GB1356848A - Noise reduction system and apparatus using a compression and ex pansion system - Google Patents

Abstract

1356848 Compandors NIPPON VICTOR KK 4 June 1971 [5 June 1970 (2) 26 June 1970 27 June 1970 31 Aug 1970] 18976/71 Heading H4R In a compander system a compressor compresses the amplitude of signals in a frequency band smaller than the total pass band of the system using a compression circuit in the forward path of an amplifier, while the expander expands the amplitude of signals in the frequency band compressed by the compressor, using a similar circuit to the compressor connected in a negative feedback circuit of the expander. Variable impedance circuits in the compressor and expander can be arranged to compensate for non-linearity or supply voltage variations. Fig. 6 shows a preferred embodiment including a bridge circuit 150 in the forward path of the compressor 20, and a similar bridge circuit 190 in the negative feedback path of the expander 21. The bridge circuits have frequency selective circuits 151 or 188 in the diagonal arm so that it has no effect when the bridge is balanced, e.g. at high signal levels. A control signal, derived from the output of the compressor and input of the expander, varies the impedance of the parallel connected NPN and PNP transistors to unbalance the bridge and provide compression and expansion of the frequencies selected by the tuned circuit 151, 188, as the signal level is reduced. Use of the parallel NPN and PNP transistors improves the linearity with respect to signal level of the variable resistance provided thereby, while an alternative variable impedance circuit, Fig. 15, uses an f.e.t. provided with variable bias from silicon diodes 223 and 224, which, in combination with a bias applied to the amplifying transistor 226 preceding the control signal rectifier 225, compensates for supply voltage variations.