GB1250924A - - Google Patents

Abstract

1,250,924. Digital transmission systems. STANDARD TELEPHONES & CABLES Ltd. 25 June, 1969, No. 32092/69. Heading H4P. Digital data words transmitted have disparity which varies in a predetermined manner. The receiver determines the variations in disparity expected and compares those with the variations actually received. Lack of identity indicates an error. As described 3-digit words comprising +, 0, and - digits are transmitted, each representing a 4-bit word. Each word has + or 0 disparity until the total disparity of all the transmitted digits reaches + 1 or more, whereupon the words are transmitted inverted, i.e. have - or 0 disparity until the total disparity reaches - 1 or more. This manner of variation of the disparity has the effect that when a word + 1 disparity is followed by a word of - 1 disparity (or vice versa), no words of other than 0 disparity lying between these words, the total disparity after reception of the second unit disparity word is the same as the disparity of that word. At the receiver, Fig. 2 (not shown), each 3- digit word is converted to parallel form to feed a unit generating a corresponding 4-bit signal which is serialized and passed to the output. A counter determines the disparity of each received word and another counter determines the total disparity. The occurrence of successive opposite unit disparity words is detected by monitoring the word disparity count and causes the outputs of the counters to be compared. If they differ, the total disparity counter is reset to the current word disparity count and an error is indicated. A further unit determines the rate at which errors occur. If this rate is excessive it is assumed that word synchronism has been lost and a control signal is then passed to a circuit which derives a word rate clock signal from a digit rate clock signal, the control signal causing the word rate clock signal to be shifted in phase by one digit period in an attempt to restore sync.