Editing Test card (section)

==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 (unit)|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 [[colorburst]] 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 [[Service contour|coverage contour]]s for new television broadcasting antennas and/or networks. In preparation for the new commercial [[ITV (TV network)|ITV]] service in the 1950s, the [[Independent Television Authority]] (ITA) tasked Belling & Lee, an [[Enfield, London|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<ref>{{cite web | url=https://www.youtube.com/watch?v=P6pq7XycgGw | title=1950s-60s ITA Test Card with Music | website=[[YouTube]] | date=7 May 2023 }}</ref> intended for potential viewers and [[DXing|DX-enthusiasts]] to test the ITA's new [[Band III]] [[Very high frequency|VHF]] transmitter network that was designed with the assistance of the [[General Post Office]] (GPO), then the UK's government-run [[Postal, telegraph and telephone service|PTT agency]]. These test cards, some featuring the [[Call signs in the United Kingdom#Call sign assignments for amateur radio|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 [[Mile|miles]] which was used as a guide to reveal the distance between the receiver, the (temporary) transmitter and a replicating landscape feature causing [[Ghosting (television)|ghosting]]. Said test cards were mainly transmitted from temporary mobile transmitters attached to [[Caravan (trailer)|caravan trailer]]s based at the predicted locations of the ITA's eventual main transmitters, such as [[Croydon transmitting station|Croydon]], [[Lichfield transmitting station|Lichfield]], [[Emley Moor transmitting station|Emley Moor]] and [[Winter Hill transmitting station|Winter Hill]].<ref name="auto2">{{cite web | url=https://transdiffusion.org/2021/05/10/to-the-north/ | title=To the North }}</ref><ref>{{Cite web|url=http://sub-tv.co.uk/atvtestcards.asp|title=sub-TV: ATV Test Cards|website=sub-tv.co.uk}}</ref> Almost a decade later, the BBC started using a modified SMPTE monochrome test card radiating from the [[Crystal Palace transmitting station|Crystal Palace transmitter]] to test its new [[Ultra high frequency|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 [[Display aspect ratio|aspect ratio]], alignment, focus, and convergence; and a single-pixel border for [[Overscan|over-scanning]] and [[Display resolution|dimensions]].<ref name="claiborne">{{cite web |last=Claiborne |first=Vickie |title=Video Testing Images |url=https://plsn.com/articles/video-digerati/video-testing-images/ |website=PLSN |date=9 November 2019 |access-date=16 March 2020}}</ref>