Test Pattern

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A test card, also known as a test pattern, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast (often at startup and closedown). Used since the earliest TV broadcasts, test cards were originally physical cards at which a television camera was pointed, and such cards are still often used for calibration, alignment, and matching of cameras and camcorders. Test patterns used for calibrating or troubleshooting the downstream signal path are these days generated by test signal generators, which do not depend on the correct configuration (and presence) of a camera. Digitally generated cards allow vendors, viewers and television stations to adjust their equipment for optimal functionality.
The test card usually has a set of line-up patterns to enable television cameras and receivers to be adjusted to show the picture correctly.  Most modern test cards include a set of calibrated color bars which will produce a characteristic pattern of "dot landings" on a vectorscope, allowing chroma and tint to be precisely adjusted between generations of videotape or network feeds. SMPTE bars—and several other test cards—include analog black (a flat waveform at 7.5 IRE, or the NTSC setup level), full white (100IRE), and a "sub-black", or "blacker-than-black" (at 0 IRE), which represents the lowest low-frequency transmission voltage permissible in NTSC broadcasts (though the negative excursions of the colourburst signal may go below 0 IRE). Between the colour bars and proper adjustment of brightness and contrast controls to the limits of perception of the first sub-black bar, an analogue receiver (or other equipment such as VTRs) can be adjusted to provide impressive fidelity. The most famous Test Card shown in the UK since 1967 and also shown in some other countries is Test Card F, which features a little girl (Carole Hersee) and a clown doll in a circle in the centre of the picture.

Source:http://en.wikipedia.org/wiki/Test_card