Colour Test Card Generator
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.
The teletext character set is similar to the standard ASCII character set, but with a few
differences:
| Teletext | ASCII |
|---|
 | [ |
 | ] |
 | ^ |
| ½ | \ |
| ¼ | { |
| ¾ | } |
| – | _ |
| ÷ | ~ |
| || | | |
In addition, a small solid block is produced by the DEL character (CTRL-backspace)
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.
| Key | Attribute |
|---|
| F1 | alphanumeric red |
| F2 | alphanumeric green |
| F3 | alphanumeric yellow |
| F4 | alphanumeric blue |
| F5 | alphanumeric magenta |
| F6 | alphanumeric cyan |
| F7 | alphanumeric white |
| Shift+F1 | graphics red |
| Shift+F2 | graphics green |
| Shift+F3 | graphics yellow |
| Shift+F4 | graphics blue |
| Shift+F5 | graphics magenta |
| Shift+F6 | graphics cyan |
| Shift+F7 | graphics white |
| F8 | flash |
| Shift+F8 | steady |
| F9 | new background |
| Shift+F9 | black background |
| F10 | double height |
| Shift+F10 | normal height |
| F11 | separated graphics |
| Shift+F11 | contiguous graphics |
| Crtl+F1 | hold graphics |
| Ctrl+F2 | release graphics |
| Crtl+F3 | start box |
| Ctrl+F4 | end box |
| Ctrl+F5 | conceal |
By default text displays as white characters on a black background. You can change the
text colour by pressing one of the following keys:
| F1 | red text |
| F2 | green text |
| F3 | yellow text |
| F4 | blue text |
| F5 | magenta text |
| F6 | cyan text |
| F7 | white text |
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.
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+F1 | red graphics |
| Shift+F2 | green graphics |
| Shift+F3 | yellow graphics |
| Shift+F4 | blue graphics |
| Shift+F5 | magenta graphics |
| Shift+F6 | cyan 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.
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.
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.
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.
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.
©
Richard Russell 2005