Patent GB-668884-A (GB)
Title: Improvements in and relating to pulse code modulation signal transmitting systems
Inventor: LEVY MAURICE MOISE
IPC: H04L25/24
Filed: 1949-09-06
Abstract: 668,884. Multiplex pulse code radio signalling. GENERAL ELECTRIC CO., Ltd. Aug. 30, 1950 [Sept. 6,1949], No. 23093/49. Class 40 (v). In a repeater station for a modulated carrier pulse code signalling system, in order to improve the signal/noise ratio, pulses occurring during a part only of each pulse interval are selected, by gating means, from the high-frequency or an intermediate frequency signal and re-shaped so that each pulse comprises a train of oscillations extending beyond the time limits of the corresponding pulse interval. In the multiplex system described, four radio frequencies are used, each carrying a group of interlaced pulse code channels. At the repeater station. Fig. 3, the superheterodyne receiver 10 including local oscillator 11, feeds I.F. signals through amplifier 12 to valve 13. In the anode circuit of valve 13 are tuned circuits LC1 ... LC4 tuned to the four intermediate frequencies and each coupled to a separate gating circuit G1 ... G4. The outputs of the latter are fed through limiters LM1 ... LM4 to band-pass filters BPF1 ... BPF4 respectively, the outputs of which are combined and fed to the I.F. amplifier 17. The retransmitted radio-frequencies, differing slightly from the received bands, are produced in the modulator 18 which is fed with I.F. signals from amplifier 17 and another carrier derived from mixer 19 to which are applied inputs from oscillator 11 and another local oscillator 20. A suitable component of the output of modulator 18 is fed to the radio transmitter 21. The gating circuits G1 .... G4 are controlled by gating pulses which are of shorter duration than the pulse intervals and are derived from synchronizing pulses, transmitted in intervals between channel pulses, and of greater amplitude than the latter. These synchronizing pulses are derived from the demodulator 22 and separator 23 and fed to the gate pulse producer 24 which comprises a ringing oscillatory circuit followed by a frequency multiplier, peak selector and squarer Fig. 6 (not shown). In an alternative circuit, for generating gating pulses, which avoids the need for synchronizing signals, a tuned circuit 30, Fig. 7, is fed with the I.F. pulses which after amplification in valve 13 are applied to tuned separating circuits as in Fig. 3. From one of these circuits, LC1, feeding gating circuit G1, I.F. pulses are fed to detector diode 36, the resulting D.C. pulses across resistance R1 being also amplified by valve 13 and passed to valve 32. The tuned anode circuit, 33, of valve 32 develops a sinusoidal oscillation at the pulse interval recurrence frequency which is converted into rectangular gating pulses applied to the gating circuits G1 &c. over conductor 14. In order to ensure the presence of at least one pulse per channel sample to excite circuit 33, the binary numeral zero may be omitted from the transmitted signals. When the trains of oscillations applied to circuit LC1, and representing pulses, overlap, in order to prevent interference with the maintenance of oscillations in circuit 33, the radio frequency oscillations are maintained in the same phase throughout the transmission. This is ensured by having a continuously running oscillator at the transmitter feeding a gating circuit which is gated on by the code pulses to be transmitted and whose output is fed through a band-pass filter and amplifier to the transmitting aerial, Fig. 8 (not shown).