US3260785A

US3260785A - Vibrato circuit

Info

Publication number: US3260785A
Application number: US316886A
Authority: US
Inventors: William H Krug
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-10-17
Filing date: 1963-10-17
Publication date: 1966-07-12
Anticipated expiration: 1983-07-12

Description

- July 12, 1966 w. H. KRUG 3,260,785

VIBRA'I'O CIRCUIT Filed Oct. 17, 1963 2 Sheets-Sheet 2 J5 J0 y 1 United States Patent 3,260,785 VIBRATO CIRCUIT William H. Ifiug, 1221 Jerome Ave., Janesville, Wis. Filed Oct. 17, 1963, Ser. No. 316,886 9 Claims. (Cl. 84-1.25)

This invention is concerned with a circuit for providing a vibrato effect in a musical instrument and more particularly with an electronic circuit for establishing a vibrato effect without modifying the action of the tone generator.

In a musical instrument, and particularly in an organ, it is desirable to provide a vibrato effect in which the tones are frequency modulated above and below the nominal frequency, at a sub-audio rate. In an electronic musical instrument in which organ-representing tones are synthesized, it is often difiicult to provide a vibrato effect by varying the frequency of the tone generator. For example, where the tones are derived from a plurality of oscil lators, it is difficult to vary the frequency of each to the same relative degree. Similarly, in an instrument which utilizes a mechanical tone generator, as rotating tone wheels, there are physical problems in varying the generator speed.

It has been proposed to provide a vibrato effect by causing a phase shift of the composite audio signal at a sub-audio or low audio rate. In the Hammond model M-l organ, the signal in which a vibrato effect is to be caused is connected with a multiple section electrical delay line having outputs at several points along the line. A mechanical switch selects different outputs sequentially along the line. The phase shift in the output signal provides a vibrato effect. The mechanical switch, which in practice is a capacitor having a plurality of stator plates with a rotor which scans them sampling the variously delayed signals derived from the delay line, is expensive and subject to mechanical problems.

One feature of the invention is the provision of a vibrato circuitjutilizing a phase shift shift network having a plurality of outputs, with an electronic switch for deriving an output signal comprising sequential signal portions from each of the outputs of the phase shift network. Another feature of the invention is that the switching is effected at a rate above the audible range and the relative period of operation of the switch is varied.

More particularly, the electronic switch comprises a free-running multivibrator having two sections, each with an input connected with an output of the phase shift network and having a common output. Yet another feature is that the relative period of operation of the multivibrator is varied at a sub-audio rate, as by a variable impedance device connected in the timing circuit thereof and controlled by an oscillator.

Further features and advantages of the invention will readily be apparent from the following specification and from the drawings, in which:

FIGURE 1 is a schematic circuit diagram of a vibrato circuit embodying the invention;

FIGURE 2 illustrates waveforms characteristic of the operation of the multivibrator switch; and

FIGURES 3 through 5 are waveforms illustrating 'operation of the circuit.

During the following description, specific values and type'designations will be given for many elements of the circuit. It is to be understood that this specific disclosure is intended merely to illustrate an operative embodiment of the invention. Many changes and modifications will be apparent to those skilled in the art.

As pointed out above, the vibrato circuit of the present invention is particularly useful in a musical instrument, as an organ, of the type in which it is difiicult to frequency modulate the tone generators directly. However, it will be understood that the circuit could be used with any type of tone generator. The nature of the tone representing signal source is not important and will not be described in detail. The source may include suitable filters or synthesizing networks, depending on the type of tone generator used.

Turning now to the drawings, a phase shift network or delay line 10 has input terminals 11 to which the composite signal from the signal source is connected. A pair of outputs are derived from the phase shift network as at 1'2 and 13, there being a phase shift between the outputs dependent on the length of the line and the frequency of the signal. The phase of the output derived at 12 is the same as the incoming signal. A multivibrator 1'5 acts as an electronic switch and has inputs connected with the two outputs of the phase shift network 10. A composite output appears at terminals 16.

A pair of variable impedance devices 17 are connected with the multivibrator to vary the relative switching period, but not the switching frequency, at a sub-audio rate. Thus, the output of the circuit is made up of varying portions of unshifted and shifted signals, providing an impression of frequency modulation.

More particularly, the audio input circuit connected with terminals 11 includes a series isolating resistor 20, 1800 ohms. The delay line 10 is made up of a plurality of sections of shunt capacity 10a and series inductance 10b. The number of sections and electrical constants of each are not critical so long as certain general characteristics described below are met. The phase shift network is terminated in a shunt resistor 21, 18,000 ohms.

Multivibrator 15 has two sections 22 and 23, each onehalf of a 12AU7 dual triode. The multivibrator is designed for free-running operation and a square wave output with cycle portions of balanced or equal duration. The cathodes of the two multivibrator sections are connected together and returned to ground or reference potential 24 through a common cathode resistor 25, 10,000 ohms. Each section has a plate load resistor, 26 and 27, 10,000 ohms, connected with a positive voltage source or B+ volts. The plate of each triode section is connected with the grid of the other section through a coupling circuit including capacitors 28 and 29, 250 pf. (picofarad). The grid of each tube is returned to B+ through

resistors

32 and 33, one megohm each.

The series combination of resistor 34, 27,000 ohms, and capacitor 35, 0.01 ,uf. (microfarad) is connected between phase shift output 12 and the plate of tube 22, and thus through capacitor 28 to the control grid of tube 23. Similarly, output 13 of the phase shifter is connected through resistor 36, 18,000 ohms and capacitor 37, 0.01 ,uf., with the plate of tube 23 and through capacitor 29 with the control grid of tube 22. The difference in value of

series resistors

34 and 36 compensates for the attenuation of the signal through phase shifter 10. The tone signal amplitudes on the control grids of the two sections of the multivibrator are the same.

The frequency or repetition rate of free-running multivibrator 15 is above the usable audio range and preferably several times the upper audible limit of the human ear. In the particular example given above, the repetition rate is of the order of 100,000 cycles per second. With the circuit thus far described, the output signal will be made up of equal portions of delayed and undelayed signal and the effect will be that of tones without vibrato. Waveform 42, FIGURE 2, illustrates the signal which might be observed at the plate of either section 22 or 23 of the multivibrator. As each section conducts, the signal applied to its control grid appears across common cathode resistor 25. Of course, the switching rate is so high that it does not affect the output signal.

The switching period of multivibrator 15 is varied or modulated at a sub-audio rate by a pair of

tubes

37 and 38 connected effectively in parallel with

timing resistors

32 and 33. Tubes 37 and 38 are triodes, each one-half of a 12AU7. Tube 37 functions as a sub-audio oscillator at a frequency determined by the values of inductor 39 and capacitor 40 which form a resonant tuning circuit connected with the control grid. The cathode of tube 37 is returned through a bias resistor 41, 6800 ohms, to a tap on inductor 39. The frequency of the oscillator may, for example, be of the order of to 7 cycles per second for a pleasant vibrato effect.

The space current through tube 37 varies sinusoidally with the sub-audio frequency of oscillation. As this occurs, the effective impedance presented by tube 37 to the timing circuit of multivibrator varies. Tube 38 has its anode circuit connected across resistor 33, and its control grid returned to ground 24. The cathode of tube 38 is returned to the top of oscillator bias resistor 41. Accordingly, tube 38 also has a sinusoidally varying space current and impedance, 180 out of phase with tube 37. As the shunt impedance in one timing circuit is increased, that of the other circuit is decreased. Thus, the repetition rate of the multivibrator remains unchanged while the relative switching period, or the length of time signals are derived from either output, varies sinusoidally at the sub-audio frequency of control oscillator 17.

The amplitude of the current flow through

tubes

37 and 38 determines the relative effect of the impedance change on the multivibrator circuit. Preferably, a maximum ratio of 9 to 1 is utilized. The switching waveform at 42 has a 1 to 1 ratio. Waveforms 43 and. 44 represent conditions of maximum and minimum impedance of

tubes

37 and 38, with switching periods wherein one tube of the multivibrator is conductive for W of the multivibrator period and the other tube is conductive for of the period. This switching period variation, together with a phase shifter having a phase shift of the order of 360 at 1100 cycles per second provides a very pleasant vibrato effect.

FIGURES 3 through 5 are waveforms illustrating the operation of a specific embodiment of the invention utilizing a delay line having 360 phase shift at 1200 cycles per second, with an audio frequency of 400 cycles persecond. A switching frequency of the order of two times the audio frequency is shown for the sake of clarity. In practice the switching frequency might be much higher, as of the order of 100 kilocycles.

In each figure, curve 50, which is positive going at the left of the figure, represents the unshifted signal appearing at output 12 while curve 51 represents the shifted signal appearing at output 13. These curves are shown with light lines. The portions of each signal coupled to the common output circuit by the switch are indicated by heavier lines. In FIGURE 3 switching is carried out at one extreme, with a 9:1 ratio of direct to delayed signal. In FIGURE 4, the ratio is reversed, 1:9 for direct to delayed signal. In FIGURE 5, the ratio is 1:1.

Curves

52, 52a and 52b illustrate the switching action in FIGURES 3 through 5 respectively and show the signal portions selected from the two outputs,

The human ear cannot respond to rapid changes in the signal and the apparent effect on the listener is a composite or average of the signal portions derived from the outputs of the delay line. This is illustrated by the heavy line curves 53, 53a and 53b. A comparison of FIG- URES 3, 4 and 5 shows that the phase of

curves

53, 53a and 53b vary with different switching ratios. As pointed out in the specification, the switching ratio is varied at a sub-audio rate. This results in the phase shift shown and an apparent frequency shift or vibrato effect.

I claim:

1. A vibrato circuit for a musical instrument, comprising: a phase shift network having a signal input terminal and at least two outputs; a switch connected with the outputs of said phase shift network for deriving a composite output signal comprising sequential signal portions from each of said outputs; and means for varying the relative period of operation of said switch to vary the signal portion derived from each output.

2. The vibrato circuit of claim 1 wherein the relative period of switch operation has a maximum ratio of the order of 9 to 1.

3. The vibrato circuit of claim 1 wherein said phase shift network provides a shift of the order of 360 at a frequency of 1100 cycles per second.

4. A vibrato circuit for a musical instrument, comprising: a phase shift network having a signal input terminal and at least two outputs; a free-running multivibrator switch having two sections, each with an input, said multivibrator having a single output; means connecting one of the inputs of the multivibrator switch with one output of said phase shift network; means connecting the other input of the multivibrator switch with the other output of the phase shift network; and a control device connected with said multivibrator for varying the relative period of operation thereof to vary the duration of the signal portion derived from each output.

5. The vibrato circuit of claim 4 wherein the multivibrator has two sections with two intersection coupling circuits and the control device includes a pair of variable impedance elements, one connected with each of the intersection coupling circuits of the multivibrator, and means for periodically varying the impedance of said elements.

6. The vibrato circuit of claim 5 wherein one of said control devices and a first impedance element form an oscillator having a bias element in the circuit thereof, and said second variable impedance element is an amplifier having a control electrode connected with said bias element, the impedance of the second element varying oppositely and in synchronism with the variations of the impedance of the first.

7. A vibrato circuit for a musical instrument, comprising: a phase shift network having a signal input terminal and at least two outputs; a free-running multivibrator switch having two sections, each with a control electrode and an output electrode; means connecting the control electrode of one section of the multivibrator switch with one output of said phase shift network; means connecting the control electrode of the other section of the multivibrator switch with the other output of the phase shift network; a common output circuit connected with the output electrodes of both sections of the multirvibrator; and a control device connected with said multivibrator for varying the relative period of operation thereof to vary the duration of the signal portion derived from each output.

8. A vibrato circuit for a musical instrument, comprising: a phase shift network having a signal input terminal and at least two outputs; a free-running multivibrator switch having two sections each with a control electrode and an output electrode and two intersection coupling circuits connected between the two sections of the multivibrator; means connecting the control electrode of one section of the multivibrator switch with one output of said phase shift network; means connecting the control electrode of the other section of the multivibrator switch with the other output of the phase shift network; a common output circuit connected with the output electrodes of both sections of the multivibrator; a pair of variable impedance elements, one connected with each of the intersection coupling circuits of the multivibrator; and control means, forming with a first of said impedance elements an oscillator having a bias element in the circuit thereof, said second variable impedance element comprising an amplifier having a control electrode connected with said bias element, the impedance of the second variable impedance element changing oppositely and in synchronism with the impedance of the first, to vary the relative period of operation of the switch and the duration of the signal portion derived from each output.

9. A vibrato circuit for a musical instrument, comprising: a phase shift network having a signal input terminal and at least two outputs; an electronic switch connected with the outputs of said phase shift network for deriving a composite output signal comprising seguential signal portions from each of said outputs; means establishing a switching rate of at least several times the audio frequency of said instrument; and means for varying the relative period of operation of said switch at a sub-audio rate to vary the signal portion derived from each output.

References Cited by the Examiner UNITED STATES PATENTS Hanert 841.25

Stefanov 33214 X Wood 33214 X Martin et al 307'88.5 X K'abell.

Cooper et al. 33214 X Weinstein.

1O ARTHUR GAUSS, Primary Examiner.

I. C. EDELL, Assistant Examiner.

Claims

1. A VIBRATOR CIRCUIT FOR A MUSICAL INSTRUMENT, COMPRISING: A PHASE SHIFT NETWORK HAVING A SIGNAL INPUT TERMINAL AND AT LEAST TWO OUTPUTS; A SWITCH CONNECTED WITH THE OUTPUTS OF SAID PHASE SHIFT NETWORK FOR DERIVING A COMPOSITE OUTPUT SIGNAL COMPRISING SEQUENTIAL SIGNAL PORTIONS FROM EACH OF SAID OUTPUTS; AND MEANS FRO VARYING THE RELATIVE PERIOD OF OPERATION OF SAID SWITCH TO VARY THE SIGNAL PORTION DERIVED FROM EACH OUTPUT.