GB436635A - Improvements in the control of attenuation in electric signalling systems - Google Patents

Abstract

436,635. Wired wireless systems. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. March 28, 1935, Nos. 9683 and 9684. Convention dates, March 28, 1934 and Sept. 6, 1934. [Class 40 (v)] The frequency characteristic of an attenuation network particularly in a multiplex system in which the signals are of different frequencies, is automatically controlled by means operated in accordance with a function of the amplitudes of two or more pilot frequencies present in the output of the network. The transmission path may also include a variable attenuator which is independent of frequency and is controlled in accordance with the level of one or more of the pilot frequencies. In the system shown in Fig. 1, the signals pass successively through an attenuator A having a variable frequency characteristic and an attenuator B having the same attenuation for all frequencies, and after amplification at V, the pilot frequencies are separated out at D1, D2, and rectified at G1, G2, to set up voltages across R1, R2. The difference between these voltages is used to control A and one of the voltages is used to control B. The network A, Fig. 2, comprises a potentiometer consisting of resistance RA, a small condenser C, the impedance of which varies considerably over the frequency range, a relatively large blocking condenser C1 and a triode VA, the bias of which is controlled by the differential voltages set up over R1, R2. The output is taken from RA. The pilot frequencies lie just above and below the signalling band. Normally the control voltages are equal, but if due to conditions in the signal path, either pilot frequency predominates, the bias applied to VA is controlled to effect compensation by varying the impedance of condenser C and triode VA in series. The attenuator B is similar to A but condenser C is omitted so that the attenuation is independent of frequency. The bias of VB is controlled by the voltage on R1 due to the high pilot frequency but it may be controlled by the sum of the high and low pilot frequency voltages. In order that the attenuators A, B may be entirely independent, the frequency which controls B should not be varied in attenuation by A, and for this purpose a series of pads, Fig. 5, controlled by relays (contacts u) operated in accordance with the control voltages on R1, R2, may be used for the attenuator A. Each pad has alternative paths, one N1 purely resistive and the other N2 having frequency characteristics which vary from pad to pad but which have the same attenuation as N1 for the frequency which controls B. Delay in the operation of the attenuators may be obtained by replacing valves VA, VB, by resistances of high temperature coefficient, e.g. uranium oxide, heated by the control voltages. In a path containing a number of repeaters, a constant frequency network may be used at all or a large number of repeaters, whereas a variable frequency network is provided at a small number of repeaters.