Patent GB-1461861-A (GB)
Title: Television display apparatus
IPC: H04N5/14
Filed: 1974-01-23
Abstract: 1461861 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 23 Jan 1974 [13 Feb 1973] 03122/74 Heading H4T In a television receiver a reference level is inserted into the video signal during at least part of a line scan period occurring during field blanking periods, a beam current measuring circuit measures the beam current of the dispay cathode-ray tube during the reference level insertion period and stores a quantity (charge) representative of the beam intensity which quantity is employed to correct departure of the reference level from a desired value, the measuring circuit additionally including a threshold circuit which, in response to a beam current intensity exceeding a pre-set level, supplies an output which "extends" the correction time either by supplying further charge to the storage means from an independent charging source or by increasing the rate at which the reference level and the measurement of the beam current is effected so that the reference level is rapidly brought to the desired level. Fig. 1 shows an embodiment in which the "extension" of the correction time is effected by supplying further charge to the storage means (capacitor 53) from an independent source. In the general operation a reference level insertion circuit 3 inserts a reference level in a video signal supplied at input 1 under the control of pulses from a pulse generator 9, the video signal, including the reference level, being supplied via a reference level correction circuit 15 and amplifier 27 to the control grid of a picture tube 31. Simultaneously with the occurrence of the reference level insertion, control pulse generator 9 supplies an output to operate a gate 37 which supplies a quantity representative of the beam current in tube 31 to a detection circuit 45 and to a threshold circuit 49. When the level of the beam current is below that at which the threshold circuit 49 responds, the output of detector 45 charges a capacitor 53 via switch 61, the charge in capacitor 53 operating via correction circuit 15 to readjust the beam current to the required level. When, however, the beam current level is so high that threshold circuit 49 supplies an output, the latter charges a second capacitor 60 and causes switch 61 to change-over from the position shown, for a time determined by the time constant of capacitor 60-e.g. several hundred line periods-so that capacitor 53 is now charged via resistor 71 from a source which is independent of beam current. During the next measurement, threshold circuit 49 determines whether this increase has been sufficient or not and change-over switch 61 then reverts to the position shown and either remains in this position or is again operated to cause further charge to be given to capacitor 53. Pulse generator comprises a shift register 81 supplied with field frequency pulses at input 75 and line frequency clock pulses at input 77 which shift the field frequency pulses, e.g. a binary "1" one section per clock pulse. Outputs 83, 85 then operate a gate 91 which supplies pulses at output 7 to control the reference level insertion and at output 41 to control gate 37. Fig. 2 shows a modification of Fig. 1 in which the capacitor 60 of the latter is replaced by a trigger circuit 99 the set input of which, 57, is connected to the output of threshold circuit 49 and the re-set input of which, 101, is connected to the field frequency input 75 of generator 9. In operation, when the trigger is set due to the threshold circuit responding to an excessive level of beam current switch 61 changes over from the position shown so that capacitor 53 is charged via resistor 71 from the independent source (as in Fig. 1). Additionally, the set output of the trigger changes over a second switch 107 in generator 9 so that a binary "1" circulates in the shift register and results in an increase in the rate at which the beam level measurement is effected. This continues until the threshold circuit is re-set when the beam current level drops below the threshold value or on the occurrence of the next field pulse at reset input 101. A modification of the embodiment of Fig. 2 in which the change-over switch 61 is omitted is described with reference to Fig. 3 (not shown). An application of the principle of the circuit of Fig. 2 to a three-gun colour television receiver in which the beam corrections are effected sequentially is described with reference to Figs. 4 and 5 (neither shown).