedance elements in the control circuit to control the rate of change of control voltage.

14. Apparatus for producing complex waves at a desired frequency comprising a generator of electrical waves of the desired complex form and of a frequency higher than the desired frequency, a frequency changer circuit connected to the generator, a pulse generator producing a series of short electrical pulses at a frequency different than the frequency of the electrical wave, and a connection from the pulse generator to the frequency changer circuit whereby the pulse generator will cause the frequency changer circuit to produce a series of pulses at the same frequency as the generated pulses and whose amplitudes correspond to the instantaneous amplitudes of the electrical wave at different phase positions.

15. Apparatus for producing musical tones including in combination, means for providing tone currents, a transducer, a circuit connecting said means to said transducer including an electron discharge valve having a control element, said circuit supplying tone currents to said transducer in accordance with the voltage applied to said control element thereof, means for providing a control voltage, and a control circuit including a keying switch selectively connecting said control voltage to said control element, said control circuit including a pair of oppositely poled rectifiers connected in series and a pair of time delay impedances in shunt with said rectifiers to control the attack and decay characteristics of the tone currents applied to said transducer.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,274,370 Kent Feb. 24, 1942 2,403,090 Larsen July 2, 1946 2,436,890 Higinbotham Mar. 2, 1948 2,464,708 Moseley Mar. 15, 1949 2,542,065 Van Wye Feb. 20, 1951 2,551,619 McWhirter et al. May 8, 1951 2,639,639 Schmidt May 26, 1953