Test card

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Test pattern
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 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 nowadays generated by test signal generators, which do not depend on the correct configuration of (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.

Test cards are also typically broadcast to a background of specially composed music (to avoid having to pay licensing fees for existing compositions), a tone, or the relayed broadcasting of a radio station owned by the same broadcaster. There is now a cult following for test-card music.


[edit] BBC test cards

BBC test cards are identified by a letter. The most famous British 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.

[edit] 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 States and the United Kingdom, 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.
The current modified SMPTE colour bar test pattern used by the CBC.
The current modified SMPTE colour bar test pattern used by the CBC.
  • 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 still broadcast a modified form of the SMPTE bars late at night.
  • 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 will the station sign on.

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 as of March 2006 analogue transmissions are still found worldwide (though the United States 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.

[edit] Test card music

Background music often plays during the broadcast of a test card. This music is usually a composition commissioned by the station itself or "royalty-free" stock music in order to avoid having to pay royalties for something that does not generate revenue.

Test card music became popular in its own right when a group of enthusiasts discovered one another and realised they were not alone in enjoying the music played during the day in the 1950s, 60s and 70s while the test card was broadcast on BBC. The Test Card Circle, formed in 1989, is a group of such enthusiasts.

[edit] Gallery

Other test cards include Convergence.

[edit] Timeline

  • 1934- The first testcard "Tuning Signals" are broadcast, the earliest being a simple line and circle broadcast using Baird's 30 line system.

Image:Baird Circle.gif

  • 1939- The famous "Indian Head" test card appears in North America for the first time.
  • 1947- The first testcard, Testcard A is broadcast on the BBC network.

  • 1948- Testcard B produced, 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, accompanied with a monotonous hum- the first but not last to be so.

  • 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 and TCE was withdrawn after only a day 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 1st July on BBC2. Only limited programmes were available in colour from the start. The full output became colour on BBC2 on 2nd December the same year. It features a picture of Carole Hersee playing noughts and crosses with doll.

  • 1969- BBC1 & ITV begin colour transmissions & usage of testcard F. The BBC1 version was simply the BBC2 version 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". TF was broadcast simultaneously on both VHF-405 lines & UHF-625 lines (the system it was designed for in the first place).
  • 1970's- Testcard G is created, but only broadcast occasionally on BBC2.
  • 1979- The IBA's (called the ITA pre-1972) ETP-1 testcard is released and is used extensively up until Channel 4 and ITV start broadcasting 24-hours.

  • 1984- Testcard F is converted to an electric duplicate.
  • 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 widescreen.

  • 2001 - Testcard is incorporated [1] into a web browser (Twibright Links) to facilitate gamma, aspect ratio and brightness calibration.

[edit] References

[edit] External links

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