Patent GB-1165621-A (GB)
Title: Improvements relating to Colour Television.
IPC: H04N9/27
Filed: 1967-02-03
Abstract: 1,165,621. Cathode-ray tube circuits; sawtooth circuits. INTERNATIONAL POLAROID CORP. 3 Feb., 1967 [4 Feb., 1966], No. 5412/67. Headings H3P, H3T and H4T. In a colour television receiver utilizing a display cathode-ray tube of the "penetration" type, i.e. a tube in which the colour to be reproduced is determined by the velocity (energy) of the electron beam, mis-registration of the rasters appertaining to different colours is prevented by changing the amplitude of the beam deflection waveforms synchronously with the occurrence of the change in velocity of the electrons in the beam which is required to cause the production of a desired colour. The invention is described as applied to a colour television system utilizing a "penetration" type tube 11 (Fig. 1) having a red producing phosphor 22 and a green-blue producing phosphor 23 such colours being produced when the beam velocity corresponds to 10 KV and to 15 KV respectively. In operation, the luminance component Y of a received NTSC type of colour signal is supplied via amplifier 31 to the tube cathode 37 and, to produce a field sequential display, the colour difference signals R-Y and G-Y derived from a decoding circuit 33 are supplied to the control grid 59 sequentially and cyclically via respective gates 53, 55 operated by the outputs of a multivibrator 57 controlled by the vertical synchronizing pulses of the system. One output of 57 also operates a switch 61 controlling the high-voltage supply to the anode 17 of the tube gun (connected internally to the final anode coating on the tube inner surface) such that, when gate 53 conducts, the potential applied to anode 17 is 10 KV whereas, when gate 55 conducts, the anode potential is brought to 15 KV. In accordance with the operation of the invention the rasters scanned during successive fields are caused to register precisely by utilizing the output of 57 which controls switch 61 to control the amplitudes of the vertical and horizontal scanning waveforms such that they are of greater amplitude when anode 17 is operating at 15 KV than when it operates at 10 KV. This is brought about in the case of vertical deflection by reducing the gain of amplifier 49 and in the case of horizontal scanning by switching in a shunt load 73 during operation at 10 KV. Fig. 3 shows the vertical scanning arrangement comprising the sawtooth generator 47 and the amplifier 49 the former utilizing a unijunction transistor 75 triggered by vertical synch pulses at 85 to produce a sawtooth wave across capacitor 87 which is fed via an emitter follower (feed-back for linearity control being provided by resistor 99) to the amplifier 49 comprising transistor 101 and valve 105 coupled to the vertical deflection coils 21. The gain of the amplifier is then controlled by a shunt path provided by a transistor 103, which during the 10 KV operation is rendered conductive by the output of multivibrator 57 supplied to conductor 121. For controlling the amplitude of the horizontal scan the shunt 73 comprises a valve 145 (Fig. 4) connected across the horizontal deflection coils 19 and rendered conductive by applying the same output of multivibrator 57 to its control grid. The actual horizontal deflection waveform is produced by supplying horizontal synchronizing pulses to the grid of valve 135. Line sequential operation instead of field sequential may be employed and the invention is applicable to systems operating on the classical colour theory instead of the "Land" system described.