Patent GB-1361539-A (GB)
Title: Wide dynamic range noise masking compandor
Inventor: BURWEN RICHARD S
IPC: H03G7/00
Filed: 1972-04-28
Abstract: 1361539 Signal companding system R S BURWEN 28 April 1972 [10 May 1971] 19800/72 Heading H4R An audio signal is subjected to preemphasis over its higher and lower frequency positions prior to being compressed using a control signal which is frequency conscious. After transmission over a noisy channel, e.g. a tape recording system, the signal is expanded using a complementary characteristic and is finally deemphasized before being connected to a reproduction unit. In the compressor of Fig. 1 the input signal 12 is preemphasized in the range below 100 Hz. and above 1À3 KHz. to produce a signal Z which is applied to a divider 16. The latter is connected via a low frequency attenuator 18, that is effective below 50 Hz. so as to particularly reduce D.C. transients produced by the divider, and, a high frequency limiter 20, that clips high level H.F. signals so as to prevent overload and thus magnetic tape saturation, to a tape recorder 24. A compression control signal Y is derived from the output of limiter 20 through an equalizer 26 that produces a 5 db. boost at 80 Hz. and a 3, 12 or 20 db. boost at 9 KHz for tape speeds of respectively 15, 7À5 and 3À75 i.p.s., a precision peak rectifier 28 that in effect stores the most recent peak signal information but has a fast attack in the event of a new peak occurring, and, a squaring circuit 30. The control signal is effective to reduce an original dynamic range of 90 db. to one of 30 db. A complementary expander (Fig. 2, not shown) incorporates a multiplier between the input tape channel and a de-emphasizer, the control signal being derived from the input channel via a path incorporating similar components to those in the compressor. The divider (Fig. 5) is compensated against D.C. level shifts caused by rapid changes in the control signal level by means of a D.C. negative feedback loop incorporating a low pass # filter 96. The main amplifier 92 is fed from a multiplier 94 which receives a control signal and a feedback signal from adder 98. The latter is applied with one input from the output of the amplifier and another input which is selectively earth or the output of an integrating amplifier 100 having a shunt CR feedback loop therearound. When the divider 90 is operating at lowgain levels a slight D.C. offset that could occur is minimized by feedback D.C. stabilization provided by the amplifier 100. Compression control voltage is derived as shown in Fig. 4. The output of active rectifier 46 is supplied to a virtual earth input of an operational amplifier 52 which is connected via a diode 54 to a unity gain amplifier 56. The capacitor 64 provides the storage for the last peak detected while the diodes 60, 54 ensure rapid attack times without distortion in the event of changes in the control signal level. The rather jagged output of the peak rectifier is passed through a non-linear filter which is effective to smooth the signal more particularly when this varies by more than a given percentage from that currently at the output of the filter. The feedback connection to capacitor 70 of the # filter enhances the attack time of the compressor under large signal change conditions. Stereo system (-Fig. 6, not shown) incorporates separate pre-emphasizers and compressors in each channel but a common control signal is used for controlling the latter. The signal may correspond to the mean level in the two channels or to the higher level.  Crescendo effects can be enhanced by emphasiz - ing the lower and higher frequency components of the signal in the expansion control path to a greater extent than those in the compression control path viz. those in equalizer 26 in Fig. 1.