Patent GB-675061-A (GB)
Title: Improvements in television transmitters
Inventor: HERRICK ROSWELL HARRY
IPC: A47B88/49
Filed: 1950-05-08
Abstract: 675,061. Television systems. ZENITH RADIO CORPORATION. May 8, 1950, No. 11453/50. Class 40 (iii). In a television system wherein the timing of the line scans of the transmitted picture is varied in accordance with a key signal to which privileged persons have access whereby they may receive and utilize the television picture signals to produce an undistorted picture, a key-signal pattern is stored at the transmitter and at all cooperating receivers and is continuously and repetitively scanned in synchronism at each of these units to produce a key signal which is used at the transmitter to effect coding of the transmitted signal and at the receivers to effect decoding of the received signals. By this means the necessity to transmit the key signal, normally by telephone lines, continuously to each cooperating receiver is removed. Fig. 1 shows a transmitter which is designed to co-operate with a receiver as described in Specification 675,062. The transmitter comprises a camera unit 1, the output of which is applied through a video amplifier 4, a synchronizing-signal and blanking-signal mixer 5, a D.C. restoring unit 6 and modulator 7 to the transmitting aerial 8. Synchronizing and blanking signals generated at 9 are added to the video signal at 5; frame and line synchronizing signals are also applied to frame-scan generator 11 and line-scan generator 13, respectively, the outputs from which control the scanning in camera unit 1. The line-synchronizing pulses are applied to unit 13 through a coding circuit 14 comprising a delay line which is switched into or out of the circuit in accordance with a control signal applied thereto from unit 20. The unit 20 applies a signal to unit 14 to switch the delay line into circuit when a frame synchronizing signal from unit 9 and a key-signal element picked off from the magnetic storage tape 27 are applied simultaneously thereto. Thus circuit 14 is actuated only during the framesynchronizing periods and causes, when actuated, a delay in the video signal relative to the line-synchronizing signals mixed therewith in unit 5, 20 preventing a proper reproduction of the object viewed by the camera unit 1 at any receiver not having access to the key signal stored on tape 27. The key-signal is generated by the chain 17, 18, 19. Frame-synchronizing pulses, Fig. 3, curve A, are applied from unit 9 to a random pulse selector 17 which selects pulses at random from the signal applied thereto. The output from 17, curve B, is applied through a switching unit 18 to unit 19 which generates bursts of key-signal oscillation, curve E, in response thereto. The key signal generated at 19 is recorded on the tape 27 through a recording head 29. The tape 27 is rotated continuously by motor 25 which is driven synchronously from the frame-synchronizing signals. The tape 27 is made to rotate once per second; a further tape 28 is made to rotate once per minute. Tape 28 carries upon it a switching signal which lasts for the time of one revolution of tape 27 and may be applied to the switching circuit 18 via switch 38. To record the key signal upon tape 27, the key 34 is closed for a short time, thereby erasing any previous signal. Then, just before the switching signal is picked off, switch 38 is closed. The switching signal, Fig. 3, curve D, conditions unit 18 to pass the pulses, curve B, applied thereto to unit 19, so that key signal is recorded upon tape 27 for a length of time equal to that taken by one revolution of the tape. The recurrent key signal is applied by pick-up head 30 to unit 20, as previously described, and is also applied over lines 32 to a distributing station. Fig. 2 (not shown) shows an arrangement utilizing a plurality of code tapes 27 on which previously determined keysignal patterns have been recorded. Synchronizing the key signal at receiver. The key-signal pattern is also recorded on a tape at the receiver and the signal picked off therefrom used to decode the coding effected at the transmitter. To ensure synchronism between the receiver tape and tape 27, a further tape 26 bearing a single synchronizing signal, Fig. 3, curve C, is utilized. This synchronizing signal is applied with the key signal to lines 32 and is thus eventually recorded at the receiver with the key signal and is also applied to unit 41 which then allows a signal from unit 43 to pass to the mixer 5 and thus be eventually radiated from the aerial 8. The signal applied to the mixer comprises a portion of a signal generated in 43 comprising a pulse signal obtained by frequency-multiplication of the equalizing pulses of the frame synchronizing signal. Distribution of key signal, Fig. 7. The incoming lines carrying the various key signal patterns 32a, 32b, 32c are applied to a plurality of circuits for coupling them to a co-operating receiver. Each circuit comprises a selector 159, a switching unit 161, a coil 165 coupling to a telephone hand-set 163 and output lines 167. The switch 161 is controlled by a signal recorded on tape 170 lasting for the duration of one revolution of the tape 27, Fig. 1. The tape 170 is driven synchronously with the tape 27 by means of motor 175 which is synchronized by the frame synchronizing signals received by units 173, 174. Thus, when switch 171 is closed to allow the signal from tape 170 to pass to switch 161, the key-signal is sent over lines 167 to the receiver where it is recorded on a further magnetic tape.