Patent GB-1390341-A (GB)
Title: Signal compressors and expanders
IPC: H04B1/64
Filed: 1971-03-12
Abstract: 1390341 Compandor; noise reducing system DOLBY LABORATORIES Inc 1 March 1972 [12 March 1971] 6747/71 Heading H4R A compandor effective only on signals lying in a restricted frequency band within a broader frequency spectrum and responsive to high level signals within the restricted band so as to restrict it even further, comprises a plurality of impedance networks connected in series across an input source and a control circuit responsive to a signal having a level above a predetermined level within the restricted band so as to adjust a reactive impedance in one of the networks thereby to further restrict the band within which signal companding takes place. The invention is based on the principle that noise reduction is so ineffective on high level signals that it is better not to introduce possible distortion by companding high level signals even when these occur in that part of the frequency range normally most susceptible to noise interference. The systems to which the invention is applicable are audio, audio multiplex, suppressed and unsuppressed carrier, single and vestigial sideband, and video systems. In the audio system case the noise to be suppressed is high frequency tape noise so that companding is concentrated on the high frequency end of the audio spectrum. In other systems it could be the low end or one or more bands within the spectrum that are subjected to companding. In any event the essential feature is that the companded frequency band be changed if a high level signal occurs within the band without-as is usually the case-altering the companding law. One corollary of the circuit action is that the contiguous band in which dynamic range modification does not occur is widened by exactly the amount by which the companded band is narrowed.  Compressor.-In Fig. 3 three impedance networks are connected in series across input terminals 24a, 24b and an output is obtained at output terminals 25a, 25b. In the third variable impedance network, the value of the inductor is chosen such that there is effectively no gain for signal transfer between the input and output terminals for signals up to a frequency f x  but for signals above this frequency there is a marked and substantially constant gain. The control circuit monitors the signal developed across the third network so as to decrease the value of the inductor and thus raise the frequency f x  to f y  in the event that the level of a signal at frequency f y  exceeds a predetermined threshold. Expandor Fig. 4.-In this arrangement the output voltage is derived from across a small value resistor 31 through which the current level is maintained low by means of the action of inductance 22 for signals subjected to expansion i.e. above f x  or f y . In this way the output signal is attenuated by a substantially constant amount at the higher frequencies. Control circuit, Fig. 7, consists of a differential amplifier 40 whose output is rectified 41 and smoothed 42 so as to act syllabically i.e. so as to compand without introducing non-linear distortion into the signal. CR circuit 43, 44 has a short time constant relative to 45, 46 so that under normal signal conditions a high degree of smoothing is obtained but in the event of rapid signal changes a fast attack can be obtained by the forward conduction of silicon diode 47 which has an appreciable forward voltage drop. The compandors may comprise capacitive rather than inductive variable impedances (Figs. 5, 6, not shown). The indeterminate values of the variable impedances at the extremes of their "travel" may be compensated by the swamping effect of suitable shunt inductors, e.g. 22a or series capacitors. The low resistance 31 may be constituted by a negative feedback amplifier or a grounded base transistor. In Fig. 4 the diodes 50, 51 and batteries 52 (which could be provided by suitably biased transformers) limit the voltage across the variable impedance so that under extreme transient conditions the time lag in the control circuit will not result in significant modification of the high level transients. The variable impedances may be electromechanical or equivalent electronic devices.