Patent GB-1118261-A (GB)
Title: Transmission time-bandwidth reduction system
IPC: H04B1/66
Filed: 1966-01-28
Abstract: 1,118,261. Data transmission time-bandwidtn reduction; time compression. INTERNATIONAL STANDARD ELECTRIC CORP. 28 Jan., 1966 [4 Feb., 1965], No. 3901/66. Headings H4F and H4R. In a time-bandwidth reduction system for time-based multi-element data signals the elements are temporarily stored in a data store and are normally written into or read out of the store at a first rate but are written in or read out at m times that rate when m successive data elements are identical, these m elements being represented by a single coded element. In a first embodiment, the signal output of television camera tube 21 is stored line-by-line in shift register 29 of n-bit capacity under the control of read-in pulses from generator 55, each line comprising n data elements at "black" or " white " level. A counter 58 detects when the n elements of a line have been stored and initiates read-out at a first rate controlled by pulses of frequency f from generator 78. The last four stages of the shift register are coupled to a four-input AND gate 102 which detects the occurrence of four successive data elements at the "white" level and thereby causes read-out pulses of frequency 4f to be applied to the register from generator 80 for rapid read-out of these four data elements. At the conclusion of a line read-out, counter 58 causes triggering of a frame-deflection staircase-waveform generator 39 in readiness for scanning and storage of the next line. The output of shift-register 29 is applied through gate 92 to transmitter 72 for transmission as a four-level composite video signal (Fig. 3V, not shown) in which (a) the first (i.e. the lowest) level represents frame and line synchronizing pulses and is derived from clamp circuit 68, (b) the second and third levels represent " black " and " white " respectively and occur as the video output from gate 92 and (c) the'fourth level represents fourfold redundancy and occurs at the output of gate 96 when AND gate 102 detects that four successive data elements are at the " white " level, gate 92 blocking the passage of the four rapidly-read-out identical data elements to transmitter 72 for the duration of the fourth level signal (i.e. for one normal read-out pulse interval). At the receiver (Fig. 2) the first level synch. signals are separated and control linedeflection sawtooth generator 166 and framedeflection staircase generator 165 of display tube 127. The residual three-level video signal isapplied to an n-bit shift-register 126 for line-by-line storage under the control of read-in pulses from a "normal-rate" generator 150, but if a fourth-level signal is detected by a threshold circuit 138 the read-in pulses are supplied at four times the " normal " rate by a generator 146 instead. When a line has been stored it is read-out at the " normal " rate, under the control of pulses from a generator 143, to the tube 127 for display and the next line is stored. In a second embodiment (Fig. 6) of the transmitting system the shift-register 29 is replaced by a cathode ray storage tube (189) employing one writing beam and two reading beams, the writing beams being scanned in synchronism with the scanning of the camera tube 21. One reading beam has a spot size four times that of the other and scans slightly ahead of the other to sense four-fold redundancy in successive elements of the charge pattern laid down on electrode (194) by the writing beam and corresponding to the video output of the camera tube. The line scanning rate of the reading beams is controlled by a dual-slope sawtooth generator and is increased from normal rate to four times the normal rate when such redundancy is sensed. The output of the other reading beam is used to derive the four-level composite video signal for transmission. In a second embodiment of the receiver (Fig. 10) the shift-register 126 is replaced by a cathode-raystorage tube (246) into which the second- and third-level video signals are written at a rate controlled by a dual-slope sawtooth generator (256) and from which they are read out to display tube 127 at a fixed rate. The fourth level redundancy signal switches dual-slope generator (256) from its slow-scan rate to its fast-scan rate. In a third embodiment of the receiver (Fig. 11) the storage and display functions are combined in a direct-viewing cathode-ray storage tube (272) into which the second- and third-level video signals are written at either the fast or the slow rate under the control of dual-slope sawtooth generator (256).