Patent GB-680783-A (GB)
Title: Colour television system
IPC: H04N11/12
Filed: 1950-09-14
Abstract: 680,783. Colour television. RADIO CORPORATION OF AMERICA. Sept. 14, 1950 [Sept. 23, 1949], No. 22651/50. Class 40 (iii). A colour television transmitter comprises means for cyclically sampling, at a rate high compared with the line-scanning frequency, the lower-frequency portions of the outputs of three colour cameras to derive a time-divisionmultiplexed signal corresponding to the outputs of the three cameras, and means for adding thereto, in an equivalent manner with respect to each channel of the multiplexed signal, a further signal comprising the higher-frequency components of the output from at least one of the cameras. The outputs from " green," " red " and " blue " cameras 12, 14 and 16 are passed through low-pass filters 84, 86, 88, respectively to the commutator 10. Filters 84 and 86 pass signals within a band 0-2 Mc/s. and filter 88 passes a band of 0-1.4 Mc/s. Commutator 10, which in practice is an electronic device, samples these signals at a rate for each colour of 3.8 Mc/s. A synchronizing generator 30 controls the operation of the cameras and of the commutator through an " interlacing " oscillator 28 and drive circuit 26. - The output of the green camera 12 is also applied to a high-pass filter 96 which transmits a " detail " signal comprising components of the picture signal above 2 Mc/s. The upper limit of the band passed by filter 96 is restricted to 3.5 Mc/s. to restrict the production of spurious low-frequency components due to heterodyning with the 3.8 Mc/s. sampling rate of the commutator. The outputs from the commutator 10 and filter 96 are combined in an adder circuit 98, the output of which is radiated by amplitude-modulation of a carrier-wave signal generated at 46. In a modification of this circuit, the outputs from all three cameras are combined and then applied to the high-pass filter to provide the " detail " signal while in another modification, the outputs from the green and red cameras are combined to provide the " detail " signal. In a further arrangement, a separate adder circuit is provided for each colour channel immediately before the commutator, and the " detail " signal is applied in parallel to these. Interlacing. The frame and line frequency of the embodiments illustrated are the same as those employed with standard black-and-white transmissions, successive frames being vertically interlaced in a conventional manner. In addition by means of the interlacing oscillator 28, which operates at half the line frequency to change the phase of the sampling by 180 degrees at the beginning of each line, corresponding lines of successive pairs of vertically interlaced frames are " horizontally interlaced," i.e. points at which samples corresponding to any one colour occur lie midway between the corresponding points in the immediately previous pair of frames. Receiver. The receiver comprises a commutator which is driven in synchronism with that at the transmitter by means of a synchronizing signal separator, an " interlacing " oscillator and a drive circuit. The video output of the receiver is cyclically switched to " green," " red " and " blue " cathode-ray tubes, the pictures produced on the screens of which are combined optically to obtain the desired colour image. It is also stated that conventional black-and-white receivers will satisfactorily reproduce the transmitted scene as a monochrome picture.