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Test pattern


A test card, also known as a test pattern in North America, 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. (Compare with 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 (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 (Carol Hersee) and a clown doll in a circle in the centre of the picture.

Test cards are also typically broadcast with library music (see below), a sine wave reference tone, or the relayed broadcasting of a radio station owned by the same broadcaster. There is now a cult following for test card music. See the "Test Card Circle" website here: [27731]

BBC test cards

BBC test cards are identified by a letter.The most famous Britishmarker test card is Test Card F which incorporates a colour photograph of Carole Hersee (daughter of BBC engineer George Hersee) playing noughts and crosses with a doll, used on the BBC and ITV from the beginning of colour broadcasts in the late 1960s. It was later updated as Test Card J, and for widescreen broadcasts as Test Card W. Test Card F has often been spoofed by comedians.

Decline

Formerly a common sight, test cards are now only rarely seen outside of television studios, post-production, and distribution facilities. In particular, they are no longer intended to assist viewers in calibration of television sets. Several things have led to their demise for this purpose:

  • Modern microcontroller-controlled analogue televisions rarely if ever need adjustment, so test cards are much less important than previously. Likewise, modern cameras and camcorders seldom need adjustment for technical accuracy, though they are often adjusted to compensate for scene light levels, and for various artistic effects.
  • Use of digital interconnect standards, such as CCIR 601 and SMPTE 292M, which operate without the non-linearities and other issues inherent to analogue broadcasting, do not introduce colour shifts or brightness changes; thus the requirement to detect and compensate for them using this reference signal has been virtually eliminated. (Compare with the obsolescence of stroboscopes as used to adjust the speed of record players). On the other hand, digital test signal generators do include test signals which are intended to stress the digital interface, and many sophisticated generators allow the insertion of jitter, bit errors, and other pathological conditions that can cause a digital interface to fail.
  • Likewise, use of digital broadcasting standards such as the DVB and ATSC eliminates the issues introduced by modulation and demodulation of analogue signals.
  • Test cards including large circles were used to confirm the linearity of the set's deflection systems. As solid-state components replaced vacuum tubes in receiver deflection circuits, linearity adjustments were less frequently required (few newer sets have user-adjustable "VERT SIZE" and "VERT LIN" controls, for example). In LCD and other deflectionless displays, the linearity is a function of the display panel's manufacturing quality; for the display to work, the tolerances will already be far tighter than human perception.
  • In developed countries such as the United Statesmarker and the United Kingdommarker, the financial imperatives of commercial television broadcasting mean that air-time is now typically filled with programmes and commercials (such as infomercials) 24 hours a day, and non-commercial broadcasters have to match this.
  • In North America, most test cards such as the famous Indian Head test card of the 1950s and 1960s have long been relegated to history. The SMPTE color bars occasionally turn up, but with most North American broadcasters now following a 24-hour schedule, these too have become a rare sight. Many Canadian Broadcasting Corporation stations broadcast a modified form of the SMPTE bars (with an additional modulated ramp at the top and a CBC logo animation in place of the Q block) late at night until late 2006, when the network moved to 24-hour broadcasting.
  • When there are in fact no standard programmes being broadcast on the channels that do not have 24-hour programming, other, more informative features such as educational shows, e.g. the BBC Learning Zone, and teletext-type programmes such as Pages from Ceefax, ITV Nightscreen and 4-Tel On View are often broadcast, the latter type acting as the better test-card substitute as they just roll continuously.
  • Australian national broadcaster SBS airs a weather map in place of a test card with music from albums sold by SBS and a ticker at the bottom of the screen during the early hours of the morning.
  • Australian community broadcaster Channel 31 in Melbourne airs Fishcam, the output of a videocamera aimed at a fish tank.
  • Some Philippine cable networks replace test cards with an advertisement showing the product, "a reason to go to sleep" and the time when the station will sign on.
  • In Singapore, since 2004, instead of showing test cards, television channels usually air radio channels while showing their station ID at the same time.
On television networks and stations in most of the Third World countries, test cards are still seen because most television networks and stations in those countries do not have 24-hour programming.

Use of test patterns and test cards is still common within television production facilities. Many of these still have analogue infrastructure, and currently analogue transmissions are still found worldwide (though the United Statesmarker is currently scheduled to require broadcasters to switch off the NTSC service in 2009—NTSC may still be a viable transmission means for cable television for several more years). Many artistic settings are still made by using test cards or test patterns in conjunction with devices like waveform monitors and vectorscopes (most modern waveform monitors include vectorscope capability), and while digital transmission eliminates many of the "analogue" effects associated with analogue television, digital broadcasting has its own set of issues.

Gallery



Image:TESTPATTERN_NBC_1024.jpg|WHO-TVmarker test patternFile:RCA Indian Head test pattern.JPG|Indian Head test cardImage:SMPTE Color Bars.svg|SMPTE color bars

Image:Philips PM5544.svg|Philips PM5544Image:Philips Pattern PM5644.png|Philips PM5644

Image:EBU Colorbars.svg|EBUmarker test cardImage:Multiburst.jpg|Multiburst test cardImage:FCC Composite.jpg|FCC Composite test cardImage:Tpulse.jpg|T-Pulse test cardImage:Telefunken FuBK test pattern.svg|Telefunken FuBK test card

Image:Zenith Test Pattern.jpg|KS2XBS "Phonevision" test patternImage:Oud testbeeld.png|Testcard from the Netherlands Public Broadcasting, used from 1978 till 1988

Image:УЭИТ.svg|Ex-USSR television test card (Ueit, ).Image:Tv.resolution.chart.0249.svg|Early soviet test card (TIT-0249BIS, ).
Other test cards include Convergence.

UK Test Card timeline

Year Notes Image
1934 The first testcard "Tuning Signals" was broadcast by BBC 1, the earliest being a simple line and circle broadcast using Baird's 30 line system, and used to synchronise the mechanical scanning system
1947 The first testcard, Testcard A is broadcast on the BBC network
1948 Testcard B. Used behind the scenes, but not broadcast
1948 Testcard C, the far superior of this and the previous, is released. Lack of specification means that there were many variants released with subtle differences
1955 The ITA Broadcasts an unlabelled testcard for the upcoming ITV service
1955 A further ITA testcard featuring a greatly simplified testcard C is broadcast
1960s The ITA "Picasso" Testcard is released
1964 Testcard D is released in 405 line format. Music as well as test tones were regularly used to accompany this image on BBC1 and ITV
1964 Testcard E is released to comply with the BBC's new 625 line standard. Numerous television vendors complained that the image made on screen was unattractive - its sinusoidal frequency gratings looked soft - and TCE was withdrawn after only five days of service
1964 Once testcard E was withdrawn, the BBC released a modified version of TCC with more specific details on the inside circle.
1967 Testcard F, the most famous and used testcard, is released by the BBC to coincide with colour transmissions that started that year on 1 July on BBC2. Only limited programmes were available in colour from the start. The full output became colour on BBC2 on 2 December the same year. It features a picture of Carole Hersee playing noughts and crosses
1969 BBC1 & ITV begin colour transmissions and usage of testcard F. The BBC1 version was simply the BBC2 version of the 35mm transparency with the letters "BBC1" electronically keyed over the top of "BBC2 COLOUR". The ITV version had the name of the station operating in that particular area, except London, which read "Thames Television/London Weekend Television". TCF was broadcast simultaneously on both VHF-405 lines and UHF-625 lines (the system it was designed for in the first place)
1970s Testcard G - a variant of the Philips PM5544 test pattern, is created but only broadcast occasionally on BBC1 as well as on BBC2.
1979 The Independent Broadcasting Authority (IBA) - Independent Television Authority (ITA) prior to 1972, introduce ETP-1/Electronic Test Pattern One to replace Testcard F within the ITV regions. ETP-1 was also extensively used by Channel 4 and S4C in the run up to the launch of these channel's in November 1982 - using 'IBA:CH4'/'IBA:S4C' captioning instead of the 'IBA' captioning used by ITV. ETP-1 became a common sight on British television in the 1980s up until ITV started broadcasting 24 hours a day in 1988. Channel 4/S4C continued to use ETP-1 - using 'NTL:CH4'/'NTL:S4C' captioning from 1990 after the Broadcasting Act 1990 saw the privatisation of the IBA's transmitter network and sale to National Transcommunications Limited/NTL. However ETP-1 disappeared in1992 when Channel 4 simply broadcast its teletext service 4-Tel on View whilst off air - it later began 24 hour broadcasting in 1997, with S4C simply broadcasting black screen and tone whilst off-air.
1984 Testcard F is converted to an electronic format
1999 Testcards J and W are released, replacing F. Testcard J is a modified version of F, with improvements including an improved centre picture and a dot in the white area at the top. W is similar but designed in 16:9 widescreen.
2007 British Sky Broadcasting create a 1080 line high definition test card for their recently launched HD service. The style is similar to Testcard F with the girl being replaced by Myleene Klass


Test patterns for photocopiers

A lesser-known kind of test pattern is used for the calibration of photocopiers[27732][27733]. Photocopier test patterns are physical sheets that are photocopied, with the difference in the resulting photocopy revealing any telltale deviations or defects in the machine's ability to copy.

In numismatics

Television has had such an impact in today's life, that it has been the main motif for numerous collectors' coins and medals. One of the most recent ones is The 50 Years of Television commemorative coin minted in March 9 2005 in Austriamarker. The obverse of the coin shows a "test pattern", while the reverse shows several milestones in the history of television.

Monoscope

Close-up, showing test card target.
Rather than physical test cards, which had to be filmed, an alternative was to use a cathode ray tube, driven backwards from a television tube, so that it generates an image rather than displaying an image. These were fragile, but had advantages over test cards, always being properly framed and in focus.They fell out of use in the 1960s, as they were not able to produce color images.

References

  1. Test card special, by Ryan Dilley, BBC News, 19 April, 2001


External links




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