A Test Card (or Test Pattern in the USA) is a picture that aired on a TV channel when nothing is being aired or when the picture broadcast needed to be fixed. However, now with devices that fix the picture automatically and with 24-hour programming, Test Cards have seen decline. The most famous one is Test Card F.
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Test cards are usually fitted with various greyscales and colors to ensure that a picture looks all right. Now, analog TVs and digital TVs can get rid of the modulation and demodulation that early analog TVs had.