Patent GB-1263981-A (GB)
Title: Method and system for band compression for video signals
IPC: H04N7/12
Filed: 1969-02-18
Abstract: 1,263,981. Television. WESTINGHOUSE ELECTRIC CORP. 18 Feb., 1969 [8 March, 1968], No. 8756/69. Heading H4F. In a system enabling the transmission of a video signal comprising a first set of lines over a low bandwidth system, the video signal is stored on a continuous loop storage medium and is repeatedly reproduced from the storage medium, the reproduced signal being sampled on different replays of the signal to provide a sampled signal wherein the consecutive signal elements are derived from different lines of said first set of lines to thereby form a second set of lines forming the image. General description, Fig. 1 (not shown).-A television camera (12) produces a fast-scan video signal corresponding to the image of a field (16) which is converted to a slow scan system (18) and stored for a short period. System (18) repeatedly plays back and samples the fast scan signal so that the horizontal lines consisting of elements 1 1  to M 1 , 1 2  to M 2  ... 1 N  to M N , Fig. 7, are converted into a slow scan signal of vertical lines consisting of elements, 1 1  to 1 N , 2 1  to 2 N  ... M 1  to M N . This slow scan signal is supplied via a low band width transmission system (20), e.g., a telephone line, to a slow to fast scan conversion system (22) which includes means for selectively sampling, and storing the slow scan input signal during many cycles of the storage medium thereby to build up a fast scan signal which may be rapidly read out and displayed at (24). Fast to slow scan conversion.-The storage medium is a magnetic drum or disc 26, Fig. 2, driven by a motor 30. The video signal produced by camera 12 has 525 lines per frame, each horizontal line having 400 picture elements, and is scanned at a rate of 60 fields (or 30 frames) per second requiring a bandwidth of 3.15 MHz and this signal is to be converted to require a bandwidth of 7À5 KHz at a picture rate of 9 frames per 2 minutes. For this reason the drum 26 is rotated at 30 revs. per second, recording track 32 carries a sync. signal of 525 pulses and track 33 carries two pulses about its circumference, the output of head 40 supplying signals to a horizontal sweep generator 56 and the output of head 41 supplying vertical sweep generator 52. The signal from camera 12 is supplied via a gate 60 which is triggered to gate a single frame during one revolution of the drum 26 to read-write switching circuits 63, 65, which are controlled so that when one is reading via head 42 or 43 the other is writing via head 49 or 34, respectively. The sync. signal supplied by head 41, which consists of 60 c/s. clock pulses is compared at 67 with the 60 c/s. supply voltage to control the speed of the motor 30. The fast scan signal which has been recorded on drum 26 during one revolution is repeatedly sampled at 81 in the manner shown in Fig. 7 to provide a frame of vertical lines during 400 revolutions. The gating and switching signals for circuits 60, 63, 65, 81 are derived from the outputs of heads 40, 41, Fig. 9 (not shown). The switching signal for circuits 63, 65 is also supplied to a lamp 89 to indicate the start of a new cycle so that the operator may present a new field 16 to the camera 12. After each revolution of drum 26 the integration circuit 75 ensures that the sampling pulse to gate 81 is delayed by one picture signal element time period so that the lines of the slow scan signal may be built up during successive revolutions without erasing a previously recorded line. Slow to fast scan conversion, Fig. 3 (not shown).-This uses a magnetic drum carrying recorded sync. signals and co-operating with a pair of read-write circuits which read and write alternately as in Fig. 2. The input slow-scan signal is sampled so as to build up a complete frame of horizontal lines of video information during 400 revolutions of the magnetic drum, Figs. 8A, 8B, which may be read off rapidly during a single revolution. Modifications.-The system of Fig. 2 is modified by including an additional clock pulse track of 209, 999 pulses per revolution on the magnetic drum, Fig. 5 (not shown), which is divided by 400 to control via a differentiating circuit (79) the output sampling gate (81). The output of the divider is a fraction less than 525 pulses per revolution and the output of the gate is a series of vertical lines in which the sampling of the vertical pulses is progressively delayed so that the last pulse of a line is delayed by one picture element period as in Fig. 2. The resulting image signal has both vertical and horizontal interlacing, Figs. 11A, 11B (not shown). A similar arrangement may be provided for the slow to fast scan system, Fig. 6 (not shown).