Patent GB-1437110-A (GB)
Title: Signal transmission system
IPC: H04N7/12
Filed: 1973-11-22
Abstract: 1437110 Television PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 22 Nov 1973 [25 Nov 1972] 54154/73 Heading H4F Television bandwidth reduction system for transmitting the video signal corresponding to an image (photograph or page of text) over a telephone channel comprises a store, e.g. a magnetic disc memory M 1 , in which the video signal corresponding to one frame scan (i.e. two fields) is stored (e.g. recorded on one track) after it has been frequency modulated FM on to a carrier. This FM recorded signal is repeatedly read-out of the store, full-wave rectified RF and connected via NAND gate N 1  to a shift register SR 1  which counts signal transitions relative to a reference value, the resulting binary number output being fed to a coincidence stage CD. Shift register SR 1  specifically has 12 stages but instead of counting to their maximum possible number 2<SP>12</SP> they are limited to count up to 3200. In addition the coincidence stage is supplied with the binary count from a similar shift register SR 2  which receives a pulse once per frame scan (i.e. once per complete read-out of the recorded FM signal from store M 1 ) derived from the vertical and horizontal sync information VS and HS from generator G 1  whose output also controls television camera CM and store M 1 . Coincidence stage CD supplies an output pulse PS whenever the binary outputs from the two shift registers are the same thus with shift register SR 1  repeatedly counting up to 3200 (i.e. X) signal transitions, output pulses PS are produced during the first frame scan that corresponds to the 1st, x+1, 2x + 1 ... signal transitions, and during the second frame scan that corresponds to the 2nd, x + 2, 2x + 2 ... signal transitions, so that after X frame scans (i.e. X read-outs of the FM recorded signal) the complete frame of video information is transmitted. The shift register SR 1  is reset to zero at the beginning of each frame scan (read-out from store M 1 ). The pulses PS are fed to trigger a square wave signal generator G 3  whose further reduced bandwidth output is low pass filtered F 1  (bandwidth up to 3 kHz), cut-off filtered F 2  (e.g. at 3 kHz) and has a sinusoidal signal at 3 kHz carrying the synchronizing information (phase inverted by field sync signal VS) added thereto in adder AD before it is transmitted over limited bandwidth channel TC. In the receiver signals at 3 kHz are cut-off F 3  and below 3 kHz passed F 4 , and the output from filter F 3  is connected to a signal generator G 5  which provides short pulses corresponding to the FM signal transitions which are stored, e.g. recorded on one track of a disc memory M 2 , together with the sync signal directly from filter F 4  and via a phase detector VC which detects field period phase inversion of 3 kHz horizontal sync generated sinusoidal signal. After X frame scan periods the complete FM video signal of a frame is stored, and this is then read-out, demodulated FDM and displayed on a monitor TV or printed in a facsimile device FP. The vertical (field) sync information is not required when store M 2  comprises a disc memory but it is when the store comprises a tape memory or memory tube. Instead of using store M 1  a camera CM incorporating a storage tube may be used. If fullwave rectifier RF were omitted then register SR 1  would have to be actuated by both leading and trailing edges of the FM signal or the video signal bandwidth transmitted would be halved. Instead of FM modulation, pulse code modulation may be used. If pulses PS are applied directly to the transmission channel TC, e.g. a coaxial cable, the system may comprise a TDM system with other information between the pulses PS.