GB771908A - Improvements in or relating to signal transmission systems - Google Patents

Abstract

771,908. Pulse code modulation systems. WESTERN ELECTRIC CO., Inc. April 5, 1955 [April 26, 1954], No. 9929/55. Class 40(5) In a pulse code modulation signalling system, noise due to quantisation errors is reduced by detecting the difference between the amplitude of a quantized pulse and its true amplitude and adding this error signal to the next succeeding signal pulse before it in turn is quantized. The system can be applied to various coding systems. In one embodiment, Fig. 2, in which the input signal is sampled at 21 and the resulting variable amplitude pulses are quantized at 24 under the control of a timing pulse generator 22, the input to the quantizer 24, and its output are both applied to a subtractor 25 from which the quantizing error signal is passed, via a delay network 26 to an adder 23 so as to be added to the next pulse before it is itself quantized. The adder 2 subtractor are stated to be amplifiers of the kind disclosed in U.S.A. Specification 2,541,276. In a modification, Fig. 3 (not shown) further noise reduction is effected by two error feed-back loops. In a second embodiment, Fig. 5, in which signal changes are transmitted by sampling the input signal at 32 and comparing the samples at 33 with the quantized output, integrated at 34, the difference between the input and output of the quantizer 35 is obtained at a subtractor 41 and this error signal is added at 42 to the comparison signal from integrator 34 so as to be subtracted from the next input signal pulse. In a third embodiment, Fig. 7 (not shown), in which the size of the quanta vary with the signal to be quantized as in Specification 686,583, the quantisation error signal is obtained as before and added to the variable reference voltage which is fed back to be subtracted from the input signal sample pulses. Specification 630,094 also is referred to.