Patent GB-732446-A (GB)
Title: Improvements in or relating to carrier telephone systems
Inventor: HALSEY REGINALD JOHN
IPC: H04J1/04
Filed: 1952-04-16
Abstract: 732,446. Carrier multiplex systems. POST. MASTER GENERAL. April 10, 1953 [April 16, 1952], No. 9603/52. Class 40 (4). In a carrier multiplex system employing quadrature type single-sideband modulator units and demodulator units, each channel is provided with means for effecting transmission of carrier frequency over the channel in response to direct-current signals and with a receiver comprising conversion means whereby the carrier frequency received is converted into direct current having a constant polarity independent of phase differences between the received carrier and the local carrier applied to the demodulator unit in the receiver. As shown, Fig. 1, audio-frequency speech signals are fed from a two-wire circuit 1 through a hybrid transformer 2, a compressor 5, and a lowpass filter 6 to a quadrature type single-sideband modulator unit 7. This comprises two phase-shifting networks 8 and 9, the phase shift through network 8 differing from that through network 9 by approximately 90 degrees, and two modulators 10 and 11 which are connected to carrier supplies 12 and 13 differing in phase by approximately 90 degrees. - The outputs of the two modulators are combined at 15, producing a single sideband with the unwanted sideband and the carrier substantially suppressed. Signalling and dialling are effected by transmitting the carrier, and this is brought about by the application of a D.C. signalling control voltage to a lead 14 so as to unbalance the modulator 10. Alternatively both modulators may be unbalanced or the carrier may be injected at a point following the modulators. A number of channels are combined at 23 to form a sub-group which is transmitted by wire or radio to the input 24 of the receiving equipment, Fig. 3, which comprises a number of demodulator units 29 equel to the number of channels. Each demodulator unit 29 includes two modulators 30, 31 and two phase-shifting networks 32, 33 similar to the networks 8, 9 in the transmitting circuit. Local carrier supplies 34 and 35, differing in phase by approximately 90 degrees and having as nearly as possible the same frequency as the carrier of the wanted channel, are connected to the two modulators 30 and 31, and the demodulator unit 29 produces a single-sideband output in the same way as the modulator unit 7. A low-pass filter 37 removes all except the wanted channel, which passes through an expander 38 and a hybrid transformer 2<1> to the two-wire speech circuit 1<1>. The received carrier, corresponding to signalling and dialling, produces outputs of D.C. or low-frequency A.C. from the modulators 30 and 31, and these are fed through devices 41 and 42, which give a unidirectional output, and combined at 43 to perform signalling or switching functions in known manner. The devices 41, 42 may be full-wave rectifiers but are preferably devices, such as a known type of magnetic amplifier, which produce an output proportional to the square of the input. One object of the invention is to avoid the use of channel filters and discriminating signal receivers.