Patent GB-1213359-A (GB)
Title: Improvements in and relating to a facsimile system
IPC: H04N1/00
Filed: 1968-11-29
Abstract: 1,213,359. Facsimile systems; digital transmission systems; transistor gating circuits. INTERNATIONAL SCANATRON SYSTEMS CORP. 29 Nov., 1968 [21 Dec., 1967], No. 56798/68. Headings H3T, H4F and H4P. The bandwidth of a facsimile transmission system is reduced by converting two-level signals representative of the black and white levels into three-level signals having half the repetition frequency. One level of the original signal is represented by the median level and the other level of the original signal is represented alternately by the maximum and minimum levels of the three-level signal. In order to achieve the desired bandwidth reduction the level of the two-level signal having the shorter pulse lengths must be represented by the median level of the three-level signal. As shown, Figs. 1 and 2, the output of a document scanner 1 comprises an amplitude modulated subcarrier, the upper waveform shown being obtained with slow scanning and the lower waveform being obtained with fast scanning, which is demodulated and smoothed in 2 and 3 to obtain two-level signals representing the copy being scanned. These signals are shaped by Schmitt trigger 4 and applied to bi-stable 5 which triggers on the leading edges. The outputs of Schmitt trigger 4 and bi-stable 5 are combined in a network 6 to produce a three-level signal in such a way that a black signal produces the median level output and a white signal produces high and low level outputs alternately depending on the state of bistable 5. The resulting signal has half the repetition frequency of the original two-level signal and is used to modulate a subcarrier which is applied to a communication channel 9. At the receiver the subcarrier is detected and filtered in 12 and 13 to the three-level signal which is applied to a network 14 to reconstitute the two-level signals which are used to remodulate a subcarrier which is applied to a recorder 16. The network 14 and remodulator 15 are combined in the form of a shunt modulator, Fig. 3 (not shown), comprising input and output transformers for the carrier signal, connections between which are bridged by a diode network comprising a series connection of two shunt arms each consisting of a pair of parallel connected oppositely poled diodes. The three-level input signal is applied to a centre tap of the primary of the output transformer and the common point of the shunt circuit is connected to a variable bias supply which is adjusted so that with median level signal input both diode shunt arms are cut off to provide maximum carrier signal output and when the three-level input deviates above or below the median level one diode in each shunt arm conducts thus shunting the primary of the output transformer to cut off the carrier output. A suitable three-level combining network 6, Fig. 11, is arranged so that the output of the bistable 5 is passed through emitter followers 34 and 58 and appears on output line 64 to provide the high and low signal outputs during the intervals when the input from Schmitt trigger 4 is down. When this latter input goes up transistor 48 conducts and the bi-stable signal is shunted from the output which is driven to the median level by a voltage obtained from emitter follower 40 by suitable adjustment of potentiometer 44.