GB874701A - Improvements in or relating to communication systems employing compandors - Google Patents

Abstract

874,701. Automatic volume control. STANDARD TELEPHONES & CABLES Ltd. April 20, 1960, No. 13777/60. Class 40 (4). [Also in Group XL (c)] The effective transmission equivalent of a channel between an instantaneous amplitude compressor and expander having inverse networks is chosen to give the compressor and expander complementary characteristics, temperature control devices being provided at each end of the channel to compensate for changes in the effective transmission equivalent arising from the use of temperature dependent non- linear resistors in the compression and expansion networks. In a two-way carrier system the transmit channel comprises a compressor as shown in Fig. 2 with shunt impedances 24, 25 across the output of a transistor 22. Output from the compressor feeds a modulator bridge 57 having a local oscillator 9, as shown in Fig. 4. The receive channel comprises an amplifierdetector with the two transistor stages 67, 68, and a full-wave detector 69, 70, as shown in Fig. 4. Automatic gain control is effected by varying the base-emitter impedance of stage 67 by a rectified signal fed back from the detector. The output of the detector over terminals 89, 90 feeds an expander, as shown in Fig. 3, with an expansion network 43, 44, in the emitter circuit of transistor 41, the output of which is shown connected to part of a hybrid 46. Temperature compensation is effected from a potential chain 59, 60, 61, 62; 62 being a block thermistor with a negative impedance coefficient and a Zener diode 64 biased into breakdown stabilizing the voltage across the chain, see Fig. 4. Separate resistors 65, 66 supply bias to modulator 7 and the amplifier detector to govern the modulation index and the degree of automatic gain control. The compression network 24, 25, the expansion network 43, 44 and the thermistor 62 at a terminal station are mounted together and should the temperature at a station increase the compression network adds attenuation which is compensated by a decreased bias over lead 17 which restores the modulation index in bridge 57. The level of the transmitted carrier wave is thereby reduced, but operation of automatic gain control restores the transmission equivalent at the far end of the channel before the expander is reached. With temperature increase the expansion network adds amplification which is compensated by reduced bias over lead 18 which stabilizes gain at a lower level in the automatic gain control circuit. The compression and expansion networks are of the kind described in Specification 832,863. The invention may be applied to a voice frequency system.