GB643574A - Improvements in apparatus or systems for transmitting electric signals - Google Patents

Abstract

643,574. Automatic volume control. SCOTT, H. H. Jan. 1, 1947, No. 98. Convention date, Jan. 19, 1946. [Class 40 (iv)] The incidence of both high-frequency and low-frequency noise in the reproduction of music is reduced in quiet passages by varying the point of cut-off at both high and low frequencies according to the strength of signal. In the simple embodiment of Fig. 4, an adjustable band-pass filter 82 comprises a tube 8 shunted across the output terminals 17, 18 and functioning as a variable capacitance or inductance at high and low frequencies respectively, the value of the reactance varying with the output voltage of the control circuit 6 according to the instantaneous strength of the signal. The band-pass filter may be replaced by lowpass and high-pass filters 69, 84, Fig. 8. A resonant circuit 10, 38 provides a fixed point of attenuation above the cut-off of normal transmission range, while the inductance 34 and reactance tube 8 provide a point of high attenuation just above cut-off and varying with the cut-off frequency. The circuit functions as a double-m-derived filter having a very sharp variable H.F. cut-off characteristic with a predetermined amount of resonance below cutoff to increase the response in that region. The high-pass filter 84 comprises a reactance tube 33 which functions as a variable resistance or inductance depending on the size of the condenser 152. The filter 7 and amplifier 100 cut out L.F. and H.F. components from the signals sent to parallel control circuits 6, 106 controlling the filters 69, 84. In Fig. 11, the signal is fed to low, band and high-pass filters 407, 207, 307 in parallel, the middle frequencies having a fixed gain in amplifier 40 while the low and high frequencies receive variable gain in amplifiers 142, 141 under the control of the circuit 6. Specifications 643,854, 645,256, 645,343 and 656,545 are referred to.