Patent GB-650886-A (GB)
Title: Improvements in or relating to electric pulse communication system
Inventor: SCULLY CHARLES THOMAS
IPC: H04B1/64
Filed: 1947-12-19
Abstract: 650,886. Multiplex pulse signalling pulse generating circuits. STANDARD TELEPHONES & CABLES, Ltd. Dec. 19, 1947, No. 33618. [Classes 40 (v) and 40 (vi)] In a pulse modulation communication system the modulator and/or demodulator have non-linear characteristics such that signal amplitude compression or expansion is combined with the modulation or demodulation of the train of pulses. The type of pulse modulator in which a saw-tooth triggering wave and the signal voltage are applied to a multivibrator, may be used, the triggering wave being of the form 5, Fig. 1. The multivibrator is designed to emit pulses 2 at the points where the normal bias level 1 cuts the sloping portions of the wave, when the signal voltage is zero. A signal voltage which shifts the bias level to position 3 causes the pulse 4 to be emitted earlier. Owing to the shape of the curve the time shift of a pulse corresponding to a given change in signal amplitude is greater for smaller amplitudes than larger. For a double pulse system, the triggering wave is of the form shown in Fig. 2. For producing a wave of the form shown in Fig. 1, a circular or linear trace on a cathoderay tube screen may be focused, through a variable density screen, on to a photo-electric cell the screen being graded circumferentially or transversely in such a way that the photo cell output is of the desired waveform, Figs, 3, 4 and 5 (not shown). Alternatively a sine wave oscillator 23, Fig. 6, synchronizes a saw-tooth oscillator 24 feeding a mixer 27, which is also supplied from the sine wave generator through a phase adjuster 25 and attenuator 26. The resultant wave from the mixer 27 is of the form shown in Fig. 1, when the two inputs to the mixer are adjusted to be in phase. A triggering wave suitable for a double pulse system may be produced by mixing a sine wave with its third harmonic so phased as to give the wave shown in Fig. 8. Over a moderate range on either side of the points 34 this wave will produce the desired type of compression. In a further arrangement the wave of Fig. 1 is produced by reducing, by about six decibels, the fundamental component of a saw-tooth wave having a straight sloping edge. The saw-tooth is generated by the gas-filled triode 34, Fig. 9, synchronized from a source connected at terminal 48. Its output is amplified by triode 35, in the anode circuit of which is a series resonant circuit 36, 37, shunted by resistance 38, which reduces the amplitude of the fundamental by the desired amount. A pulse modulator suitable for use with the triggering wave of Fig. 1 comprises a resistance bridge 63 ... 66, Fig. 10, to one diagonal of which is fed the triggering wave from terminal 59, and the signal wave is fed through transformer 67 to the other diagonal. The combined voltage across resistance 66 is applied to a biassed off rectifier 68, which conducts when the input overcomes the bias, and is connected to an amplifier 77. A biassed shunt rectifier 79 limits the input to a level exceeding that passed by rectifier 68 and duration modulated pulses are thus formed at the amplifier input. The amplifier may be designed to differentiate the pulses and select the resulting leading edge phase-modulated pulses. For demodulation, a synchronizing signal may accompany the phase modulated pulses. This is selected to synchronize a generator producing the triggering wave of Fig. 1, which is mixed with the signal pulses to convert them to amplitude modulated pulses, Figs. 12 and 13 (not shown). For demodulating duration modulated pulses applied to terminal 109, Fig. 14, they are fed to three filters 111 ... 113. Filter 111 is the usual low-pass filter producing the compressed signal at terminal 114. Filter 112 and amplifier 116 provide a harmonic of the pulse recurrence frequency, of constant amplitude, unmodulated by the signal. Filter 113 provides the same harmonic, amplitude-modulated by the signal. The combined output of these last two circuits, rectified by rectifier 120, provides a voltage proportional to signal amplitude which is added to the compressed signal from filter 111. For the production of amplitude modulated pulses having amplitude compression, the duration modulated pulses from Fig. 10 are applied to valve 127, Fig. 16, and thence through rectifier 139 to charge condenser 141 connected to the grid of valve 142. At the end of the charging period a pulse from a train of pulses applied to terminal 153 is applied to the suppressor grid of valve 142 to give an output pulse of amplitude dependent upon the potential of condenser 141. A pulse, delayed by circuit 154, subsequently discharges the condenser which is prevented from charging negatively by the shunt rectifier 156. In order to allow time for the operation of valve 142 and the discharge of condenser 141, the depth of modulation of the duration modulated pulses must be suitably limited. Alternatively, the duration modulated pulses are produced only during alternate periods of the triggering wave and full use of the signalling time may be made by duplicating the circuits to provide two interleaved channel pulse trains. The demodulation of the amplitude modulated pulses thus produced is accomplished by a similar circuit, Fig. 17, in which the signal pulses are applied to terminal 125 and the triggering wave of Fig. 1, to terminal 153. The valve 142 then delivers expanded duration modulated pulses to the amplifier 151 and low-pass filter 157 to reproduce the signal at terminal 152. The suppressor grid bias of valve 127 is automatically adjusted to maintain constant the amplitude level of the incoming pulses. For this purpose, the incoming pulses and the triggering wave amplitudes are compared in valve 158, and when the former exceed a predetermined value, a pulse is fed through valve 159 to increase the charge in condenser 177 connected to the suppressor grid of valve 127. The invention may be applied to a pulse multiplex communication system, a single modulator being fed with the triggering wave, controlled by the output of a frequency multiplier fed from a master pulse generator, and successively gated portions of the channel signals. The gating circuits are controlled by pulses derived from the master pulse generator through tappings on a delay network, Fig. 18 (not shown).