Patent GB-657681-A (GB)
Title: Improvements in and relating to pulse code modulation systems
Inventor: LEVY MAURICE MOISE
IPC: H03M1/00
Filed: 1948-01-26
Abstract: 657,681. Multiplex pulse code signalling. GENERAL ELECTRIC CO., Ltd. Jan. 26, 1948, No. 2301. [Class 40 (v)] A signalling system uses pulse code modulation to convey coarse information regarding a magnitude, and amplitude, phase or duration modulation of a recurrent pulse to convey fine information. As described in a multiplex system, in each channel the repeated groups of code pulses are transmitted simultaneously, using different radio frequencies and two other simultaneous recurrent pulses, of other radio frequencies, are modulated to convey finer information, the channel pulses being interlaced. In a circuit for handling the odd-numbered channels for a 20-channel system, the channel signals are applied to a known distributer and time modulator TM, Fig. 2, to produce time - modulated pulses such as P1, P3, Fig. 1 (a), shown in channels CH1, CH3, respectively. All these pulses are applied to gating circuits GA, GB and GC to which are applied gating waveforms A, B, C, Figs. 1 (d), (c) and (b) from generators MA, MB, MC, respectively. The B pulses are of twice the frequency of the A pulses and the C pulses twice that of the B pulses. Each gating circuit comprises a valve VI, Fig. 3, to the control grid T1 of which are applied the time-modulated pulses and to the suppressor grid T2 of which are applied the gating pulses. A pulse is repeated at the anode, if and when both input pulses are present simultaneously, and shock excites the resonant circuit LC. The first positive half-wave from the latter renders valve V2 conducting, to charge condenser C1, connected between its cathode and earth. The cathode potential, Fig. 1 (f), remains positive until a pulse, Fig. 1 (g), applied to the cathode T3 of a diode D subsequently discharges condenser C1. The cathode of valve V2 is connected also to the control grid of valve V3 so that if it is positive when a pulse, Fig. 1 (e), occurs in the time interval CH2, this pulse is repeated at the output terminal T5. The output terminals T5, T6, T7 of the gating circuits GA, GB, GC are connected respectively to radio transmitters RTA, RTB, RTC feeding different radio frequencies fl, f2, f3 respectively to the aerial AE. The three code pulses thus available define the position of the pulses P1, P3, &c. to the nearest half-cycle of oscillation C. Two more time modulated pulses are transmitted on radio frequencies f4, f5 from transmitters RTD, RTE, the first of which occupies one of ten possible positions within the said half cycle and the second of which can represent any position within one-tenth of the half-cycle. These pulses are produced by circuits TM1, TM2, respectively, similar to the circuit of Fig. 3, which however produce amplitudemodulated pulses which are subsequently converted to time-modulated pulses, in known manner, for application to the radio transmitters. Circuit TM1 superposes the pulses from circuit TM on a stepped wave, Fig. 1 (k), applied at terminal T8 and whose period equals a half-cycle of wave C. This stepped wave is produced by the addition of the saw-tooth waves, Figs. 1 (i) and (j), and results in the production of condenser charges having one of ten discrete amplitudes. Circuit TM2 superposes the pulses from circuit TM on a sawtooth wave, Fig. 1 (h), applied at terminal T11 and whose period is one-tenth of a half-cycle of wave C. The subsequent conversion of the condenser charges to time-modulated pulses is under the control of a saw-tooth wave of period equal to the channel period and having the cycles occurring in the odd channel periods suppressed. The modulation of the fine information pulses may alternatively be amplitude or duration modulation.