GB1305622A - - Google Patents

Abstract

1305622 Companders DOLBY LABORATORIES Inc 10 July 1970 [21 July 1969] 32684/70 Divided out of 1279634 Heading H4R Compressor or expander comprises a straight through path and means for combining therewith the output of a further path which takes its input either from the input or the output of the compressor or expander. The further path output may be combined to add to, or oppose, the signal in the straight through path. The further path includes a variable gain circuit and a variable cut-off filter for restricting the signals passing through the further path. The filter presents all pass characteristics at low level and introduces a high pass and/or low pass characteristic when the output level rises. The filter and gain control device may comprise a series arm with a capacitor and resistor in parallel feeding a shunt connected variable impedance which can be a pair of oppositely poled biased diodes, or the source drain path of a FET whose gate is fed with a D.C. proportional to signal level. A sharper filter cut off to provide more discrimination against noise and less noise modulation is given by the circuit of Fig. 3 which comprises a fixed high pass filter formed by C2 and R4 followed by the variable filter formed by C1 and the source drain path of FET 17. The further path also includes a clipper 21 to eliminate large transients which can pass through the gain control circuit due to the time delay of the smoothing circuit 20. A stronger low and mid frequency limiting characteristic can be provided by increasing the impedance of the filter circuit. In Fig. 4 (not shown) this is provided by removing R5 and returning R4 to the output of a unity gain amplifier fed from the output of the filter circuit. This has the effect of making R4 infinite during the time that the FET 17 is not conducting and lowering its impedance as the FET conducts. A further development, Fig. 5, includes two further paths one, 13, dealing with high frequency plus midband noise, and the other, 24, dealing with low frequency plus midband noise. An alternative, Fig. 6 (not shown), combines the two filter circuits of Fig. 5 to feed one FET.