Patent GB-679737-A (GB)
Title: Improvements in or relating to signal transmission systems using pulse code modulation
IPC: H03M1/00
Filed: 1949-12-21
Abstract: 679,737. Thermionic distributers. WESTERN ELECTRIC CO., Inc. Dec. 21, 1949 [Dec. 24, 1948], No. 32718/49. Class 40 (iii). [Also in Group XL (c)] In a pulse code modulation transmitter (see Group XL (c)), the continuous outputs of five coded signal channels from valves 611, 612, 613 and two others, Fig. 7 (not shown), are converted into a code group of five successive pulses, being sampled in succession by a distributer, to which they are applied with different delays, so that each pulse group comprises successive pulses corresponding to the same signal amplitude. For this purpose delay lines 621, 622, 623, &c. are arranged in the output circuits of valves 611, 612, 613, &c. and the delayed signals are passed through valves 611, 671, 662, 672, &c. and switches 641, 642, &c. to the gating valves 811, 912 ... 915. The master oscillator 410, operating at group recurrence frequency controls the local oscillator 412. The latter synchronizes a multivibrator square pulse generator 413, the pulses from which are further squared in valves 416, 417, 418 and fed to output valves 419, 420, 421 which supply pulses of suitable polarity, and suitably timed by networks 450, 460, 461, to control the remainder of the equipment. The output from network 460 is fed to a code element timing circuit, the input valve 803 of which feeds pulses to a valve 822 in the cathode circuit of which is a ringing circuit 825, 826 tuned to five times the pulse recurrence frequency. The sine wave output from resistance 827 is squared in limiter valves 823, 824 and fed to the output valve 830. Successive positive gating pulses for application to the screen grids of valves 811, 912 ... 915 are derived from the anodes of valves 821, 922... 925 respectively, the grids of these valves being driven negatively in succession. They are connected respectively to the cathode condensers 841, 942 ... 945 of valves 851, 952 ... 955 which are simultaneously charged positively by the application of a positive pulse to the valve grids derived from valve 830, differentiating circuit 804 and valves 840, 850, and occurring once per code element period. The cycle of gating pulses is initiated by a positive pulse from network 450, differentiated and delayed by circuit 861 and applied to valve 831, which is rendered conducting to discharge condenser 841. The valve 821 is thus rendered non-conducting to apply the gating pulse to the screen grid of valve 811, and a positive pulse from its cathode which is fed through the differentiating delay circuit 962 to the grid of valve 932. Just prior to valve 932 becoming conducting, condenser 841 is recharged to terminate the first gating pulse and condensers 942 ... 945 recharged to make up for any leakage losses. Upon valve 932 becoming conducting condenser 942 is discharged and a gating pulse applied to valve 912 by valve 922. In a similar way successive gating pulses are applied to valves 913, 914, 915 to complete the code group cycle. Depending upon whether coded signal channels are producing positive or negative outputs at the termination of delay networks 621, 622, 623, &c. respectively, the valves 811, 912...915 respectively produce output pulses or do not produce output pulses when the gating pulses are applied to their screen grids. These output pulses across the common anode resistance 918 are fed to a gating circuit allowing short pulses derived from valve 830 of the code element timing circuit to be fed to a radio transmitter, Fig. 11 (not shown).