US2468205A - Volume controlled sound reinforcement system - Google Patents

Description

April 26, 1949. E. w. KELLOGG 2,468,205

VOLUME CONTROLLED SOUND REINFORCEMENT SYSTEM Filed Dec. 31', 1.946

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film/4R0 WKELLOGG wwwkm xsu SQ Patented Apr. 26, 1949 VOLUME CONTROLLED SOUND REINFORCE- MENT SYSTEM Edward W. Kellogg, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1946, Serial No. 719,452

6 Claims.

This invention relates to signal transmitting and receiving systems, such as public address and sound reinforcement systems, and particularly to the automatic volume control of such systems.

General systems of this type are well-known in the art, reference being made to Anderson U. S. Patent No. 2,189,306 of February 6, 1940, and to Stanko U. S. Patent No. 2,338,551 of January 4, 1944. The type of public address system shown in the Anderson patent is one wherein the gain of a variable gain amplifier is at a stable value during no-signal transmission and is increased by the signal during the transmission thereof. If there is incipient singing while speech is being transmitted, it is promptly stopped when the speaker pauses between sentences. This is accomplished by rectifying a portion of the signal and applying it to the grid of the variable gain amplifier.

In a large proportion of sound reinforcement systems, the microphone is acoustically coupled to the loud speakers, and if the gain or amplification exceeds a certain critical value, feedback or singing occurs. tion on the usefulness of the system, and various expedients have been proposed, to permit operation at higher gain without singing. The Anderson circuit prevents sound from the loud speakers from sustaining high-gain condition by utilizing only the low frequency components of the voice for the gain control, which frequencies are present in the voice, but not needed for intelligibility, and are not transmitted to the loud speaker. Thus, the output of the speaker contains no frequencies which will be picked up by the microphone and transmitted to the gain controlling rectifier.

It has been found, however, that in many instances, the first parts of many words are frequently lost in transmission because they consist of unvoiced components, such as certain consonants, which have no strong low-frequency components, and are, therefore, incapable of raising the gain. These initial sounds, which are very important to intelligibility, are thus reproduced at low amplification, and being usually of lower intensity to start with than the remainder of the word, may fail to be heard.

The present invention, therefore, is directed to a circuit which will permit the unvoiced components to increase the gain of the amplifier, but only as a transient condition during the time it takes for the voiced components to act through the main gain control rectifier to increase the This places a serious limitat the amplifier in the gain of the variable gain amplifier. The general problem of causing high as Well as low frequency components of speech to exercise a desired influence on a variable gain amplifier has been recognized by Miller in his U. S. Patent No. 2,312,260 of February 23, 1943, wherein he utilizes an equalizer to vary the spectral energy distribution of the signal as applied to the rectifier; but, the Miller patent does not deal with the problem of stability.

The present invention accomplishes the result of an increase in gain by the unvoiced components by utilizing an additional rectifier on which is impressed the higher frequencies of the signal, but applies the voltage derived by rectification of the high frequency components in such a way that it is effective toward increasing gain only during the interim required for the low frequency components to act on the gain control.

The principal object of the invention, therefore, is to facilitate the transmission of signals in a system in which feedback can occur.

Another object of the invention is to provide an improved method of controlling the gain of a variable gain amplifier with the signal currents in a system Where the input and output are acoustically coupled.

A further object of the invention is to provide an improved non-feedback public address system, wherein the variable gain amplifier is controlled by both the voiced and unvoiced components of words.

A still further object of the invention is to provide an improved control circuit for a Variable gain amplifier.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawing, in which the single figure is a schematic circuit diagram of a public address system embodying the invention.

Referring to the diagram, a microphone 5 is connected over conductors 6 to a preamplifier I having dual output circuits. The outputs of the amplifier 1 are passed into two filters, filter I0 including two series condensers H and I2 and a shunt inductance l3, and passes frequencies above 500 cycles only, while filter i5 is composed of two series inductances l6 and I! and a shunting condenser l8 and passes fre quencies below 500 cycles only. Although a 500 cycle cut-ofi point is given as an illustration, it is to be understood that the cut-off point may be varied, while it may also be desired to use a band-pass filter at l which passes frequencies between 100 and 150 to 500 cycles. In the use of such a band-pass filter, any background noise having frequencies below 100 or 150 cycles will not be transmitted to vary the gain of the variable gain amplifier.

The filter I0 is coupled by a transformer to an amplifier tube 2|, and'a'lso to an amplifier tube 22. The signal is amplified in amplifier tube 22 and then impressed on a variable gain amplifier 24, composed of push-pull. tubes 25 and 26, over a transformer 21. The output of the variable gain amplifier is coupled over transformer '29 to an amplifier feeding a loud speaker 3|.

The amplifier 30 may also include a compressor, if desired. Thus, the signal is transmitted from the microphone 5 to a loud speaker 3| over the filter Ill and amplifiers 22, 24, and 30, this signal being composed, however, only of frequencies above the cut-off of the filter In, as for example those above 500 cycles.

The low frequency portion of the signal passed by filter I5 is impressed on an amplifier 33 over transformer 32 and then over conductor 34 and transformer 35 to a full-wave rectifier 31 composed of tubes 38 and 39. The output of the "rectifier is connected toa ripple removing filter composed of resistor 40, condensers 4| and a resistor 49, and then over conductor 42 and biasing battery 44 to the gain control electrodes of the tubes 25 and 26 of the variable gain amplifier 24. The portion of the circuit just described is comparable to the circuit shown in the aboveidentified Anderson Patent No. 2,189,306, although a limiting device, composed of -a rectifier tube 41 and biasing battery 48, is provided between the ripple filter and the variable gain amplifier to prevent the voltage on the grids of the variable gain amplifier from going beyond a predetermined operating value.

As mentioned above, however, the circuit so far 7 described does not provide the necessary increase in gain. whenever the first word starts with an unvoiced consonant. To permit the gain to be increased by such unvoiced consonants, the remainder'of the circuit shown has been provided. This portion of the circuit consists of the amplifier 2| connected to the filter H) in which the high frequency components of-the signal are amplified and which are then impressedover con- :ductor 52 and transformer 53 on a full-wave rectifier 54 consisting of tubes 55 and'56-.- The output of the rectifier 54 is transmitted over a ripple removing filter composed of a. resistor 60, condensers 6|, and discharge resistor 62, to the conductor 42 over a condenser 64. The circuit composed of amplifier 2| and rectifier 54 thus receives the signal as transmitted to speaker 31, and will contain the signal frequencies above 500 cycles. The voltage produced by these signal frequencies will be impressed through condenser 64 on the grids of tubes 25 and 26. However, since the direct current output of the rectifier 54 is impressed on the grids through condenser 64, the positive voltage impressed on the control grids of amplifiers 25 and 26 can last only for a brief period, for as soon as condenser 64' is charged, no further current can flow to sustain the voltage across discharge resistor 49. Thus, the-continuation of input torectifier 54 will not maintain amplifier 24 in the high'gain' condition. However, the gain change is produced over a sum-- oient period to give the low frequency control circuit, composed of amplifier 33 and rectifier 31, time to increase the gain of the variable gain amplifier with the low frequency components of the signal. The time that is required for the voltage increase caused by the high-frequency components to leak away, is determined by the capacities of condensers B4 and 4| in relation to resistor 49.

The above circuit, therefore, provides the nonfeedback condition as disclosed in the abovementioned Anderson patent, and also provides gain changes for all portions of the signal, thereby preventing the loss of certain portions of the signal not having the required energy content to operate the normal gain control rectifier.

I claim:

1. In combination, a variable gain amplifier, a signal source connected to the input of said amplifier, a loud speaker connected to the output of said amplifier, filter means connected between said amplifier and said signal source for selectin'g predetermined frequencies from said signal source for impression on said amplifier, a rectifier connected to said filter means for producing a voltage in accordance with said predetermined frequencies in the output of said microphone, means for connecting the output of said rectifier to said variable gain amplifier for controlling the gain thereof in accordance with said predetermined frequencies, a second filter means connected to said signal source, a second rectifier connected between said second filter means and said and said variable gain amplifier for limiting the gain control of said variable gain amplifier by said predetermined frequencies to a certain initial time period.

2. In a signal transmission circuit, a microphone for detecting signals, a variable gain amplifier' connected to said microphone, a loud speaker connected to said variable gain amplifier, a rectifier, means connected between said microphone and said variable gain amplifier, for selecting predetermined frequencies in the output of said microphone for impression on said variable gain amplifier and on said rectifierfor varying the gain of said variable gain amplifier, means connected between said rectifier and said variable gain amplifier to limit the varying of said variable gain amplifier by said predetermined frequencies to a predetermined time period, and means for selecting other frequencies in the output of said microphone for continuously varying the gain of said variable gain amplifier.

3. A signal transmission circuit in accordance with claim 2, in which said first mentioned means includes a low pass filter, and said last mentioned means includes a high pass filter and a second rectifier.

4. A signal transmission circuit comprising a variable gain amplifier, an input circuit for said amplifier selective to a predetermined rangeof signal frequencies,a-=rectifier circuit connected to said input circuitandselective to a different range of signal frequencies, said rectifier circuit being connected to said variable gain amplifier for continuously varying the gain thereof; a rec tifier circuitconnected to said input circuit and adaptedtovary the gain of said variable gain amplifier, said second mentioned rectifier circuit being sensitive to the same range of frequencies as said input circuit, and an electrical element connected between said second mentioned rectifier circuit and said variable gain amplifier for varying the gain thereof for a short initial time period.

5. A signal transmission circuit in accordance with claim 4, in which said input circuit and said second mentioned rectifier circuit are selective to a high frequency range and said first mentioned rectifier circuit is selective to a low frequency range.

6. A signal transmission circuit in accordance with claim 4, in which said input circuit and said second mentioned rectifier circuit are selective to an audio frequency range above substantially 500 cycles.

EDWARD W. KELLOGG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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