Patent GB-651115-A (GB)
Title: Electric pulse communication system
IPC: H03M3/02
Filed: 1948-11-26
Abstract: 651,115. Pulse code modulation systems; pulse discriminating circuits. STANDARD TELEPHONES & CABLES, Ltd. Nov. 26, 1948, No. 30705. Convention date, Jan. 20. [Classes 40 (v) and 40 (vi)] In a pulse code modulation communication system, pairs of code pulses are generated representing the variations in amplitude of a signal wave, the time interval between a pair of code pulses taking one or other of two fixed values according as the signal amplitude at a time corresponding to a given pair of pulses is, or is not, greater than the amplitude of a decoded approximate replica of the signal wave at the time corresponding to the previous pair of code pulses. Transmitter.-In the embodiment described, a main sine-wave generator 18, having a frequency of 56 kc/s., supplies a cusper circuit 32 ... 36, the output wave 37 of which is shaped in circuit 39 into pairs of rectangular pulses 38, which are fed to the radio transmitter 40, 41. The pulse pairs may assume two different spacings under the control of the signal wave source, such as the microphone 1, and indicate whether the demodulated signal wave provided from pulses 38 is of greater or less amplitude than the signal wave provided from the microphone 1, at regular intervals whose period is that of the generator 18. The pulses 38 are demodulated in circuit 14, which is described later, and the resulting signal wave is applied to the grid of valve 11. The cathode resistance 9 of valve 11 is included in the cathode circuit of valve 4 to the grid of which is applied the signal wave from the microphone after passage through a level control circuit 2. The valve 11 is biassed slightly below cut-off when its grid is at earth potential and the current in resistance 9 corresponds to zero signal on the grid of valve 4. A signal appears at the anode of valve 11 when the amplitude of the signal from demodulator 14 exceeds that applied to valve 4 from the microphone 1. When this condition obtains, the amplifier 16 is allowed to pass a pulse, when gated open by a pulse from circuit 17 controlled by the generator 18, and occurring at the bottom of the sine wave output of the generator. The output pulse 20 is widened in the flip-flop circuit 21 which feeds a phase inverter 23 providing two outputs of opposite polarity which are fed to the balanced cathode follower valves 26, 27. The outputs of valves 26, 27 bias the cusper diodes 35, 36 to reduce the pulse spacing of pulses 38. When the demodulated signal amplitude does not exceed that applied to valve 4 the pulse pairs are of normal spacing. Receiver.-The radio receiver 43, Fig. 2, feeds pulses 44 to a pulse shaper 45, providing rectangular pulses 48 which are fed to the demodulator 81. The latter is similar to circuit 14, Fig. 1, and its output passes to the signal reproducer through low-pass filter 82. The pulses 48 are superposed in valve 51, upon a sine wave produced by the ringing circuit 49 and appear, after clipping and inversion in circuit 52, as pulse pairs 53, 54, of two different amplitudes depending on their spacing. The amplitude discriminator circuit, comprising valves 57, 58, directs pulse pairs of one spacing to shaper 64 and valve 68 and the others to shaper 65 and valve 69. Valves 68, 69 are series-connected and, when rendered conducting, share the negative and positive charges of condensers 77, 76, respectively, with that of a main condenser 78. The latter with the shunt resistance 80 has a discharge time constant of the order of half a second, and across it is built up a step wave 83, Fig. 1, simulating the original modulating signal wave. The pulses 53, 54 are applied to the grids of both valves 57, 58. Those of lower amplitude cannot pass the biassed-back valve 57 but appear at the anode of valve 58 which is connected to shaper 64. The pulses of higher amplitude appear at the anode of valve 57 and are passed to shaper 65 and also to the suppressor grid of valve 58 which is thereby cut off. The fidelity of the system at different frequencies and amplitude ranges is discussed in the Specification. Specifications 600,244 and 627,462 are referred to.