Patent GB-678159-A (GB)
Title: Improvements in or relating to systems for the transmission of stationary or animated coloured images
IPC: H03K4/39
Filed: 1948-06-07
Abstract: 678,159. Colour television. PHILIPS ELECTRICAL, Ltd. June 7, 1948 [June 10, 1947], 15321/48. Class 40 (iii). A colour television system comprises means including a pair of plane mirrors, for projecting a plurality of images of the object to be televised on to the photo - sensitive screen of a television camera tube in respective component colours, the images being scanned by a single cathoderay beam to develop a single television signal which is transmitted to a receiver, whereat the plurality of images is reproduced on the screen of a cathode-ray tube in their respective component colours, and at the receiver optical means comprising a pair of plane mirrors used for projecting and combining the component images into a single coloured image on a screen. First embodiment, Fig. 1. Three images 3<SP>1</SP>, 61, 71 of the object 1 to be televised are projected on to three separate portions of the mosaic electrode of an iconoscope 10 by means involving a pair of parallel plane mirrors 4, 5. The portions of the mosaic corresponding to the images 3<SP>1</SP>, 61 and 7<SP>1</SP> are covered with blue, red and green filters respectively so that currents, set up in the resistor 20 as the mosaic is scanned by the electron beam 14, successively represent the red, blue and green components the light from the object 1. The receiver may comprise a similar arrangement wherein the iconoscope is replaced by a receiving cathode ray tube having a screen appropriate portions of which fluoresce red, blue and green. The three images formed on the screen of the cathoderay tube are superimposed on a ground glass screen by a projection system involving a pair of parallel plane mirrors similar to those shown in Fig. 1. Fig. 2 (not shown), illustrates a receiver using a Schidmt optical system in conjunction with a pair of parallel mirrors, the correction plate being mounted around the neck of the cathode-ray tube. Second embodiment, Fig. 3. At the transmitter four symmetrically spaced images of an object 33 are projected on to the mosaic electrode of a television camera tube by means of an optical system comprising a lens 34 and a pair of orthogonally arranged plane mirrors 31 and 32. The mirrors intersect in a line passing through the scanning centre of the mosaic screen and coinciding with the optical axis of the system. The portions of the screen corresponding to the four images are sensitive either to four different colours or to three different colours, two of the portions having in the latter case the same colour sensitivity. Signals derived from scanning the mosaic are transmitted to a receiver wherein the scanned images are reproduced on a cathode-ray tube and re-constituted by projection on to a screen by an optical system similar to that shown in Fig. 3. In the system of Fig. 1 variations in the anode potentials of either the transmitting or receiving cathode-ray tubes would cause expansion or contraction of the images reproduced on the screen of the receiving cathoderay tube and would thus prevent accurate superposition of the three component images by the parallel mirrors. This effect and other symmetrical distortions which may occur are obviated by the system of Fig. 3. If these distortions are small, the system may be modified as illustrated in Fig. 4 (not shown), to use the whole aperture of the lens 34. In this arrangement the line of intersection of the mirrors 31 and 32 passes outside the lens 34 and intersects the plane of the object 33 in the optically conjugate point of the scanning centre of the mosaic electrode. Interlacing of colour components, Fig. 5. Fig. 5 illustrates a system of the type described with reference to Fig. 1 and comprises 11 lines per frame, lines 1 to 4 representing the red, lines 5 to 7 representing the blue, and lines 8 to 11 representing the green components of the image. As shown, the mirrors S1 and S2 are displaced one-sixth of the spacing between the lines below line 4 and above line 8 respectively, thus resulting in interlacing of the three colour components. In addition normal doubler interlacing may be provided. Modified scanning arrangements. In order to reduce the effects of non-linearity of the scanning waveforms and also enable the signals transmitted to be received by a normal blackwhite receiver, the horizontal and the vertical scanning in the embodiment of Fig. 3 may be effected by waveforms as shown in Figs. 6a and 6b. Means for deriving the waveform of Fig. 6a is illustrated in Fig. 7 (not shown), and comprises two pairs of gas-filled valves which conduct alternately and are switched over by the fly-back of the saw-tooth waveform applied thereto. The frequency of the applied waveform is twice the line (or frame) scanning frequency and the valves are so connected that the currents through the deflection coils in alternate saw-tooth strokes are in opposite directions. Any lack of linearity of the scanning waveform will therefore distort the component images equally and allow exact superposition of the reproduced images to take place. The use of such waveforms and, in the case of the system of Fig. 1, waveforms such as shown in Figs. 9a and 9b allows the transmitted signals to be received by a normal black-white receiver. Four-colour systems. The embodiment illustrated in Fig. 3 may be made to operate as a four-colour system in which case the component colours are chosen to be complementary pairs K1, K2 and K3, K4 as shown in the colour triangle, Fig. 13. Double interlacing is used, one set of lines being used for the colours K1 and K2 and the other set for the colours K3 and K4. Thus in parts of the image having only one of the component images, the definition corresponds to that obtained with half the total number of lines used; however, in the more common substantially white parts of the picture all the scanning lines provide a nearly accurate representation of the colour and the definition is increased accordingly.