Patent GB-1204180-A (GB)
Title: Improvements in or relating to two-level video-representative signal encoding and decoding systems
IPC: H04N1/415
Filed: 1967-11-16
Abstract: 1,204,180. Facsimile communication bandwidth reduction. SOUTHERN PACIFIC TRANSPORTATION CO. 16 Nov., 1967 [16 Nov., 1966], No. 52260/67. Heading H4F. In a system for encoding two-level videorepresentative signals, e.g. black and white facsimile signals, derived by scanning simultaneously a plurality of lines of data, there are transmitted two signals, the first representing a "typical" " line of data and formed of black when black occurs in any line, and white in the intervening intervals, and the second representing which of the lines are most nearly like the " typical " line. In a receiver the binary black and white signal representing the " typical " line is re-established from the first signal and applied to plural gates corresponding to the scanning lines, and the gates are made transmissive in responsive to the second signal only for those lines which are most nearly like the " typical " line. The gate outputs control the reproduction of the data. In Fig. 1 photo-cells 10, 12, 14 scan three lines of a document and have their outputs applied through clipping circuits, which establish two signal levels corresponding to black and white. The outputs pass to an OR gate 22, which produces an output whenever black occurs in any line, and also to different circuits 24, 26, 28 which also receive the output of the OR gate. The difference circuits produce an output whenever a scanned area is white whilst an output occurs from the OR gate. If the output of gate 22 is regarded as a " typical " line, the circuits 24, 26, 28 which do not produce an output indicate that the associated line is the same as the typical line. The outputs from circuits 24, 26 and 28 are integrated in circuits 32, 34, 36 and applied to threshold detectors 40, 42, 44. Operation proceeds cyclically under the control of sampling generator 38 which periodically resets the integrating circuits to zero and also actuates an encoder 30 which forms a signal representing the sequence of black and white signals produced by gate 22 during the sampling interval. The encoder may produce a single analog signal or any other form of signal which requires less bandwidth than the originating signal. Threshold detectors 40, 42, 44 may be set to produce an output when the difference and integrating circuits indicate that white exists for more than 50% of the sampling interval. Thus detectors not producing an output correspond to lines most nearly like the " typical " line. The parallel outputs of the detectors are translated to a single analog signal by an encoder 46. The signals from encoders 30 and 46 are then transmitted one after the other during a sampling interval by gates 48 and 50 which are controlled by a oneshot circuit 52 triggered by generator 58. Each signal may be allocated half of the interval. Provision is also made to transmit a clock signal from generator 38. At a receiver, Fig. 2, the two encoded signals are separated by an arrangement of gates 64, 66 and one-shot circuit 69 operating in the same manner as the corresponding arrangement at the transmitter. A sampling generator 62 is synchronized by the clock signal. A decoder 68 responds to the first encoded signal to reestablish a signal corresponding to the signal from OR gate 22 and this is applied to the input of three gates 70, 72, 74. Analog to digital converter 76 responds to the second encoder signal and forms parallel output-signals corresponding to the outputs of threshold detectors 40, 42, 44. These output signals provide second inputs for gates 70, 72, 74 and function such that a gate becomes transmissive where the corresponding threshold detector produced no output. The signal representing a typical line is thus transmitted where the original lines are most nearly like the typical line. Nothing is transmitted for the other lines. The gates control the reproduction of the data, e.g. by styli 77, 78, 80. The reproduced data differs from (or is degraded with respect to) the original data but is said to be adequately readable.