1,120,541. Compandors. R. M. DOLBY. 25 July, 1966 [11 Aug., 1965; 11 Oct., 1965; 18 Jan., 1966], Nos. 34394/65, 43136/65 and 2368/66. Heading H4R. A compressor comprises a straight through signal path the output from which is added to the output of another path consisting of an amplifier and a limiter. An expander comprises a straight through path from the output of which is subtracted the output of a second path, connected to the output of the subtracting device and including an amplifier and a limiter. The arrangement introduces less distortion since a large part of the signal is always obtained from the straight through path. A number of further paths may be provided, each including a filter so that the compression or expansion is limited to a particular frequency band. Fig. 3 shows an RC filter and diode limiter for use in a system of this type. An alternative form of limiter, which requires a smaller drive voltage, is shown in Fig. 5. In a system where a carrier is transmitted, e.g. in a colour television system using a colour sub-carrier, the further paths may include filters tuned to reject the carrier frequency, Fig. 6 (not shown). Audio compressor, Fig. 7, has four paths for noise reduction, each covering a portion of the audio spectrum and each including a linear limiter, e.g. 76, 78, 80 or 82, which may be of the form shown in Fig. 8. In the limiter of Fig. 8, the operation is such that at low input levels, transistors Q1 and Q2, and diodes D1 and D2 are cut off and the signal passes from the input to the output unattenuated. As the signal level increases transistor Q1 and Q2 reach a point where they conduct, the dynamic resistance of the diodes is reduced, and the signal is attenuated. In the circuit shown, the control voltage has a time constant of decay which is fairly long. In order to reduce the time constant of decay, but still provide adequate smoothing, the smoothing circuit of Fig. 9 may be used. To provide a further reduction in decay time the circuit of Fig. 10 may be used, in which the emitter follower provides a fast charge or discharge for C9, via the diodes D7 and D8, for large changes in signal while for small changes, e.g. ripple, diodes D7 and D8 are virtually non-conducting and the time-constant is determined by C9 and R10. In order to allow the use of a longer attack time in the linear limiter and yet still control large amplitude transients, a non-linear limiter 87 may be included in the path to clip symmetrically the signal passing therethrough. A modification of the circuit of Fig. 7 includes only limiting paths 1 and 4 and variable cut-off frequency filters, Fig. 11 (not shown), are arranged in the paths so that as the signal level increases the top and bottom cut off frequencies of the respective bands are shifted respectively down and up. An expander may be used alone as a means for reducing rumble or high frequency tape noise by arranging paths including respectively low-pass, for rumble, and high-pass, for high frequency noise, filters. In such an arrangement lowlevel signals, such as rumble or H.F. noise, will be attenuated, while if higher level signals are present in these frequency bands the attenuation will be reduced.
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