Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setrier frequency the less pronounced the Moirébecomes.In order to keep interference with the pictureas low as possible, the phase position of thecolour carrier frequency from line to line isshifted forward by 90°. In this way, bright anddark pixels only overlap after four lines. Atnormal distances the eye cannot detect thisinterference.This compensation is known as "Fourth LineOffset Procedure".If one subtracts the two signals from one another,the brightness information is eliminatedand the colour information is present at doublethe amplitude on the output of the circuit.CVBS(PALsignal)2 Hdelay14122 Hdelay14+-Y+1. ZeileChrominancebandpassC1Y2. Zeile12 3 4 5f1Chroma3. Zeile12 3 MHz 5f4. ZeileThis fourth line offset now permits the filteringout of colour components from brightness information,and the reverse. The phase shiftfrom line to line causes a counter phasing ofthe colour carrier every two lines.If a two-line delay line is inserted into the circuitand the direct and delayed signals areadded together, the colour information is removedand the brightness information is presenton the output at double the amplitude.Theoretically this circuit supplies error freeresults. A condition for this, however, is thephase and amplitude equality of the chromasignal over 3 lines, which is only the case forthe same and equally saturated colours. Inpractical terms picture joining can occur, inwhich cross colour and cross luminance interferencecannot be removed by the comb filter.Therefore, another circuit is integrated, inwhich three lines in sequence, from the pointof view of both phase and amplitude, arecompared.If a deviation is detected that goes beyond aspecific difference, the comb filter function isautomatically switched off and the separationof the two components is implemented traditionallywith chroma trap and band pass.Document Q 2500 88 © Loewe ProCollege