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The most fundamental job of a video decoder is to separatethe color from the black and white informaTIon for video compositesignals. This task has been achieved many ways sincethe introducTIon of color television over 50 years ago. Manydifferent separaTIon methods have been used through theyears. With the availability of new cost effecTIve technologies,the consumer has been seeing a gradual improvement inpicture quality and detail. Advances in display tube technologyand semiconductor processes have pushed the technologicalenvelope providing sharper, more robust video. Butseparating the chrominance from the luminance information isespecially challenging due to the fact that the signals overlapeach other in the frequency spectrum. How do you separatethem, while minimizing display artifacts?Composite Signal ConstructionThe composite video signal is constructed with 3 basic elements:- Luminance Information from DC to 5.5MHz (B&W Detail)- Chrominance Information modulated onto a carrier (at3.58MHz or 4.43MHz)- Synchronization Information (Horizontal and Vertical Sync)The three analog elements of a composite video signal carryall the information necessary to display a two dimensionalpicture on a cathode ray tube (CRT) television.Luminance (a B&W World)The luminance signal carries the black and white parts of thepicture. This component of the composite video signalrequires the most bandwidth (typically to 5MHz), and signalintegrity, to convey sharp and clear images. Edge information,brightness, and contrast of the image are entirely containedin the luminance portion of the signal. Until 1947 thebroadcast video signal was only black and white. To maintaincompatibility with the installed equipment of the time, color orchrominance information was added to the luminance signalto create the color composite signal as we know it today. Figure1 shows the specified bandwidths for NTSC and PAL.
