Patent GB-1044003-A (GB)
Title: Improvements relating to sound recording and transmission systems
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H03G9/02
Filed: 1962-06-07
Abstract: 1,044,003. Compander systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. June 5, 1963 [June 1, 1962], No. 22179/62. Heading H4R. In a sound recording or transmission system using compression and expansion the relatively high frequency components of the signal are separated from the relatively low frequency components and compressed and expanded separately, the compressed high-frequency components being combined with the low-frequency components for recording or transmission and then separated out for expansion after transmission or replay. Fig. 1 shows the transmission or recording compressor in which a Norton filter 1 separates the high frequencies, e.g. above 1À9 kc/s., from the low frequencies, the low frequencies being transmitted unchanged via the combining amplifier 4 while the high frequencies are compressed so that the range 0 to - 30 dB. at the input occupies the range 0 to - 15 dB. at the output, the compression being accomplished by feeding the high-frequency components to a divider 5 to which is also fed a control signal derived from the high-frequency components by rectification, filtering, and square rooting in units 6, 7, and 8, the resulting compressed highfrequency components being added to the low frequencies in amplifier 4 before recording or transmission. The expander is shown in Fig. 2 and comprises a filter 2 to separate the low and high frequencies, the low frequencies being passed to the output via amplifier 15 in which the expanded high frequencies are added, the expander comprising a multiplier 10 in which the high frequencies are multiplied by a signal derived, via filter, rectifier, filter and limiter 14, 11, 12 and 13 respectively, from the envelope of the high frequency signal components. The filters 1 and 2 used to separate the low and high frequency components of the signal are Norton filters having a Butterworth response characteristic the circuit values of the filters being given on Figs. 3 and 4 (not shown). For optimum performance the cross-over frequency in filter 2 in the expander is lower (i.e. 1À15 kc/s.) than that in the compressor (i.e. 1À9 kc/s.). Derivation of the control signal for the divider in the compressor of Fig. 1 is accomplished by rectifying the high-frequency output of the Norton filter and passing the rectified signal through an elongator, Fig. 7, which has a short rise time and a slow decay and allows the compressor to follow the transients occurring in the signal. The non-linear filter 7 shown in detail in Fig. 8 is included to reduce distortion which is said to occur for rapid variations of the envelope of a tone of about 2 kc/s. The function generator 8 may comprise a diode circuit which extracts the square root of the applied voltage over the range of 0 to 30 dB. below peak level, the dB. range thus being halved over this range of levels. The divider, shown in Fig. 12, comprises a pair of Zener diodes which shunt the signal path in the output circuit of a push-pull amplifier, the resistance of the diodes being varied by the D.C. control applied to the slider of resistor P 1  from the square root circuit 8, Fig. 1. P 1 , R 0 , and C provide respectively resistive, voltage, and capacitive balance of the circuit. Control signal for the multiplier in the expander is derived in a similar manner to that used in the compressor, the filter 12 corresponding to the filter 7 in the compressor and being substantially the same. The filter 14 is included to prevent cross-modulation, and comprises the circuit of Fig. 11 in which a longtailed pair amplifies the input signal and negative feedback is applied over the long-tailed pair via a third valve, the negative feedback being most effective at the frequency (1270 c/s.) of the series LC circuit and at high levels where the diodes OA202 are conducting. The multiplier, Fig. 13, comprises a circuit in which the transfer from the input transformer T1 to the output transformer T2 is varied by varying the impedance of the Zener diodes with the control current applied to the slider of potentiometer P. The impedance of the diodes being reflected through transformer T2 and appearing in series with the secondaries of transformers T1 and T3.