Patent GB-905433-A (GB)
Title: Improvements in or relating to telecommunication systems
Inventor: CLARK ADRIAN PERCY
IPC: H04L27/20
Filed: 1958-09-03
Abstract: 905,433. Electric selective signalling systems. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. Sept. 3, 1959 [Sept. 3, 1958], No. 28333/58. Class 40 (1). An electrical signalling system, for the transmission of information in binary form, is characterized in that each signal element comprises the same integral number of cycles of sine wave current, the two binary values being distinguished by differences in phase. To eliminate the effects of large variable phase shifts produced in a signal passing over telephone lines, a basic sine wave is employed and data is transmitted by phase modulation, i.e. successive elements are compared, by the receiver, for a phase change. The Transmitter (Fig. 1).- The square wave output W1, Fig. 3, of a generator O, Fig. 1, triggers a strobe generator S1, at its falling edge, to produce pulses ts1. The information to be transmitted over the telephone lines is read out of a storage device (not shown) as a modulating wave A, which is applied, with the strobe pulse ts1, to a strobe mixing gate SG whose output is waveform tsA, Fig. 3. A toggle DT, changing over each time a rising edge is encountered, transforms wave tsA into form B, which when applied to a bridge ring modulator BRM modulates a sine wave W2, from the generator O, to change or not change phase between successive elements to indicate " 1 or " 0 " respectively, i.e. waveform W3. Wave W3 is transmitted along the telephone wires. The Receiver (Fig. 2).-The incoming wave W3 is applied to an amplifier limiter circuit ALC1 to produce an output W4, Fig. 4, representing in amplified and square waveform a narrow slice of wave W3 taken midway between the positive and negative peaks. Wave W3 is also applied to a delay circuit DC and an amplifier limiter circuit ALC2 to produce waveform W5, closely similar to, and accurately in phase with, W4, but with a delay of one signal element. A phase detector PD compares the phase of W4 and W5 to produce an output W6 which is positive when waves W4, W5 are in antiphase and negative when they are in phase. If no spurious signals or distortions were produced on the telephone lines, wave W6 would be the inverse of transmitter input A. However, since distortions and spurious signals are present and have to be eliminated, waveform W6 is applied to a low-pass filter LP, the output being W7 which, when applied to a slicing circuit SC operating similarly to the amplifier ALC1, produces wave W8. This is applied to output toggle OT, arranged to give a regenerative effect under the control of a strobe pulse ts2 which has to be derived from the incoming signal. Thus, W8 is also fed to a differentiating circuit DCG including a gate, to suppress the negative pulses and to pass the positive pulse W9 to a circuit RC comprising two tuned circuits in cascade, to produce waveform W10 which is phased to rise from zero at each rising edge of the positive-going waveform W9. W10 is squared by circuit SQ to give form W11 which, since SQ operates in a manner similar to ALC1 and ALC2, is not appreciably affected by the varying signal levels of W10. Wave W11  triggers a strobe generator S2 whose output pulses ts2 are arranged to sample waveform W8. The pulses occur in the middle point of the signal elements which they are sampling. Thus, if W8 is negative on the occurrence of a pulse, the output waveform C goes negative and remains negative until the next pulse. Similarly, C goes positive if W8 is positive. The wave C is an exact copy of receiver input A but is somewhat delayed. The inverse waveforms W3, W4, W5, W8 may also pass through the apparatus. Specification 905,432 is referred to.