Patent GB-775918-A (GB)
Title: Signal transmission circuits including compandors
IPC: H04B3/20
Filed: 1955-02-23
Abstract: 775,918. Automatic volume control. WESTERN ELECTRIC CO., Inc. Feb. 23, 1955 [Feb. 24, 1954], No. 5422/55. Class 40 (4). A two-way transmission circuit comprises a transmission channel having a volume range compressor, a receiving channel having a volume range expander incorporating a variable attenuator, and a cross-control circuit responsive to signals in the transmission and receiving channels which controls attenuation in the expander so as to diminish echoes. In the embodiment of Fig. 2, the compressor is shown with a backward acting control of a variable attenuator 12 by means of a syllabic rectifier A. A rectifier B applies a positive voltage to a capacity 26 in the cross control circuit in proportion to the output from the compressor. The expander has a normal forward acting control of a variable attenuator 18 by means of a rectifier C which also supplies negative voltage to the capacity 26. The potential of capacity 26 also controls the attenuator 18 and when outgoing speech from the compressor puts a positive potential on the capacity 26 the attenuator 18 is biased so as to give maximum attenuation to speech received by the expandor. If, however, speech received by the expandor is of sufficient volume for rectifier C to overcome the potential from rectifier B, a break in transmission will occur. To prevent charge leaking from capacity 26 through the rectifiers B and C, cathode followers 21 and 22, with varistors 23 and 24, are used. Potentiometers P in the cathodes are adjusted to give zero potential across capacity 26 under idle conditions. A positive potential from rectifier B will, cause varistor 23 to conduct to keep the grid of 21 at earth potential. Varistor 24 will not conduct so that the grid and cathode of 22 will become positive and a positive potential will be applied to rectifier C to block the leakage path. A negative voltage is similarly applied to rectifier B when capacity 26 has a negative charge. Resistances R1, R2 control the release times of the cross control.. In the embodiment of Fig. 3 a variable attenuator 36 in the expandor is balanced between the cathodes of cathode followers 41 and 42. When speech is received by the expandor, rectifier C makes the grid and cathode of 42 positive with respect to the cathode of 41 so that the loss of attenuator 36 is reduced according to normal self-control of the expandor. When speech is transmitted through the compressor, rectifier B tends to make the grid and cathode of 41 positive to increase the loss in attenuator 36; and if the cathode of 41 becomes positive with respect to the cathode of 42 an additional loss (20 db.), due to resistors 39 and varistors 38, is added. An optimum input to the compressor, while the cathode followers are being balanced, can be determined to obtain a maximum loss for echo signals. In both embodiments the output of the expandor is reduced if the expandor output passes via a hybrid transformer as an echo signal to the compressor because the compressor will increase the loss in the expandor and reduce the expandor output. Specification 527,936 and U.S.A. Specification 2,328,951 are referred to.