Patent GB-693710-A (GB)
Title: Improvement in colour television reproduction tube
IPC: H04N9/26
Filed: 1950-11-15
Abstract: 693,710. Colour television tubes. RADIO CORPORATION OF AMERICA. Nov. 15, 1950 [Nov. 30, 1949], No. 27917/50. Class 39(i) [Also in Group XL(b)] A colour television receiver comprises a single cathode ray tube having a directional' screen 8, Fig. 1, consisting of a plurality of like groups of fluorescent elements, the elements of a group fluorescing in a plurality of different colours, the arrangement being such that the colour of response is determined by the direction of approach of the beam, a control electrode 10 for modulating the beam in accordance with the incoming video signal, magnetic deflecting means 18 for causing the beam to scan the screen and magnetic (as shown) or electrostatic means for applying a rotating field to the beam at a region between the gun and the deflecting means to cause the beam to approach the screen from different directions in dependence upon the colour component to be reproduced. The directional screen may comprise a plurality of hollow hexahedrons, Fig. 1B, two adjacent inner faces R of which are coated with a red fluorescing material, two B with blue and two U with green fluorescing material. The beam is bent outwardly by the first rotating field produced by coils 21, 23, 26, 28 and inwardly by the second rotating field produced by coils 21<SP>1</SP>, 23<SP>1</SP>, 261, 281 180 degrees out of phase with the field of the first set of coils. The beam is focused on aperture 14 by focusing cylinder 12 and refocused by coil 16 at the plane of the screen 8, which has a high positive voltage. The coils for producing the first rotating field are arranged in two pairs 21, 23; 26, 28 connected in series with variable condensers 30, 32 respectively, and the two pairs are connected in parallel to one end of the transformer secondary 24. The two parallel circuits are tuned to opposite sides of resonance by adjustments of the condensers in such a way as to displace their magnetic fields by 90 degrees. The coils 211, 231, 261, 281 are similarly arranged and connected to the other end of the transformer secondary 24. The centre of curvature of the screen 8 is at the aperture 14, and the surface of the tube wall is frosted so that the screen light from the differently coloured phosphors appears to the observer to originate at the same point. A diaphragm 42, Figs. 1, 1A, with apertures 44 is provided to interrupt the beam when it would tend to strike more than one coloured.phosphor simultaneously. Alternatively circuits may be provided to key the beam on at appropriate intervals, Fig. 2 (not shown). The incoming wave detected at receiver 46 comprises repeated red, blue and green video signals that are time division multiplexed at a repetition frequency of 3.8 Mcs for each colour. As the scanning proceeds, the phase of these signals may vary to produce a desired dot pattern. The signals are applied to control grid 10 via lead 70, and to scanning circuit 17 via synchronisation separator 48. To synchronise the phase and frequency of the multiplexing operations at the transmitter and receiver a burst of 3.8 Mc energy is transmitted on the ' back porch' of the horizontal blanking pulse. The horizontal synchronisation pulses from separator 48 are differentiated by condenser 50 and resistor 52 and the resulting negative pip ' reflected by diode 54 and inverted by phase inverter 56. The ' positive pip ' so derived keys a flip-flop multivibrator 58 which gives a positive rectangular pulse of a duration equal to the back porch ' interval. This positive pulse is applied to grid 60 of a gating tube 62 to the second grid 64 of which is applied the incoming signal. The 3.8 Mc burst can thus be selected, passed to amplifier 66 and thence via cathode coupling to 3.8 Mc oscillator 68. The oscillator 68 is pulled into step with the incoming 3.8 Mc burst and its output is applied to primary winding 20 of transformer 22. To ensure that the proper colours are produced by the proper signal components the tube 4 may be rotated or phase rotation means 72 suitably adjusted. In a modification, Fig. 2 (not shown) the rotating fields are electrostatic. The synchronised 3.8 Mcs oscillator is connected to a phase splitter comprising condensers and resistances to provide three outputs having a separation of 60 degrees between phases. Suitable inverting and non-inverting amplifier arrangements for the three phases produce a separation of 120 degrees between phases and the phases are combined and the resulting three phase wave amplified and applied to a further normally cut off amplifier. The latter amplifier is made conductive only at the positive peaks of the three phases and while it is conductive video signals applied to a second grid thereof are amplified and applied to the control grid of the cathode ray tube, which is normally biased to visual extinction. The video signals include a keying component to energise the beam only when it will not impinge on two differently coloured fluorescent materials simultaneously. The synchronised 3.8 Mcs from the oscillator is also supplied through an isolation amplifier to a phase splitter and the two outputs with 90 degrees relative displacement are separately amplified by push-pull amplifiers and thence passed to plates inside the cathode ray tube for rotating the beam. An alternative form of directional screen comprises a transparent plate with a number of groups of phosphors, e.g. in a three colour system a group comprises three differently coloured spots at the corners of an equilateral triangle, and a multi-apertured plate, each aperture overlying the centre of the triangle and a portion of each spot.