Patent GB-602849-A (GB)
Title: Multi-channel electric pulse communicating systems
IPC: H04B7/17
Filed: 1945-12-21
Abstract: 602,849. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. Dec. 21, 1945, No. 34766. Convention date, April 7, 1944. [Class 40 (v)] In a multi-channel pulse signalling system, each channel comprises a train of time-modulated pulses, the channel trains being interleaved and further modulated by an identifying signal which is different for different channels. The channels may be different programmes of a broadcasting station and may be rebroadcast from a further station fed through directional relay stations. In one embodiment, at the transmitter, a base wave generator 20, Fig. 1, controls the timing of pulses generated in the channel pulse time modulators 11, 12, 13, &c., suitable phase displacing networks 22, 23, &c, being provided to produce interlacing of the channel pulse trains. The modulated pulses are shaped to widths characteristic of each channel by circuits 16, 17, 18, &c., before mixing and transmission. At the receiver pulse width selectors 31, 32, 33, &c. select the separate channel pulses and are followed by demodulators 36, 37, 38, &c. In a second embodiment, the pulses of each channel are doubly modulated with intelligence in addition to the channel identifying signal. The two sub-channels thus formed may be used for stereophonic sound or television and sound. At the transmitter, the signals of sub-channel 1X, Fig. 9, for example, time modulate pulses in modulator 11, which are given a characteristic width in shaper 16, and then amplitude modulated by the signals of sub-channel 1Y. At the receiver the pulse width selector 210 selects the channel pulse train of characteristic width which synchronize a base wave generator 214, the output of which is frequency multiplied at 190, phase adjusted at 192 and used as a demodulating wave for pulses from circuit 210 passed through peak clipper 212 to demodulating circuit 183 which feeds telephones 222 through low-pass filter 221. For the second sub-channel, e.g. 1Y, the output of base wave generator 214 is adjusted in phase at 223, clipped and shaped at 224 and applied as de-blocking pulses to the selector 226. The output of the latter comprises only channel 1Y pulses, the amplitude modulation envelope of which is filtered at 229 and fed to telephones 230. For selecting channel pulses distinguished by a characteristic amplitude, a clipper 245, Fig. 12, clips the channel pulses 1 ... 5 at a level 245a just exceeding the amplitude of the pulse of the wanted channel such as channel 3, and inverts and amplifies the clipped-off pulses as at 245b. The pulse train is mixed with pulses 245b in mixer 246 to produce pulses 246a which are clipped at level 247a in circuit 247 to yield pulses of channel 3 only. In another embodiment, pulse recurrence frequency modulation is substituted for time modulation in some of the channels and the resulting pulses are amplitude modulated. Moreover, in some channels the channel identifying signal is a tone frequency modulation outside the range of frequencies used for intelligence communication. As described, subchannels 1X and 2X, Fig. 13, use pulse recurrence frequency modulation at 250, 255 respectively and sub-channel 3X uses time modulation. Sub-channels 1Y, 2Y, 3Y all use amplitude modulation of the pulses in their related subchannels 1X, 2X, 3X respectively. The pulses of main channel 1 are pulse recurrence frequency modulated by an identifying tone signal from source 251, while the pulses of channels 2 and 3 are shaped to characteristic widths. A radio receiver 267 for main channel 1 feeds its pulse output to a gating circuit 269 from which channel 1 pulses are fed through peak clipper 277 to the recurrence frequency demodulator 272 and telephones 276 for sub-channel 1X. The gating pulse generator 270 is free running at a pulse period slightly greater than the channel pulse period, but is synchronized to the latter on receipt of the identifying signal by filter 273. Pulses from the gating circuit 269 are also fed through amplifier 291 and filter 292 to select their amplitude modulation envelope forming sub-channel 1Y for telephones 293. Specifications 538,036 and 554,456 are referred to.