GB645256A - Improvements in apparatus or systems for transmitting electric signals - Google Patents

Abstract

645,256. Automatic volume control. SCOTT, H. H. Jan. 2, 1947, No. 193. Convention date, Jan. 23, 1946. [Class 40 (iv)] Noise -suppressing transmission systems of the type described in Specifications 643,574, 645,343 and 656,545, are modified by the use of filters in combination with fixed or variable gain amplifiers. A low-pass filter 207, Fig. 7, provides an m-derived half section giving a sharp cut-off at say 2,500 cycles/sec. and high attenuation above cut-off and feeds a fixed gain amplifier 40, while a high-pass filter 307 feeds an amplifier 41, the gain of which is varied by a control circuit 6 the. output of which is proportional to the intensity of the mid-audio frequencies, whereby when the mid-audio frequencies are strong, the highfrequency response is high, while in quiet passages, the high frequency signals are attenuated to cut out high frequency noise such as needle scratch. Alternatively an amplifier with a wide frequency range may be in parallel with an amplifier having a narrow range reversed in phase, the second amplifier being effective at low volume to reduce the overall range of the system. In Fig. 9, one control circuit 6 controls a band-pass filter 169 and another control circuit 106 controls a variable amplifier 41, the filter 169 and amplifier 41 adding high and low frequencies, respectively to the output when the signals of the intermediate frequencies are strong. Specifications 643,662, [Group XL (b)], and 643,854 also are referred to.