Patent GB-684664-A (GB)
Title: Colour television image reproduction
IPC: H04N9/16
Filed: 1951-10-02
Abstract: 684,664. Cathode-ray tubes. RADIO CORPORATION OF AMERICA. Oct. 2, 1951 [Oct. 5, 1950], No. 22939/51. Class 39(i) [Also in Group XL(b)] A cathode-ray tube for use in a colour television system comprises a double-sided fluorescent screen, one side of which may be made to fluoresce in each of the three component colours while the other side produces white light in response to excitation by an electron beam. The invention is applied to a colour television system employing the mixed-high-frequency principle, i.e. a system wherein the higher frequency components of the video signal contain information concerning picture detail only, reproduction being effected at the receiver by separating the received video signal into its higher and lower frequency components, causing the higher frequency components to produce a black and white picture upon one side of the cathode-ray tube screen and the lower frequency components to produce a coloured picture upon the other side of the cathode-ray tube screen, the two images thus formed producing single composite image. First embodiment, Fig. 1. The receiving tube 11 comprises a double-sided fluorescent screen consisting of a transparent mica support 13, on one side of which is a screen 14 which produces a black-and-white image in response to excitation by an electron beam from the gun 20, and on the other side of which is a screen comprising lines of phosphor, successive lines emitting red, green and blue light in response to excitation from the gun 23. Video signals from the receiver 27 are applied via high pass filter 33 and low pass filter 32 to the electron guns 20 and 23 respectively. In addition synchronizing signals are applied via circuit 28 to the deflection-waveform generator 29 which supplies the deflecting coils 22 and 25 to produce the normal scanning raster on both screens of the cathode-ray tube. A further deflection coil 26 is supplied from circuit 31 with a signal having a frequency equal to the colour repetition rate. Thus in a dot-sequential colour television system wherein the colour channels are cyclically sampled at 3.6 Mc/s, circuit 31 supplies a signal of this frequency to the coil 26. The electron beam from gun 23 is deflected over the width of a group of three lines by this additional deflecting signal and. due to the synchronism between the deflection and sampling rates, irradiates a line corresponding to a particular colour at the time that a sample due to that colour is being received. Thus the higherfrequency components are reproduced in black and white on one side of the screen and the lower frequency components are reproduced in colour on the other side of the screen, the two images being viewed in superposition as indicated in the Figure. Second embodiment, Fig. 2. The colour-reproducing surface of the double-sided screen comprises a large number of tetrahedral portions such as 38. The three exposed faces of each tetrahedron fluoresce with light of the three component colours respectively and are so arranged with respect to the electron guns 45, 46 and 47, that electrons from gun 45 produce red light only, those from gun 46 produce green light only and those from gun 47 produce blue light only. The higherfrequency components of the received signal are applied via filter circuit 33 to the electron gun 52. while the lower-frequency components are applied via filter circuit 32 to a switching circuit 54 which directs these components through circuit 55, 56 or 57 to the appropriate one of the electron guns according as to whether the instantaneous signal being received corresponds to a red, green or blue component of the picture. Third embodiment, Fig. 3. The receiving tube comprises, as previously, a double-sided screen, the colour reproducing surface of which consists of a large number of phosphor dots arranged in groups of three, the elements of each group corresponding to respective ones of the three primary colours. The cathode-ray tube also includes an apertured electrode 66, having an aperture arranged in front of each group of phosphor dots. In a dot sequential system, the beam from the electron gun 72 is subjected to a field which rotates at the sampling frequency by means of a signal applied from circuit 75 to the deflecting coil 74. The direction of incidence of the electron beam upon the plate 66 consequentially varies in synchronism with the sampling frequency and the phosphor dots in each group are so arranged with respect to the corresponding aperture that the red, green and blue phosphor dots are successively energized once in each sampling period,