Patent GB-1385944-A (GB)
Title: Method and apparatus for maintaining the operation of an image- display device within controlled limits
IPC: H04N5/59
Filed: 1972-04-07
Abstract: 1385944 Cathode-ray tube circuits D E SUNSTEIN 7 April 1972 [9 April 1971] 16128/72 Heading H4T A control signal produced in response to a decrease in the E.H.T. supply to a cathode-ray tube caused by excessive average beam current is employed to reduce such current automatically to a predetermined maximum value. This arrangement prevents beam defocusing which, in addition to being objectionable may in a colour television receiver, also result in a reduction in colour saturation, incorrect colour rendition and when a beam indexing tube is employed, loss of index signal information. Fig. 1 shows the one embodiment applied to a known type of beam indexing colour television receiver in which the control signal is produced across a resistor 50 (shunted by an averaging capacitor 52 to provide a suitable time constant) in series with the shunt regulator for the H.V. supply source (EHT) 46, and supplied to a control circuit 54 which controls, via the brightness control circuit 18, the maximum level of the luminance signal "Y" supplied to the control grid of the display tube 32. The control circuit 54 comprises, as shown in block 54A, (Fig. 2) a pair of transistors 140, 142 with the collector emitter path of transistor 140 connected between the manual brightness control potentiometer 102 and ground the base of transistor 142 being supplied from the resistor 50A. During normal brightness reproduction the current shunted by the regulator 48A (and resistor 50A) is such as to produce about 10 volts across resistor 50A and this maintains transistor 140 in heavy conduction so that the lower end of the brightness control potentiometer 102 is returned to ground. When the reproduced image goes "towards white" the proportion of shunted current is reduced and the drop in voltage across resistor 50A drives transistor 140 towards its cut-off to an extent dependent on the magnitude of the shunted current and when the latter is such that this voltage drops below about 0À5 volts (close to the point at which the shunt regulator would cease to exercise control so that the EHT voltage would drop) transistor 140 cuts-off and in effect, introduces a high resistance in series with the brightness control potentiometer 102. This, which is equivalent to operating potentiometer 102 in a direction to reduce the brightness, operates, in known manner, to cause the diode 86 to clip the luminance signal "Y" (the synch. pulses of which are positively-going at the cathode of diode 86) farther in the direction towards peak brightness and thus limit the maximum average brightness to a value at which the current loading on the EHT circuit 46A is below that at which the EHT voltage is reduced. In order to avoid loss of indexing signal information potentiometer 124 in D.C. restorer 20A is initially adjusted so that a desired minimum beam intensity is maintained. Instead of employing the control signal to operate on the manual brightness control, a second luninance channel comprising a variable gain I.F. amplifier stage, second detector, "Y" filter and a variable gain "Y" amplifier may be provided and the control signal employed to adjust the gains of the I.F. and "Y" amplifiers (Fig. 5, not shown). The control signal may be derived across a resistor in the cathode of the display tube (Fig. 3, not shown) or from an additional winding on the horizontal deflection transformer (Fig. 4, not shown), instead of from the series resistor in the shunt regulator circuit.