Patent GB-1390343-A (GB)
Title: Signal compressors and expanders
IPC: H03G9/02
Filed: 1971-03-12
Abstract: 1390343 Compandor; noise reducing system DOLBY LABORATORIES Inc 1 March 1972 [12 March 1971] 7959/72 Divided out of 1390341 Heading H4R A compandor comprises two signal paths linking an input terminal to an output terminal via a mixer wherein the companding law in both paths is linear, the output is taken from one or the other of these paths virtually exclusively at the operating limits of a variable component in the mixer and, the components of the mixer are such, together with the gains/attenuations in the paths, that low level signals are amplified/ attenuated i.e. companded, to a greater extent than high level signals. The paths may include filters for equalization purposes or for restricting the action of the compandor to particular frequency ranges. The filters must be isolated, e.g. by buffer amplifiers, from the mixer in order that the latter will not affect their frequency determining properties. This ensures that the paths are linear with respect to dynamic range. Compressor, Fig. 1, comprises a first path 11 incorporating an amplifier 13, a second direct path 14, and a mixer consisting of a variable resistor 17 and a fixed resistor 16 with the output being taken from the junction of the resistors. The companding law for each path is linear as indicated by dotted lines 11a, 14a respectively in Fig. 2. Thus when input signal level is very low, the voltage generated across resistor 17 is too low for the control circuit to decrease the value of 17 and the output signal is derived essentially via the path 11, see the full line 25 on Fig. 2. On the other hand at high input signal levels the impedance of 17 is reduced to zero and the output signal is derived essentially via the path 14, see the full line 26 on Fig. 2. At intermediate settings of 17 i.e. at intermediate input signal levels, the companding law for the circuit-as opposed to either path alone-is represented by the smooth curve 27. The filter components 30, 31, 32, are used for restricting compression to a high(hiss) frequency band. Inductor 32a is utilized if carrier signals are being transmitted by the circuit. Expander, Fig. 3, comprises a low pass filter 12 in the first path and the gains of the two paths are transposed relative to Fig. 2 so that low level signals more particularly in a high frequency range are attenuated relative to high level signals. Control circuit may take the form shown in Fig. 3, where differential amplifier 40 derives a signal from across the variable impedance 17 in the mixer and submits it via a rectifier 41 to series connected CR circuits. The first has a short time constant and the second a long time constant so that control is normally effected syllabically. However in the event of sharp transients in signal level the fast attack time of the first CR circuit predominates due to the forward breakdown of silicon diode 47. The derived signal may be limited by connecting a pair of parallel opposition diodes across resistor 17 (Fig. 5, not shown). In carrier systems the control circuit and controlled impedance may be replaced by non- linear devices such as diodes which perform a similar function in dependence upon signal level. The inherent distortions of this method are only suitable for carrier working since the non-linearity can be compensated by the inverse actions of the compressor and expandor. The filters used in the paths are of course tuned in this case. More than one path may be connected between the input and mixer and it is possible that compression and expansion in different frequency ranges may take place simultaneously (Fig. 6, not shown).