Test card

A test card, also known as a test pattern or start-up/closedown test, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast (often at sign-on and sign-off), and may be accompanied by a bleep sound. Used since the earliest TV broadcasts, test cards were originally physical cards at which a television camera was pointed, allowing for simple adjustments of picture quality. Such cards are still often used for calibration, alignment, and matching of cameras and camcorders. From the 1950s, test card images were built into monoscope tubes which freed up the use of TV cameras which would otherwise have to be rotated to continuously broadcast physical test cards during downtime hours. Electronically generated test patterns, used for calibrating or troubleshooting the downstream signal path, were introduced in the late-1960s, and became commonly used from the 1970s and 80s. These are generated by test signal generators, which do not depend on the correct configuration (and presence) of a camera, and can also test for additional parameters such as correct color decoding, sync, frames per second, and frequency response. These patterns are specially tailored to be used in conjunction with devices such as a vectorscope, allowing precise adjustments of image equipment. The audio broadcast while test cards are shown is typically a sine wave tone, radio (if associated or affiliated with the television channel) or music (usually instrumental, though some also broadcast with jazz or popular music). Digitally generated cards came later, associated with digital television, and add a few features specific of digital signals, like checking for error correction, chroma subsampling, aspect ratio signaling, surround sound, etc. More recently, the use of test cards has also expanded beyond television to other digital displays such as large LED walls and video projectors.

Technical details

Test cards typically contain a set of patterns to enable television cameras and receivers to be adjusted to show the picture correctly (see SMPTE color bars). 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 (100 IRE), 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 color burst signal may go below 0 IRE). Between the color bars and proper adjustment of brightness and contrast controls to the limits of perception of the first sub-black bar, an analog receiver (or other equipment such as VTRs) can be adjusted to provide impressive fidelity. Test cards have also been used to determine actual coverage contours for new television broadcasting antennas and/or networks. In preparation for the new commercial ITV service in the 1950s, the Independent Television Authority (ITA) tasked Belling & Lee, an Enfield-based British electronics company best known for inventing the Belling-Lee connector just over three decades earlier, with designing a series of Pilot Test Transmission test cards and slides intended for potential viewers and DX-enthusiasts to test the ITA's new Band III VHF transmitter network that was designed with the assistance of the General Post Office (GPO), then the UK's government-run PTT agency. These test cards, some featuring the G9AED call sign assigned by the GPO for said transmissions, featured a squiggly line in a circle in the middle of the test card with an on-screen line gauge indicated in miles which was used as a guide to reveal the distance between the receiver, the (temporary) transmitter and a replicating landscape feature causing ghosting. Said test cards were mainly transmitted from temporary mobile transmitters attached to caravan trailers based at the predicted locations of the ITA's eventual main transmitters, such as Croydon, Lichfield, Emley Moor and Winter Hill. Almost a decade later, the BBC started using a modified SMPTE monochrome test card radiating from the Crystal Palace transmitter to test its new UHF network which it eventually launched as BBC Two in 1964. Test cards are also used in the broader context of video displays for concerts and live events. There are a variety of different test patterns, each testing a specific technical parameter: gradient monotone bars for testing brightness and color; a crosshatch pattern for aspect ratio, alignment, focus, and convergence; and a single-pixel border for over-scanning and dimensions.

History

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