NOISE REDUCTION SYSTEM

摘要

1431481 Compressors and expanders PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 27 March 1973 [30 March 1972 8 Feb 1973] 14589/73 Heading H4R In a non-linear circuit for a noise reduction system the current through a series combination of a first impedance Z 1 , a transistor T 1 and a second impedance Z 2 is amplitude controlled by a signal input to the transistor control electrode V i and utilized in a network G to control a second transistor T 2 so as to make the impedance Z 2 dependent on the amplitude and frequency of the series current. The circuit and its complement are used for recording and playback of video and/or audio signals. As shown the network is switchable S between compression and expansion modes a, b and includes identical buffer amplifiers B 1 , B 2 , high pass filter F 1 , non-linear filter F 2 , amplifiers A 1 , A 2 and a rectifying detector D which generates a control voltage to vary the cut off of filter F 1 and/or the gain of amplifier A 1 according to the amplitude and frequency of the input V i . In a simplification (Fig. 6, not shown) B 1 , B 2 and S are omitted and A 2 is a frequency-dependent amplifier, either or both F 1 and A 1 being controlled by the detector D. Circuit details for both arrangements are described (Figs. 7, 11, not shown). In a modification (Figs. 4 and 5, not shown) in one mode the circuit is as Fig. 10, but is switched to the complement by converting T 1 to a constant current configuration and inserting an impedance Z 1 1 and transistor (T 5 ) in parallel with Z 2 to form two limbs in parallel with respect to the supply but in series with respect the the input so that the current ratio in the two limbs varies according to the input.