Patent GB-1300463-A (GB)
Title: Television display arrangement
IPC: H04N9/24
Filed: 1970-03-02
Abstract: 1300463 Cathode-ray tube operating circuits; television PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 2 March 1970 [5 March 1969] 9898/70 Headings H4F and H4T  In an arrangement for stabilizing the beam current of a television cathode-ray display tube at the black level in the video signal, a blackerthan-black level is introduced periodically into the video signal so that the beam is suppressed and a magnitude proportional to the cathode-current which flows during the occurrence of such blacker-than-black level is subtracted from a magnitude proportional to the cathode current during the occurrence of the black level in the video signal. The difference between the two currents is then a true measure of the actual beam current associated with picture black and is not influenced by "perturbation signals" which inter-alia depend on picture content and occur between the cathode and control grid during the occurrence of the black level. Fig. 1, shows the general arrangement in which the composite television signal is supplied via input 23 to terminal 21 of a combination device 15 and thence via output terminals 13, 17 to the cathode and the control grid respectively of the display tube 1, the D.C. level being restored prior to input 23 or by restoration means in the device 15. During line blanking periods the switches 11 and 93 are operated from the positions shown under the control of a line frequency pulse generator 77 so that the cathode circuit is now completed via a current amplifier 97, a change over switch 41 and a second combination device 31 to an input 25 of the combination device 15. Switch 41 is operated by the output of a 2:1 divider 57 supplied from generator 77 to change over during alternate line blanking intervals. During such alternate blanking intervals a gate 61 supplies line and when appropriate, field fly-back pulses from respective inputs 73, 71 to a further input 27 of combination device 15 which then operates to produce a blackerthan-black blanking pulse in the output between terminals 13 and 17 whereas during the intervening blanking intervals (gate 61 closed) the normal black level in the video is effective. Thus the beam of the display tube is cut-off during alternate blanking intervals and reduced to the level determined by the black level in the video signal during the intervening blanking intervals so that the combination circuit 31 is supplied with the cathode current due to the "perturbation signals" alternately with the cathode current determined by the black level in the video signal plus the cathode current determined by the "perturbation signals". Circuit 31 operates to subtract the former current from the sum of the two latter currents and supplies input 25 of combination circuit 15 with a true measure of the beam current corresponding to the black level in the video signal. The input at 25 is then utilized in a control loop including switch 11 via the mid-point of a floating power supply (contained in circuit 15) to maintain the beam current at such value. Instead of operating change-over switch 41 during alternate line blanking intervals it may be operated during each line blanking interval during alternate field blanking intervals. The switches and other devices in Fig. 1 employ transistor circuitry details of which are described with reference to Figs. 2, 3 and 4 (not shown).