GB1292655A - Signal transmission method - Google Patents

Abstract

1292655 Facsimile system; pulse modulation circuits MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 5 Nov 1969 [7 Nov 1968 (4)] 54318/69 Headings H4F and H4L In a facsimile system in which a binary signal (pulse width modulated) signal is transmitted, e.g. corresponding to a scanned document having only black and white markings, to reduce the transmission time the scanning speed is increased (e.g. doubled or trebled) above that normally employed which results in the video signal having a frequency above the maximum picture frequency which can be transmitted by the system and to nullify this drawback the widths of the shorter pulses (corresponding to narrow markings on the document) are increased to decrease the video signal frequency. In addition the widths of the longer pulses may be decreased, both the increase and decrease in widths being effected according to a predetermined programme. As described one set of parallel shift-registers is employed to reduce the bandwidth, the white and black pulse widths being stored alternately, but to decrease the possibility of any errors arising, separate sets of parallel shift registers may be employed. At the receiver the pulse trains are modified in a complementary manner to recover the original signals. In an embodiment described the line scanning speed is increased by n times (e.g. trebled) and the frame scanning speed by n/m times (e.g. 3/2 times) where n is greater than m, only the signals from every mth line scanning (e.g. every other line scanning) being used. Thus the modified signals from each line scan may be extended over two line scan periods. Alternatively, when every line is transmitted, the modified signals may be extended into the blanking intervals.