Patent GB-1163016-A (GB)
Title: Improvements in and relating to Television Systems
IPC: H04N7/087
Filed: 1966-09-26
Abstract: 1,163,016. Television; multiplex radio signalling. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 26 Sept., 1966 [29 Sept., 1965], No. 42867/66. Headings H4F and H4L. A television system for the transmission of pictures via an existing broadcast television system without disturbance of the normal television programme comprises at the transmitter, means for inserting an additional video signal from a separate camera during n line periods of a field of the normal television signal, and means whereby the line scan is performed at the same rate as for the normal television signal while the field scan is changed so that the information of a complete television picture formed by the additional signal is transmitted in m fields of the normal television signal, and at the receiver, means for display and storage of the additional video signal. The injection of the additional video signal is preferably performed during part of the vertical blanking period of the normal television signal, a code signal being transmitted at the start of the additional video signal so that only receivers having corresponding decoding means respond to the additional video signal. In the first embodiment, Fig. 1, the apparatus for adding an additional television signal in the studio to the normal television signal comprises block 1 which has inputs 2, 3 and 4 for field synchronizing signals from block 8, line synchronizing signals from block 7 and the video signal from block 6, and an output 5 at which the additional video signal is derived. With switches 10, 13, 15 and 18 occupying the positions shown the auxiliary apparatus 1 is operative, but with the switches in the other positions the normal video signal is fed via conductor 11 to mixing circuit 19, the line and field synchronizing signals being fed via adder 16 to mixer 19 for transmission. Apparatus 1 includes a first counter 24, comprising monostable multivibrators 29 and 30, which is supplied with line and field synchronizing pulses and produces a square-wave pulse 27 having a duration of n line periods, the leading edge of which being x line periods from the leading edge of the field pulse. Pulse 27 is used to drive switch 31 to connect the output of a separate camera 34 which produces the additional video signal to output 5. In the detailed embodiment described x = 8 and n = 5 so that the additional video signal is transmitted during the vertical blanking periods. If the additional video signal is not transmitted during the vertical blanking periods switch 31 suppresses the normal video signal for the n line periods in which the additional video signal is transmitted. The line synchronizing pulses of the normal television signal are fed directly via conductor 36 to the separate camera 34, but new field synchronizing pulses are derived in a second counter 25 for the camera 34. Counter 25 comprises a counting circuit 38 and an amplifier 39. Line synchronizing pulses are applied to input 40 and pulses which reset the counting circuit 38 to the zero position are applied to input 41. With switch r 1  positioned as shown counter 38 counts 312 line pulses and then delivers a pulse through conductor 37 to synchronize the field pulse generator of camera 34, field pulses being applied to input 41. When switch r i  is changed over, the start of the counting circuit 38 is taken over by the output pulses from the counter 25 instead of the field pulses. The counting circuit 38 counts for example 312 line pulses and is then reset to zero, after which a new count of 312 line pulses is started, and thus the field synchronization signals for camera 34 are shifted each time by half a line period with respect to the preceding field synchronization. Thus, during each field the scan in camera 34 is shifted by half a line period (specifically one line period for two fields) so that the n lines of additional video signal from camera 34 passed by switch 31 are slightly different for each field. The batches of n lines passed by switch 31 shift from top to bottom of the additional television images, and after 625 fields the complete image is transmitted. To start transmission of the additional television picture push-button 45 is depressed causing bi-stable multivibrator 44 to change over. This energizes relay r which switches over switches r 1  and r 2 . The transmission is automatically terminated by coincidence stage 43 which causes bi-stable multivibrator 44 to change back when an output pulse of the counting circuit 38 and a field synchronizing pulse from terminal 2 coincide which is after 625 normal fields. The Specification includes details of an accurate counter 24, Fig. 2 (not shown). The additional picture to be transmitted may include a beam pattern at its top forming a code which is transmitted together with the additional video signal, Fig. 3 (not shown). This code, which may alternatively be added electrically, may comprise a given frequency to which filters in selected receivers are tuned. Camera 34 may comprise three colour cameras and the code signal may thus comprise a beam pattern having a given colour. The signal from the separate camera may be added to a normal television signal from an auxiliary transmitter, Fig. 5 (not shown). A receiver for reproducing the additional video signal from the separate camera includes similar first and second counter Circuits (24<SP>1</SP>) and (25<SP>1</SP>), Fig. 4 (not shown), decoding circuit (58), coincidence stages (76) and (77<SP>1</SP>), bi-stable multivibrator (78) and relay (rs), and is such that batches of n lines pass from top to bottom of the screen of a cathode-ray tube (55) to display the television image. The scan of this image is too slow for normal observation and thus there is arranged in front of the tube (55) a photographic camera (67), the shutter 68 of which is opened at the start of the scan of the image and is closed at the termination of a scan of m fields. Alternatively the display tube may comprise a storage tube, or have a long luminescent time display screen.