Patent GB-679901-A (GB)
Title: Improvements in or relating to keyed frequency modulation carrier wave communicationsystems
Inventor: EARP CHARLES WILLIAM
IPC: H03K7/04
Filed: 1951-01-10
Abstract: 679,901. Multiplex pulse signalling; frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 10, 1951, No. 707/51. Class 40 (v). A multiplex pulse train comprising a plurality of interlaced trains of position modulated pulses is transmitted over a communication medium by frequency modulating a carrier wave by a control wave having rectangular steps or pulses with a plurality of boundary edges, each corresponding to a pulse of the combined train. In one embodiment with six communication channels, at the transmitter, Fig. 1 (not shown), a multivibrator circuit with two stable positions has the synchronizing pulse 46, graph A, and the position modulated pulses 48, 49, 50, of channels one, three and five, applied to one valve, and the synchronizing pulse 51, graph B, and the channel pulses 53, 54, 55 of the remaining channels, applied to the other valve. The multivibrator output therefore comprises the pulse having leading and trailing edges 56, 57, graph C, corresponding respectively to the synchronizing pulses 46, 51, and the pulses 58, 59, 60 derived from the pulse pairs 48, 53, 49, 54 and 50, 55 respectively. This wave frequency modulates the carrier oscillator so that an oddnumbered pulse is signified by a frequency change from F1 to F2, for example, and an even-numbered pulse by a change from F2 to F1. At the receiver, Fig. 4 (not shown), the wave of graph C is recovered and differentiated to give the alternate positive and negative synchronizing and channel pulses of graph D. These are separated by limiting and inverting circuits into the pulse trains of graphs E and F containing the even and odd channel pulses, respectively, one train being delayed to bring the recovered synchronizing pulses 67, 68 into time coincidence. A coincidence circuit selects the synchronizing pulses, 70, 73, graph G, which are shaped and used after suitable time delays to gate the individual channel pulses for demodulation. Since graphs A, B must have the same number of pulses, when an odd number of channels is to be transmitted, a third synchronizing pulse may be generated, a double .or triple coincidence circuit being provided at the receiver to provide the gating pulse source. The invention may be used in the transmission of the index pulses of the systems described in Specifications 673,354 and 673,805. In one embodiment of Specification 673,805, however, the index pulses can be of either sign and the signs may occur at random. In this case the multivibrator referred to is replaced by an integrating device, Fig. 5 (not shown), which produces, for example, from the synchronizing pulses 114, 115, graph H, and positive and negative pulses 116 ... 121, the stepped waveform, graph J. In this waveform each vertical edge of a step indicates, depending upon its direction, a positive or negative pulse and the carrier frequency is modulated thereby. At the receiver the discriminator is arranged to produce the inverse of the wave, graph J, which when differentiated produces the pulses of graph H.