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13th June 2000

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The Television Test Card

The television Test Card is transmitted to enable engineers to check the performance of television receivers and aerial installations. Though some of the tests require specialised knowledge or equipment, many of them are very straightforward and can help the untrained viewer to decide whether or not they are getting the best possible performance from their television system. However, only skilled television engineers should carry out adjustments to the receiver that are not normally available to the viewer. Television sets contain high voltages and no untrained person should remove the back of a receiver even if it has been disconnected from the electricity supply.

Test Card F - JPEG Image

Click on the picture to download a high resolution copy of Test Card F from the BBC ftp site (141k)

Historical

With the introduction of colour, the BBC broadcast two test cards. The current one, Test Card F, carries the picture of a girl (Carol Hersee) at a blackboard and was broadcast by scanning an image on a photographic colour transparency. Test Card G is an electronically derived test card. Today only Test Card F is usually transmitted. This is now also fully electronically generated.

Up to Test Card menu

When is the Test Card transmitted?

With more programmes being transmitted throughout the day, the 'window' for the transmission of the Test Card is greatly reduced. It is not scheduled, and is usually transmitted where there is a gap in the programme schedule. This is likely to be early in the morning or during the night. You are most likely to see it on BBC 2 at weekends, before the first programme of the day.

Up to Test Card menu

Picture Geometry

It is important that the receiver is adjusted to give the correct picture size. The normal picture aspect ratio for analogue television is 4:3 (four units wide and three units high). A new test card is under development for digital television which has an aspect ratio of 16:9 (sixteen units wide and nine units high).

Test Card Arrow Head At the edge of the cathode ray tube the tips of the arrow heads at the sides and bottom should be just visible, along with most of the colour bars at the top. The picture size and width is adjusted with the appropriate controls. The white grid lines should be displayed as straight horizontal or vertical lines and also give a check on linearity. Curved lines at the edge of the picture are corrected by adjusting the pincushion correction control, the orientation by rotating the whole scanning coil assembly. The circle should appear truly circular and the background squares should be square and of equal size. The linearity controls adjust this.

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Synchronisation

The castellations on the right-hand borders of the test cards provide a check of the correct operation of the line synchronisation circuits. As the castellations at the right hand side are alternately black and white, faulty line-synchronisation will show up as a shift of complete bands of the picture. This will affect the centre circle so that it looks rather like a cogwheel. The castellations on the left-hand side of the test card serve a similar purpose and have the additional feature in that they do not line up with those on the right. If the line synchronisation is faulty the shifted bands of the picture will correspond with either the right hand or the left hand castellations, showing whether the disturbance is caused by picture information before or after the line-synchronising pulses.

Up to Test Card menu

Picture Definition

Test Card Grating 1.5 MHz Poor definition may be a function of channel tuning adjustment, incorrect tube focus, or faulty video-frequency response. The test card provides a good indication of any deficiencies. Fine detail can be checked on the frequency gratings. The gratings correspond to the frequencies shown.
2.5 MHz
3.5 MHz
4.0 MHz
4.5 MHz
5.25 MHz

On some 625-line black and white receivers all the gratings may be clearly visible. On colour receivers the 4.0 and 4.5 MHz gratings will probably be indistinct and show coloured patterns. This effect is called cross-colour and cross luminance, and is quite normal. The effect is related to the way colour pictures are transmitted and received.

Test Card Corner Focus Tube corner focus checks are provided by the corner gratings.
Poor response at low frequencies causes streaking. This means that large areas of black affect the appearance of adjacent areas of white or vice versa. The letter box is used for this and smearing will be detected if there is a problem. Test Card Letter Box

Incorrect response at very high frequencies can take the following forms:

  • Reduced response which will make the picture less sharp than it could otherwise be.
  • Enhanced response which produces unnaturally sharp outlines.
  • Very sharp cut-off which shows up as ringing.

It is very difficult to assess the extent of reduced response at high frequencies without suitable test equipment, but ringing shows up as repetitions of any abrupt boundaries between black and white areas. Look for multiple repetitions to the right of any bright-to-dark vertical edges. It can be difficult to tell the difference between ringing and short-delay ghosts but one useful technique to distinguish between the two is to vary the tuning with the AFC switched off whilst watching the effect on the repeated edges. If the repetition is caused by ghosts the repeated image remains stationary but if it is formed by ringing the repeated images will move to the right or left as the tuning is altered.

Up to Test Card menu

Grey scale

Test Card Grey Scale The group of rectangles on the left of the circle have a range of grey tones from black to white. They represent a contrast of ratio 30:1.

The brightness and contrast controls on the receiver should be adjusted so that the black spot in the darkest square is just visible and the peak white spot in the white square is visible.

When the test card is displayed on a colour receiver the grey scale will indicate if any adjustments are required to the grey-scale tracking. On a correctly-adjusted receiver there should be no hint of colour in any part of the grey scale. If there is an overall colour cast the colour background controls should be adjusted. If the whites in the grey scale are coloured or tinted, the tube drive controls need to be adjusted.

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Colour Saturation Control

The saturation control on a receiver controls the depth of colour in a picture. It is adjusted to give realistic flesh tones The girl's face on the test card is used as a guide.

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Colour Purity

Colour purity errors can show up on the test card as patches of colour appearing in the overall grey background squares. This occurs when stray magnetism upsets the deflection of the electron beams in the television display tube causing the beams to hit the wrong colour phosphors. To avoid purity errors, devices containing strong magnets such as hi-fi loudspeakers should never be placed alongside a receiver unless they are properly shielded and designed for such positioning (ie the centre speaker of a surround sound system).

Permanent magnetisation in the display tube is best remedied by careful use of a degaussing coil. Colour purity adjustment of a CRT must be carried out in accordance with the manufacturers manual, ideally on a plain raster.

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Convergence

Colour television pictures are displayed by accurately laying red, green and blue colour pictures on top of one another. Poor convergence shows up as colour fringing along edges. Convergence in the central portion of the screen should be excellent, but nearer the edges this will often be slightly less than perfect even on a properly adjusted receiver. Look for any colour fringing on the white cross of the blackboard near the centre of the screen.

Though test cards provide an excellent means of checking convergence, any adjustments should be made by using a cross-hatch generator and by following the receiver manufacturer's instructions.

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Chrominance/Luminance Delay Error

In the colour receiver the coded colour signals are first filtered from the black and white so that they can be decoded. The filter used to achieve this has a delaying effect on the colour signals and, in order that the final composite picture is correctly displayed, the black and white signal must be delayed by an equal amount. If this delay is incorrect the colour signal will produce a separate picture displaced from the black and white one. On the test card look for chrominance/luminance delay error on the yellow buttons on the green suit of the clown (name unknown). If there is an error the yellow colour of the buttons will probably be displaced to the right of the near-white image of the buttons.

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Reference Generator Faults

The reference generator in the decoder must regenerate a signal with the same frequency and phase as the colour subcarrier used in the PAL coder at the studio. To make this possible a short burst of sub-carrier is transmitted in the television signal. The colour burst is fed to the reference generator via a "burst gate" circuit. If the gate does not open at the correct time, and for the correct period, picture signals can upset the reference generator and confuse it's operation. Red and blue castellations on the left hand edge of the picture will disturb the reference generator if the gating is incorrect. The effect will be to produce horizontal bands of saturation changes across the screen in line with the coloured castellations.

Up to Test Card menu

Colour Bars

The colour bars at the top of the picture provide an overall check that the colour decoder is operating correctly, and provide a useful test signal for the service engineer with an oscilloscope.

Test Card Colour Bars

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