Test card 检验图

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 programme 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. Test cards are typically broadcast with a sine wave, radio (if associated with the television channel) or music (usually instrumental).