Patent GB-669617-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems
Inventor: TREADWELL CYRIL GORDON
IPC: H04J3/10
Filed: 1950-03-21
Abstract: 669,617. Multiplex pulse code signalling. STANDARD TELEPHONES & CABLES, Ltd. March 21, 1950, No. 7016/50. Class 40 (v). A pulse code modulator comprises a device for generating a train of pulses including a first circuit from which the pulses of a code group representing a signal amplitude are derived, a second circuit through which the remaining pulses are diverted, means for blocking the second circuit by the code pulses in the first circuit and means for terminating the generation of pulses when the device has generated a number equal to the number of elements in a code group. As described, the coder receives the amplitude-modulated pulses of a multiplex interlaced channel train derived from a master pulse generator and delay line distributer feeding the separate channel circuits and providing terminating pulses for the coder. A long synchronizing signal also is derived from the master oscillator and interlaced with the channel pulses. A common decoder is provided at the receiver and feeds a delay line channel distributer controlled by the received synchronizing pulse, Fig. 1 (not shown). The amplitude modulated pulses for coding are applied to two parallel circuits 27, 28, 29 and 30, 31, Fig. 2. In the first circuit the amplifier 27 is biased to respond only to pulses of amplitude greater than 16 units, in the case of a fiveunit coder giving 32 possible amplitude steps. The remaining portion of such a pulse is doubled in amplitude and fed back to the input through a 0.2 microsecond delay network 28, isolating rectifier 29, 0.8 microsecond delay network 32 and normally open gate circuit 33. The output pulse from circuit 27 is also fed through an amplifier 35, to a flip-flop multivibrator 36 feeding a code pulse through the differentiator and limiter circuit 37 to the radio transmitter, and a pulse to block the gate circuit 31 in the second parallel circuit. The second parallel circuit comprises the 0.2 microsecond delay network 30 and the normally open gating circuit 31 which doubles the amplitude of pulses applied thereto. Input pulses to the codes having an amplitude equal to or less than 16 units pass through this second parallel circuit and back to the input, after doubling, through the delay network 32 and gating circuit 33. Thus, a code group of pulses is developed at the output of amplifier 27 until the operation is terminated by a pulse applied over conductor 34 to block the gating circuit 33, at the time when the fifth generated pulse reaches it. A decoder comprises five pentodes 74 ... 78, Fig. 3, normally biased to anode current cut-off at their commoned suppressor grids. Their biased back control grids are connected to separate tappings on a delay network 100, to the input of which are applied the received groups of code pulses from conductor 101 so that when the last pulse code element is applied to valve 74 the others are simultaneously applied to the other four valves, respectively, if present in the group. At the same time a pulse is supplied simultaneously to all the suppressor grids from conductor 82, so that those valves which have a pulse on their control grids conduct to contribute current to an output pulse in the common anode transformer 79. The screen grid resistances 95 ... 99 and anode supply potential divider 109 ... 113 are so adjusted that the anode currents are proportional to 1, 2, 4, 8 and 16 units respectively, when the control and suppressor grids are at earth potential. These grids are prevented from rising above earth potential by rectifiers 90 ... 94 and 119. The amplitude of the resultant output pulse is thus proportional to that of the corresponding input pulse at the coder. Specifications 535,860 and 661,020 are referred to. Reference has been directed by the Comptroller to Specification 665,072.