Patent GB-709245-A (GB)
Title: Improvements in and relating to colour television systems
IPC: H04N3/34
Filed: 1952-03-14
Abstract: 709,245. Colour television. BRITISH THOMSON-HOUSTON CO., Ltd. March 14, 1952 [March 17, 1951], No. 6677/52. Class 40 (3) In a three-colour television system the low frequency components of the green signal together with the mixed high frequency components of the red, blue and green signals are transmitted continuously as modulations of a main carrier whilst the low frequency components of the red and blue signals are transmitted alternately, for example, in a predetermined line sequence, as modulations of a sub-carrier. In a first embodiment, Figs. 1 . . . 3, the colour signals from the camera 6 at the transmitter, Fig. 2, are divided into high and low frequency components by filters It  . . . 16 having passbands as indicated and all the high frequency components together with the low frequency green components are passed to an adding unit 20 whilst the low frequency red and blue components pass respectively to gated amplifiers 27 and 28. The amplifiers are operated in a predetermined sequence at line frequency by switching generator 29 the outputs modulating a subcarrier in unit 37 before combination with the remaining signals in adder 20. The sub-carrier signal is derived from a master oscillator 34 by 2:1 division in stage 35 and has a frequency equal to a multiple of one half line frequency. A control signal at twice line frequency is also derived via stages 40, 41 to synchronize a master pulse generator 23 which adds synchronizing and blanking signals to the combined signals in adder 20, times the camera operation, and controls switching generator 29. The main carrier is generated in stages 1, 2 and modulated by the combined signal in stages 3, 22 before radiation. Fig. 1 depicts the spectrum of the radiated signal. The preferred line switching sequence for the red and blue low frequency components is red, red, blue, although other sequences, such as RRRBB and RRRRBBB, arranged so that an integral number of complete groups is scanned in each picture (525 lines), may be employed. Such sequences eliminate line crawl in the reproduced picture. It is necessary, however, to broaden the reproduced red and blue lines to eliminate the resulting horizontal bar and line structures and this is effected by defocusing the cathode-ray tube spot or by high frequency spot wobbling. Alternatively a tube producing an elliptical spot may be used. At the receiver, Fig. 3, the sub-carrier signal and side-bands are selected by filter 50 and demodulated by detector 55. The green low frequency signals and mixed high frequency signals are selected by filter 49 and passed through amplifier 51 to that one of three guns in a cathode-ray tube 53 which produces a green image. The mixed high frequency signals are also selected by filter 59 and amplifier 60 and combined with the demodulated red and blue signals from detector 55. The signals are then passed to the red and blue guns of tube 53 through gated amplifiers 57, 58 operated in the sequence of the transmitter under the control of switching generator 68. The synchronizing signals are separated in a conventional manner in unit 63 and applied to control the sweep generators 64, 65 and switching generator 68. Three separate cathode-ray tubes may replace tube 53. In a second embodiment, Figs 4 . . . 6 (not shown), the transmitter is modified to effect vestigial side-band modulation of the sub-carrier so as to enlarge the range of frequencies available for the green low frequencies and mixed high frequencies. The green low frequency and high frequency signals are not separated after generation but are fed directly to the adding unit. The red and blue low frequency signals are modulated on to the sub-carrier in alternation, and to avoid line crawl the red and blue scanning rasters in the camera are displaced downwards by one line during alternate fields. A corresponding displacement is added at the receiver. The receiver is generally similar to that of Fig. 3 differing mainly in that the mixed high frequency signals are applied to the green cathode-ray tube only. Specification 704,791 is referred to.