US2763720A - Loudspeaking systems and amplifying methods - Google Patents

Description

A. J. SANIAL LOUDSPEAKING SYSTEMS AND AMPLIFYING METHODS Filed NOV; 22, 1950 LOUDEPEAKIING fiYSTEMS AND AMPLIFYING METHODS Arthur ll. Sanial, Flushing, N. Y., assignor to Audio Equipment Company, lino, Elmhurst, N. Y., a corporation of New York Application November 22, 1950, Serial No. 197,006 2 Claims. (Cl. 179-1) This invention relates to loudspeaking system-s and amplifying methods and refers more particularly to audio amplifying systems provided with microphones and loudspeakers.

In prior art public address systems or similar electronic voice amplifying loudspeaker systems the amplification or gain of the system must be maintained below a certain point predetermined by critical amplification at which the sound from the loudspeaker will feed back to the microphone in such phase as to start oscillations or so-called singing. As a general rule, the closer the microphone and the loudspeaker are placed to each other, the lower is the value of which the overall gain must be maintained to prevent such oscillations.

An object of the present invention is to improve loudspeaking systems by increasing the amount of permissible amplification.

Another object is to reduce the tendency to an acoustic feedback between a microphone and a loudspeaker of an audio amplifying system.

A further object is to eliminate back oscillations for a specific range of amplifications in loudspeakiug systems.

Other objects of the present invention will become apparent in the course of the following specification.

-n accomplishing the objects of the present invention it was found desirable to provide a new circuit combination whereby the main amplifying channel is so acted upon as to allow more gain before the acoustic feedback occurs. This is accomplished by the use of a pulse signal of a recurrence frequency which is above the frequency reproduction range of the loudspeaker system, the pulse signal being injected into the amplifying channel in such manner as to cause a slight increase in amplification. However, the decrease in average amplification is less than the decrease in instantaneous amplification, so that the gain of the amplifier can be increased to a value which is above that prevailing before the introduction of the pulse signal.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawing showing by way of example only, preferred embodiments of the inventive idea.

In the drawing:

Figure l is a circuit diagram illustrating an amplifying system constructed in accordance with the principles of the present invention.

Figure 2 illustrates diagrammatically the pulse signal of the system of Figure 1.

Figure 3 illustrates diagrammatically a somewhat different system.

Figures 4 and 5 show diagrammatically different pulse signals.

Figure l of the drawing includes a diagrammatic representation of the standard circuit of a loudspeaking system which has a microphone 10, a loudspeaker 11, or other suitable input and output transducers and an amplifying channel extending between the microphone and ates Patent 0 the loudspeaker and including amplifier tubes 12, 13 and 14, condensers 15 and 16 and the ground 17 and amplification control 30.

If the gain of this amplifying channel is set somewhat above the critical value, and when no sound signal is going through the system, ordinarily noise or other disturbances either in the acoustic fields of the microphone lid and the loudspeaker 11, or in the electrical circuits therebetween, will have the tendency to start the system into regeneration, which will quickly build up into oscillations or acoustic feedback.

In accordance with the present invention this drawback is avoided by the introduction of a pulse signal of a recurrence frequency above the frequency reproduction range of the loudspeaker system and of a duty cycle substantially less than a sine wave signal of the same repetitive frequency. The means used for that purpose include a pulse generator output with circuit impedances and amplitude suitably adjusted; the means shown in Figure 1 include pulse generating circuits 18 of any suitable type, a resistance 19, an amplifier tube 20, resistance 21, attenuator 22 and conduits 23a which introduce the pulse signal generated by the circuits 18 into the plategrid coupling circuit of one of the amplifier stages, such as the amplifier tube 12.

The signal 24 which is indicated in Figure 2, is preferably a negative pulse signal with the amplitude and phase adjusted so as to drive the grid to or beyond cut off momentarily, thus reducing the gain for a short period.

A positive pulse of sufficient amplitude may be also used, and so may a pulse consisting of positive and negative peaks (or negative and positive peaks) following each other at short time intervals equal to /2 or less of the pulse recurring frequency. The reason positive pulses may be used as well as negative pulses is that with sulficient pulse amplitude it has been found to have the effect of momentarily reducing the gain of the amplifier. This is due to the pulse driving the tube beyond saturation in which case the mutual conductance is less than under normally applied A. C. voltages which sweep over the linear portion of the tube characteristic.

All such pulses will have the effect of momentarily reducing the gain, without any appreciable effect on the average gain of the system.

It is apparent that the pulse must be of a peaked nature, namely, with a duty cycle which is less than a sine wave of the same frequency as the pulse recurrence frequency.

In order to attain the optimum effect it is advisable to make the duty cycle as short as is consistent with the effective reduction of the instantaneous gain of the amplifier to prevent the building up of regeneration and acoustic feedback.

In accordance with the present invention, the system is further improved through the provision of a means for filtering out the pulse signal beyond the amplifier stage in which the action occurs. This filtering may be effected by any suitable circuit elements or filter networks, which separate unwanted signals in one frequency range from desired signals in a different frequency range.

In the system shown in Figure l the condenser 23 is used as a filter and its constants are of a value to attenuate the pulse signal in succeeding stages by a suitable substantial amount, without appreciably affecting the signal frequencies.

The filter condenser 23 prevents unnecessary loading by the pulse components of the succeeding stages which are then free to amplify the main signal without any substantial interference from the pulse signal.

It is apparent that the described introduction-of the pulse signal will prevent acoustic feedback to an extent which will permit the use of a gain value which is equal to or higher than the critical value.

Hence a lower sound input value can be used at the microphone to obtain the same output from the loudspeaker than before the i-ntroductionof the pulse signal, or, conversely, with the same microphone input more out put can be obtained-from the loudspeaker with the use of the pulse signal than without it, provided that the power capacity of the system is dequate.

The system shown in Figure 3 includes the same main microphone-loudspeaker amplifying channel and a pulse generator output which includes pulse generator circuits 24 connected in series with a condenser 25 and an attenuator 26. Conduits 27 inject the pulse signal 28 to the grid circuit of the amplifier stage 13. This system may utilizethe pulse signal 28 illustrated in Figure 4 which is a positive pulse of sufficient-amplitude, or the pulse signal 29 of Figure 5, consisting of positive and negative pulse signal groups with each group of the pulse signals having a duty cycle substantially less than a sine wave of the same repetitive frequency, with both the relative amplitude of positive and negative swings and the phase with respect to that of the element of the electronic tube in which the pulses are injected, being so adjusted that greater gain may be used in the amplifying system without acoustic feedback.

In other respects this system is similar toone previously described and operates in the same manner.

Obviously, the signals 28 or 29 may be used in the system illustrated in Figure 1.

It is apparent that the examples shown above have been. given solely by way of exemplific'ation and not by way of limitation and that they are subject to wide variation and modification WithlI-ltht: scope of the present invention. All such variations and modifications are to be included within the scope of the present invention.

What is claimed is:

1. In an electronic-acoustic one way amplifying system, in combination witha microphone, a loudspeaker and a single amplifying channel therebetween; said system having an acoustic feedback path between the microphone and the loudspeaker; means regulating the average gain of the amplifying system, said amplifying channel including means generating repetitive pulse signals and and means adjusting the polarity and amplitude of said pulse signals; and means connected with said adjusting means for injecting into said channel pulse signals of a recurrence freqency which is above the frequency reproduction range of the loudspeaker system to reduce instantaneous gain therein without appreciably reducing average gain and to prevent the occurrence of oscillation due to acoustic feedback at the amplification value at which acoustic feed-back would start before the injection of said pulse signals.

2. In an electronic-acoustic one way amplifying systern, in combination with a microphone, a loudspeaker and a single amplifying channel therebetwecn comprising a plurality of interconnected electronic tube circuits, said system having an acoustic feedback path between the microphone and the loudspeaker; means regulating the average gain of the amplifying system; said amplifying channel including means generating repetitive positive and negative pulse signal groups, each group having a duty cycle substantially less than a sine wave signal of the same repetitive frequency and having an amplitude and phase reducing the instantaneous gain of at least one of said electronic tube circuits without appreciably reducing. average gain, and means connected with the lastmentioned means for injecting into the last-mentioned electronic tube circuit pulse signals of a recurrence frequency which is above the frequency reproduction range of the loudspeaker system to reduce instantaneous gain therein without appreciably reducing average gain and to prevent the occurrence of oscillation due to acoustic feedback at the amplification value at which acoustic feedback would start before the injection of said pulse signals.

References Cited in the file of this patent UNITED STATES PATENTS 2,021,638 Ramlau Nov. 19, 1935 2,166,775 Varley July 18, 1939 FOREIGN PATENTS 601,990 Great Britain May 18, 1948 619,073 Great Britain Mar. 3, 1949

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