Patent GB-997946-A (GB)
Title: Television system
Inventor: WHITING KENNETH WALLACE
IPC: H04N7/12
Filed: 1961-07-10
Abstract: 997,946. Television band-width reduction. PYE Ltd. July 3, 1962 [July 10, 1961], No. 24903/61. Heading H4F. High-speed television signals occupying a wide band of frequencies are converted to lowspeed signals occupying a narrower band of frequencies, suitable for transmission, e.g. by cable, by applying the high-speed signals directly to a gate circuit which is controlled by gating pulses to transmit a short duration sample during each line, the phase of the gating pulses being changed progressively so that different portions of each line of the high-speed signals are passed in successive frame periods and substantially the whole of the image detail is transmitted by the end of a given number of frames. In effect only a vertical strip of the image is transmitted in each frame interval, successive adjacent vertical strips being transmitted in successive intervals so that eventually the whole frame is transmitted. Reproduction is effected by means of a cathode-ray tube the screen of which has a long afterglow. The line scanning frequency is the same as the frame scanning frequency of the high-speed signals but the tube is scanned in the vertical direction, whilst the low-speed scan runs in synchronism with the phase modulation frequency of the gating pulses. Fig. 1 illustrates an embodiment of a converter for 405 line, 50 frames per second signals which are applied to a gate 3 and a syn. separator 2 from which the 10 kc/s. line pulses control a sawtooth generator 4 and the 50 c/s. frame pulses control a dekatron divider 8. The divider is arranged to produce one pulse every five or ten seconds. The 50 c/s. pulses also control a staircase generator 16 which is recycled by the pulses from divider 8. The output from the staircase generator is then combined with the 10 kc/s. sawtooth signals in a mixer 5 and applied to control a multivibrator 6. The multivibrator is triggered by the 10 kc/s. sawtooth and develops a rectangular pulse the width of which is determined by the amplitude of the staircase waveform. The gating pulses for gate 3 are obtained by differentiating the output of multivibrator 6 to obtain the phasemodulated edge which is amplified in stage 10, differentiated again and fed through cathode follower 11. The samples from gate 3 are passed through a filter 12 having an upper pass limit of 10 kc/s. and then combined in mixer 14 with synchronizing signals derived in stage 15 from the original 50 c/s.. from pulses and the pulses at the output of divider 8. In place of filter 12 an integrating and storage circuit, Fig. 2 (not shown), may be used to obtain square wave pulses which persist throughout the sampling interval. At a receiver the received low-speed signal is applied through amplifier 20 to modulate a cathode-ray tube 21 having a screen with a long afterglow. The signal is also applied to sync. separator 22 from which the 50 c/s. pulses pass via a squarer 23 to control (a) a line scan (vertical) generator 29 and (b) a frame scan (horizontal) generator 24 in the form of a staircase generator. The phase modulation pulses (occurring every 5 or 10 seconds) are applied via a squarer 26 to control (a) a blanking generator 28 and (b) a switching stage 27 to recycle generator 24.