GB1250492A - - Google Patents

Abstract

1,250,492. Digital facsimile transmission systems. XEROX CORP. 26 July, 1968 [27 July, 1967], No. 35821/68. Headings H4F and H4P. Non-synchronous binary data, e.g. from a facsimile scanner, is converted to 3-level form by means alternately passing either the data itself or its negative inverse to an output for transmission. The probability of the true data being transmitted is preset. In one embodiment, Fig. 1 (not shown), binary data A is fed to a first gate (14) and its negative inverse B to a second gate (16). The gates are enabled alternately by D, E at randomly-selected ones of the positive-going transtion points in the data A. The combined output of the gates is a duobinary signal C. The selecting signals may be derived from a noise generator 80, Fig. 7, whose output passes via limiter 81 direct to gate 83 and inverted to gate 84. These gates are enabled by positive bits in the binary data, and their outputs feed bi-stable 85 controlling gates 87, 88 corresponding to the first and second gates (14, 16) mentioned above. In a second embodiment, Fig. 5 (not shown), binary data signals are fed to two gates (65, 66) enabled alternately at randomly-selected positive-going transition instants in the data by the output of a bi-stable (64). The bi-stable is set and reset by a pulse generator (63) enabled when the data signal is zero and generating pulses at a rate much higher than the bit rate. When the data signal goes positive the bi-stable's current state is retained to maintain one or other gate open. The output of one gate is added to the inverse of the other to give the three-level signal. The transmitted signals may be reconverted to binary form by feeding them to two Schmitt triggers whose outputs are summed.