GB1151543A - Improvements in or relating to Electrical Signalling Systems - Google Patents

Abstract

1,151,543. Digital transmission systems. MINISTER OF TECHNOLOGY. 7 July, 1967 [8 July, 1966], No. 30745/66. Heading H4P. Digital data is transmitted by a code in which each of n unique characters is represented by a corresponding unique signal. If two successive characters are identical, a further unique signal is substituted for the signal which represents the second character. The signals are transmitted serially and each lasts one character period. In one embodiment each of a series of input 5-bit characters selects a corresponding frequency (total of 32). The resulting succession of frequencies is passed to a switch. A comparator, fed both direct and via a one-character delay with the input characters, controls the switch such that normally the succession of frequencies passes to the transmission channel. If the comparator detects identity between successive characters the succession is interrupted by the switch and a 33rd frequency replaces that frequency which represents the second of the identical characters. There is thus a frequency change at each character interval. At the receiver 33 filters separate the signals to 33 channels which feed a selection unit. This selects the signal of maximum strength in each character period to prevent errors resulting from possible overlap of signals from adjacent periods. The 33 output leads from the selection unit are connected back to the filters such that when a frequency has been detected the corresponding filter is inhibited for one character period. The leads are also connected to the remainder of the receiver. Those corresponding to the 32 frequencies feed a decoder which generates 5-bit characters and that corresponding to the 33rd frequency controls a switch. The switch is fed, both direct and via a onecharacter delay, by the 5-bit characters. Normally the decoder output is passed direct to the receiver output but when the 33rd frequency is detected the switch is reversed and the delayed signal, which is the same as the character just passed to the output, is passed to the output, thus reconstituting the original train of characters. In a modification the delay and comparator at the transmitter are fed by the 32 frequency channels. The receiver may be modified similarly. A 34th frequency may be used for the third character in groups of three identical characters.