Patent GB-1390342-A (GB)
Title: Signal compressors and expanders
IPC: H03G9/02
Filed: 1971-03-12
Abstract: 1390342 Compandor; noise reducing system DOLBY LABORATORIES Inc 1 March 1972 [12 March 1971] 7958/72 Divided out of 1390341 Heading H4R A compandor effect only on signals lying in a restricted frequency band within a broader spectrum and responsive to high level signals so as to restrict the band even further, comprises a first signal path which is essentially linear in so far as its effects on dynamic range are concerned connecting an input terminal and one terminal of a variable coupling device, a second signal path connecting the input terminal to the other terminal of the coupling, and an output terminal connected to said other terminal. The second path in conjunction with the coupling device is effective to define the frequency band in which companding is to take place, the coupling device being intrinsically non-linear as regards signal amplitude or being controlled by an external control circuit in accordance with signal amplitude so as to cause the required bandwidth restriction. The invention is applicable to audio systems in which tape hiss reduction is required, as well as to carrier and video systems. Compressor, Fig. 2, consists of a direct first path between input 10 and one side of variable coupling resistor 20, and an amplifier 18 followed by (and not preceded by) a series capacitor 17 and a resistance chain 16, 15 forming the second path between the input and the other side of the resistor. The latter may be an FET or a photo resistor whose value is controlled inversely as the level of an input signal such that as its impedance reduces, the frequency, above which input signals over the second path are amplified, is increased thus restricting the band subjected to compression. Expander, Fig. 3, similarly consists of a direct first path but in this case the capacitor 25 is in shunt with the second path so that attenuation rather than amplification of higher frequencies occurs. Compander, Fig. 3.-By utilizing amplifier 26 as an active second path, the circuit acts as an expander or a compressor in dependence on whether the amplifier gain is less or greater than unity. The diodes 40, 41 and bias source 42 serve to restrict the voltage developed across the resistor so that in the event of rapid transitions in signal voltage distortion is not introduced by the control circuit. The latter includes two time constant circuits following an input signal voltage rectifier, the circuits being separated by a resistor/diode network which breaks down in the event of rapid signal voltage transitions thereby reducing distortion still further. In other arrangements a variable reactor, e.g. a capacitor, may be used as the coupling device in which case a control voltage therefor is developed by a control circuit coupled across a resistor in the second path (Fig. 4, not shown). Both paths may include amplifiers, the relative gains of which determine the circuit's operation as a compressor or an expandor. The first and/or second paths may include filters, delay circuits or tuned circuits, the latter being utilized more particularly for amplifying or alternating sidebands of a carrier signal (Fig. 5, not shown) in conjunction with a variable resistor forming the coupling device.