IMPROVEMENTS IN OR RELATING TO DATA TRANSMISSION APPARATUS

1,213,569. Digital transmission systems. INTERNATIONAL BUSINESS MACHINES CORP. 19 Aug., 1968 [15 Sept., 1967], No. 39593/68. Heading H4P. In a data transmission system the transmitter includes means for determining the ratio of the number of data bit changes : the number of data characters in a stream of data to be transmitted, and for inserting a number of synchronizing bits in the data word stream if the ratio becomes so low that the receiver might lose synchronism. In the embodiment described the ratio must not fall below 1 : 16 over 96 characters. The data bits pass to a unit which delivers a pulse at each bit transition. The transitions are counted in a reversible counter of capacity 6. A second reversible counter, of capacity 16 Î 6 = 96, is stepped forward at character rate by clock pulses, but when neither it nor the transition counter is at zero it is stepped back by 16 in the duration of each character. Under these same conditions the transition counter is stepped back once per character. If the character counter ever becomes full when the transition counter is empty, i.e. if the ratio falls below 1:16 over any period of 96 characters, 6 sync. bits are inserted in the data character stream to be transmitted in order to maintain the ratio of the number of bit changes transmitted : the number of data characters transmitted equal to. 1 : 16.