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Colour Test Card Generator

Teletext Codes


The teletext screen has 24 rows each of 40 characters (the 25th row is used for the fastext coloured-key labels). Each character can be a conventional alphanumeric symbol or a 'control code'; control codes (usually) display as spaces but affect the attributes of subsequent characters on the row. These attributes include coloured text and graphics, separated graphics, background colour, flashing, double-height (text and graphics), held-graphics and reveal/conceal.

Details of the UK teletext system can be found in the Broadcast Teletext Specification (September 1976) published jointly by the British Broadcasting Corporation, the Independent Broadcasting Authority and the British Radio Equipment Manufacturers' Association.

Character set

The teletext character set is similar to the standard ASCII character set, but with a few differences:
left arrow[
right arrow]
up arrow^
In addition, a small solid block is produced by the DEL character (CTRL-backspace)

Control codes

Control codes are entered using the function keys, in conjunction with the Shift and Ctrl keys. Normally control codes display as a space (in the current background colour) but see the Held graphics section for exceptions. Control codes affect subsequent characters in the row; all rows start off as steady, single-height alphanumeric white characters on a black background.

Control codes summary

F1alphanumeric red
F2alphanumeric green
F3alphanumeric yellow
F4alphanumeric blue
F5alphanumeric magenta
F6alphanumeric cyan
F7alphanumeric white
Shift+F1graphics red
Shift+F2graphics green
Shift+F3graphics yellow
Shift+F4graphics blue
Shift+F5graphics magenta
Shift+F6graphics cyan
Shift+F7graphics white
F9new background
Shift+F9black background
F10double height
Shift+F10normal height
F11separated graphics
Shift+F11contiguous graphics
Crtl+F1hold graphics
Ctrl+F2release graphics
Crtl+F3start box
Ctrl+F4end box

Coloured text

By default text displays as white characters on a black background. You can change the text colour by pressing one of the following keys:
F1red text
F2green text
F3yellow text
F4blue text
F5magenta text
F6cyan text
F7  white text

Background colour

The background colour is normally black. To change the background colour you must first select the required colour as the current text (or graphics) colour, then enter the new background code F9.

For example to select a blue background you can enter F4 followed by F9 (which will together occupy two consecutive character positions on the screen). Remember to change the text colour back to something different, since blue text on a blue background isn't very useful! To switch back to a black background enter Shift+F9.

Block graphics

The teletext graphics capability is limited to low-resolution block graphics, where each character cell is occupied by six small blocks in an arrangement of two columns by three rows. For obvious reasons these blocks are sometimes referred to as sixels. To select a graphics mode insert one of the following controls:
Shift+F1red graphics
Shift+F2green graphics
Shift+F3yellow graphics
Shift+F4blue graphics
Shift+F5magenta graphics
Shift+F6cyan graphics
Shift+F7  white graphics
Once a graphics mode is selected each of the 64 possible patterns of sixels corresponds to an alphanumeric character code. Although you could work out which character needed to be entered to produce the required pattern, this has been made much easier by providing mouse control. Hold down the Ctrl key and click (or hold) the left mouse button to set the sixel pointed to by the mouse and click (or hold) the right mouse button to reset the sixel.

Blast-through text

Capital letters display as text even when in a graphics mode. This is handy as it allows you to incorporate text without the overhead (and wasted space) of switching from graphics mode to text mode and back again. This is called blast-through text.

Separated graphics

Normally graphics characters display as contiguous blocks (in other words, there are no gaps between the sixels). By entering F11 you can switch to the separated graphics mode; in this mode the graphics blocks are smaller and are separated by small gaps. This can give a shaded appearance and improve the subjective quality of certain types of graphics.

To switch back to contiguous graphics on the same row, enter Shift+F11.

Held graphics

Generally all control codes are displayed as spaces, in the current background colour. In the held graphics mode, which can be selected by entering Ctrl+F1, control codes are displayed as a copy of the most recently displayed graphics symbol. This permits a limited range of abrupt display colour changes.

The held graphics character is displayed in the same contiguous/separated mode as when it was first displayed. If there has been a change in the text/graphics mode or the normal/double-height mode since the last graphics character was displayed, the held graphics character is cleared and control codes once again display as spaces.

To switch held graphics mode off, on the same row, enter Ctrl+F2.


Text or graphics may be made to flash by entering F8; the characters alternate between the selected colour and the current background colour. To switch back to steady characters, on the same row, enter Shift+F8.

All rows begin as steady, single-height, white alphanumeric characters on a black background.

Double height

Text or graphics may be displayed in double-height, where one row contains the top halves of the characters and the next row contains the bottom halves. To select double-height, press F10; to switch back to single-height, on the same row, press Shift+F10. Wherever you have single-height characters the second row of the pair will contain spaces (in the current background colour).

If a line contains a double height code anywhere then the line contents will completely define the look of that and the following line. Any characters in the following line are completely ignored (including double height codes). The background colour of the second line always follows the first (even before the first double height code).


The conceal code (Ctrl+F5) causes subsequent text in the row to be hidden (displayed as spaces in the current background colour) until the next text colour or graphics colour control code is encountered. To reveal the hidden text press the Tab key.


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© Richard Russell 2005