GB1316425A - Colour television reproducers - Google Patents

Abstract

1316425 Television BRITISH BROADCASTING CORP 28 May 1970 [29 May 1969] 27225/69 Heading H4F Signals 11R, 11G, 11B to be fed to the guns 10 of a conventional shadow mask tube are combined at 14 with a pulse occurring during the back porch of the line blanking period, the pulse having an amplitude which would produce mid-grey on the tube, and the signals are clamped, 16, during the pulsed portion to a reference value 22 also corresponding to midgrey. To align the receiver potentiometers 28G, 28B are adjusted to vary the voltages at grids 27 of the electron guns, effectively varying the D.C. level of the signals in the G and B channels, until the mid-greys in a reproduced test card are uncoloured, Fig. 2b. Potentiometers 17G and 17B are then adjusted to alter the gains of the G and B channels and give uncoloured black and white portions. Since the signals are clamped at the reference level, the G and B curves tend to rotate about the fixed point, Figs. 2c and 2d. Finally a ganged potentiometer 13 is adjusted to give correct reproduction of black. The pulses supplied to adders 14 are derived at 20 from the sync. signal. The amplitude of the maximum expected video signal is 7/3 times the amplitude of the sync. signal so that pulses of amplitude corresponding to mid-grey are easily derived. A variable resistor 21 further adjusts the amplitude of the added pulses to a value up to 60% of peak white, but preferably 20-50% of peak white. Adjustment of resistor 21 provides a brilliance control having the same grey scale tracking as for the video signals.