Patent GB-685661-A (GB)
Title: Colour television system
IPC: H04N11/12
Filed: 1950-09-19
Abstract: 685,661. Colour television. RADIO CORPORATION OF AMERICA. Sept. 19, 1950 [Sept. 24, 1949], No. 22983/50. Class 40 (iii). In a colour television system of the dotsequential type, i.e. a system wherein signals representing the three colour components of the picture are sampled at a high rate at the transmitter and are reconstituted at the receiver by a distributer operating in synchronism with the sampler at the transmitter, the constants of the system are so chosen that, either with or without the use of auxiliary circuits, the elemental areas corresponding to any one colour in any one line of any one picture scansion (i.e. two interlaced frame) lie midway between the elemental areas corresponding to the same colour in the same line of the preceding scansion. In the embodiment illustrated in Fig. 1, this is achieved by choosing the sampling frequency to be an integral multiple of the line frequency and changing its phase by 180 degrees at the end of each line. Alternative arrangements comprise changing the phase of the sampling frequency at the end of each frame, Figs. 6 and 7 (not shown), and arranging the sampling frequency to be an odd harmonic of half the line frequency, Figs. 4 and 5 (not shown). In Fig. 1 the three colour channels in the output from the camera 2 are coupled to the valves 16, 18 and 20 constituting the sampler. The sampler valves are gated in sequence by respective signals at approximately 3.8 Mc/s. derived from the keying oscillator 24 which is synchronized to operate at a harmonic of the line frequency. The output of 24 is applied in opposite phases to the valves 30 and 35 having a common anode and rendered alternately conducting at the line frequency by a bi-stable trigger stage 46 which is triggered by the trailing edges of the synchronizing pulses supplied by synchronizing-signal generator 94. The output from valve 30 or 35 is applied to a phasesplitting network 48 which delivers three outputs spaced by 120 degrees in phase and used to gate respective ones of the valves 16, 18 and 20. A three-colour time-division multiplex signal is consequently developed in the common anode load of the sampling valves and this is applied, together with synchronizing signals from the circuit 94 to the mixer valve 90 and thence to the transmitter 156. The signal applied to the trigger circuit 46 and corresponding to the trailing edges of the line synchronizing pulses is also inverted at 118 and then applied to a flip-flop circuit 122 which supplies a short gating pulse which occurs during the " back-porch " interval of the pedestal for the synchronizing pulses. This signal is applied to valve 128 to which is also applied one of the outputs of the phasesplitter 48. The valve 128 consequently delivers a short burst of oscillations at the sampling frequency during each " back-porch " interval; these are clamped at an appropriate level by the diode 138 and then supplied to the transmitter 156 for transmission together with the other signals as synchronizing signals. At the receiver, Fig. 2 (not shown), a gating signal is developed from the trailing edges of the line-synchronizing pulses and used to control the application of the signal occurring during the " back-porch " to synchronize an oscillator operating at the sampling frequency. The oscillator output is applied to a phasesplitting network delivering three outputs phased 120 degrees apart; these are applied respectively to three valves forming a distributer and having a common input for the received video signals and separate outputs applied to separate cathode-ray tubes reproducing in their respective component colours. In the arrangements so far described points of corresponding colour in adjacent lines of succeeding frames are either displaced by 0 degrees or 180 degrees with respect to one another. In the embodiment shown in Fig. 8 the corresponding displacement is arranged to be 90 degrees or 270 degrees points of the same colour in any one picture scansion lying midway between those of the previous scansion, as before. Frame blanking pulses from a synchronizing- signal generator 250 are applied to the colour cameras 252, 254, 256 and to a ring trigger circuit 270 whose output leads 274, 276, 278, 280, thus become positive in turn for the duration of successive frames, and thus render gate valves 290, 292, 294, 296 successively conducting. Oscillator 306 generates a signal of the phase-splitter 308 giving four outputs phased 90 degrees apart. These are passed in succession by the gate valves 290, 292, 294, 296 to the phase-splitter 314 giving three outputs phased 120 degrees apart, and applied to respective cameras to control the sampling. As in the embodiment shown in Fig. 1, circuits 122<1>, 128<1>, 138<1> derive a burst of oscillation at the sampling frequency during the " backporch " interval of the line-blanking pulses and combine this with the video-signal output from the cameras in circuit 322 together with the line and frame synchronizing pulses, the output from circuit 322 being applied to transmitter 156<1>.