GB583629A - Improvements in or relating to signalling arrangements for electric pulse communication systems - Google Patents

Abstract

583,629. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Nov. 9, 1944, No. 22121. Convention date, Oct. 11, 1943. [Class 40 (v)] In order to signal at a low frequency over a communication system using a pulse train for normal working of recurrence frequency greater than the highest frequency present in the transmitted intelligence, pulses are removed from the train at regularly recurring intervals leaving only pulses which recur at the required low frequency. The invention is described as applied to a multiplex intercalated pulse system in which at the transmitter a master synchronizing wave source 10, Fig. 1, controls pulse modulators 11, 12, 13 for the separate channels. through phase shifters 14; 15, their outputs being applied to a common line or radio link 18. At the receiver, the incoming pulses are applied to a control oscillator 20 and channel selectors 21, 22, 23. The synchronizing control output is applied also to the channel selectors through phase-shifters 24, 25 and the selected channel pulses are fed through condensers 27, 27A, 27B to demodulators 31, 32, 33. For low-frequency signalling over the first channel, for example, the output of modulator 11 is diverted by switch 17 through a frequency divider 16 controlled by the source 10 which provides a wave 30, curve b, Fig. 2, which is added to the pulses 1 of the channel to increase the amplitude of every fourth pulse. The latter pulses only are applied to the link 18 after clipping above the level 31. The frequency division factor is normally much greater than four. At the receiver, the wave 32, curve c, Fig. 2, is added to the pulses for channel selection and a subsequent clipping at level 33 yields the channel pulses only. When these are signalling pulses of low recurrence frequency they are not passed by condenser 27, but are fed through low-pass filter 28 to the signalling buzzer 29. Alternatively, the circuit 28, 29 may be connected to the output of demodulator 31, and instead of providing filter 28, the buzzer may respond only to low-frequency signals and act as a choke to the normal higher frequencies. The other channels have similar switches 17A, 17B, frequency dividers 16A, 16B, filters 28A, 28B and buzzers 29A, 29B, or a single frequency divider with appropriate switching may be used for all the channels.