Patent GB-726323-A (GB)
Title: Improvements in or relating to colour television system
IPC: H04N11/12
Filed: 1952-01-29
Abstract: 726,323. Colour television. SYLVANIA ELECTRIC PRODUCTS, Inc. Jan. 29, 1952 [Feb. 10, 1951], No. 2382/52. Class 40 (3). In a colour television system, the green signal is transmitted and received continuously whilst the red and blue signals are translated alternately at a frequency lying in the higher frequency spectrum of the green signal and with a band width substantially less than that of the green signal. The translation is effected by modulating the red and blue signals on to a subcarrier at 90 degree phase points. Reference is also made to the use of pulse sampling. In a first embodiment the red and blue signals are employed directly whilst in a second embodiment they are first combined with the green component to produce signals of the form red minus green and blue minus green. In the transmitter of the first embodiment, Fig. 1a, three camera units 10, 11 and 12 generate green, blue and red signals respectively, the green signal being delayed in a device 15 and passed directly to signal adding device 16. The blue signal is passed through a low-pass filter 17 having a pass-band extending from zero to an upper limit between 0.1 Mc. and 1.5 Mc. (0.25 Mc. as shown) and is then applied to modulate a sub-carrier signal in a square-low modu. lator 18. The red signal is passed in a similar manner through a filter 21 to a modulator 22. The sub-carrier signal is generated by a synchronizing signal generator 19 and is selected to have a frequency (3.58 Mc. in the particular embodiment described) equal to a multiple of one half the line scanning frequency of 15750 c/s. Unit 23 shifts the phase of the sub-carrier signal applied to modulator 22 by 90 degrees. Bandpass filters 20, 24 covering both the upper and lower side-bands of the modulated sub-carrier signals follow the modulators 18, 22 and pass signals to adder 16. Band-pass filters 27, 28 select the high-frequency components of the blue and red signals and apply these also to adder 16. The combined image signal from adder 16 then modulates the transmitter 25. Synchronizing signal generator 19 also generates standard synchronizing signals and adds these to the transmitted signal together with bursts of oscillation at the sub-carrier frequency occurring during line and frame blanking intervals to synchronize a sub-carrier oscillator at the receiver. The delay introduced in the green signal by device 17 equals the delay encountered by the red and blue signals in their respective paths 17, 18, 20 and 21, 22, 24. At the receiver, Fig. 1b, the combined image signal is derived from a detector in the receiver unit 30 and applied through a delay network 31 to the green input electrode G of a three-colour image reproducing cathode-ray tube 32. The signal is also applied through a band-pass filter 33 covering the range occupied by the'sub-carrier signal and side bands to two modulators 34, 35 supplied with signals at the sub-carrier frequency from a unit 37. Unit 37 includes an oscillator and a filter for selecting the bursts of sub-carrier oscillation. The oscillator output is applied directly to modulator 34 and through a 90 degrees phase shifter to modulator 35. The resulting demodulated signals correspond to the red and blue components modulated on to the sub-carrier at the transmitter and are passed through filters 38, 39 to adders 44, 45 where they are combined with the high-frequency components of the received signal passed through filter 46. The resultant signals are then applied to the red and blue electrodes of tube 32. Adders 40, 41 and phase invertors and attenuators 42, 43 insert a portion of each signal in the other channel to eliminate cross-modulation products. Delay network 31 may be omitted if sufficient delay is provided in delay device 15 at the transmitter. In the second embodiment, Figs. 2a and 2b (not shown), the green signal is transmitted as shown in Fig. 1a but is also combined in reverse phase with the red and blue signals so that the two sub-carrier modulations correspond to red minus green and blue minus green. At the receiver, the green signal is applied to the control electrodes of all the cathode-ray tube guns whilst the demodulated sub-carrier signals are applied to the cathodes of the corresponding colour tubes so as to cancel out the minus green components and redevelop the individual component colours.