Patent GB-600272-A (GB)
Title: Improvements in or relating to multichannel radio broadcasting systems
IPC: H04J3/08
Filed: 1945-04-07
Abstract: 600,272. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. April 7, 1945, No. 8659. Convention date, April 7, 1944. [Class 40 (v)] In a multichannel radio broadcasting system, a single carrier frequency is radiated from an omni-directional aerial and modulated by interleaved trains of pulses representing separate programmes, the pulses of one channel being given an identifying characteristic distinct from those of the others for synchronizing radio receivers to the transmission sequence. The identifying characteristic may be the pulse duration or a time (phase or frequency) modulation by a frequency outside the modulation frequency range of the other channels. The channel pulses may be modulated in phase or recurrence frequency. General. The studios 1, 2, 3, N, Fig. 2, providing the channel-modulating signals are connected through switches 30, 37, 38, &c. to pulse phase modulators such as 32 supplied with a base sine wave of pulse recurrence frequency from source 35 through phaseshifters t2, t3, t4, tn, except in the case of channel 1. The modulators feed pulse width shapers 34, 42, 43, 44, &c. connected to a common carrier translator 50 and omnidirectional aerial 12 of the type described in Specification 538,036. An alternative system, Fig. 7 (not shown), utilizes pulse recurrence frequency modulation. Pulse modulation. One form of time modulator 32a, Fig. 3, is fed with the base wave through a phase-shifter 36a, producing the correct phase-shift to interleave the channel pulses with those of other channels, and comprising a condenser resistance network C, R1, R2. The base wave output is fed to the primary 72 of transformer 73 and the modulating signals to primaries 83, 84 from input 82. The secondaries 74, 75 feed valves 76, 77 connected to effect full wave rectification of the input wave and producing a cusper wave. The cusp time positions are modulated in pairs by the modulating signal, in push-pull manner towards and away from each other, decreasing or increasing the time interval between successive cusps, Fig. 4 (not shown). The pulse shaper 34a clips the modulator output between two selected levels converting the cusps into substantially rectangular pulses. It comprises two oppositely directed and biassed diodes 101, 102, the bias voltages being set by potentiometers 107, 109 which control the width and position of the pulses. Synchronizing signal. In the embodiment of Fig. 2, channel 1 is used as a synchronizing channel, switch 30 being left open and pulse width shaper 34 being set to produce wider pulses 1, curve 6a, Fig. 6, than those of the other channels 2; 3, &c. Channel receiver. At the receiver, Fig. 5, the pulses in the synchronizing channel are selected and delayed to form gating pulses for the selection of the pulses of a desired channel. The received signals, curve 6a, Fig. 6, are detected at 120 and the resulting video pulses, curve 6b passed to the synchronizing channel selector 122 of the type described in Specification 592,789, the output from which, curve 6c, is delayed at 125 and mixed at 126 with the channel pulses to produce curve 6d. The mixer is responsive only to pulses above a predetermined threshold level 127 so that only the channel pulses which coincide with the gating pulses are passed to the demodulator 130. The resulting amplitude modulated pulses are applied to filter 132, the selected envelope energy of the pulses being fed to the loudspeaker 134. The demodulator 130 may be of the type shown at 130a, Fig. 5A. In an alternative receiver, for use with transmissions having synchronizing signals of the type comprising a modulation of the pulses of one channel by a special frequency, the demodulated and filtered synchronizing frequency is used to control the frequency of a control wave generator 143, Fig. 5A. Generator 143 controls a gating pulse generator 144, the output from which is applied through a D.C. restorer circuit 150 to one of the grids of a biassed back mixer valve 142. To another grid of valve 142 are applied the signal pulses from detector 120. Pulses from the selected channel pass through a phase reverser 151 to the control grid of a demodulator 154 of the type described in Specification 600,291. The connection of a tuned circuit 156 to another grid of the valve preferably tuned to a harmonic of the cadence frequency (period T, curve 6b, Fig. 6) causes the output pulses from the valve to be amplitude modulated in accordance with the time modulation of the pulse input. A low-pass filter 160 passes on the modulation frequencies defining the signal envelope and the synchronizing frequency is selected by a filter 161. A signal channel is selected by mixer 170 fed with signals from detector 120 and gating pulses from generator 144, delayed appropriately by device 145. Demodulation of the selected channel pulses may take place in circuit 184 which is also fed with a suitably phased demodulating wave provided by a harmonic generator 180 fed from generator 143, as described in Specification 600,291.