Teletext

Special characters

Within Teletext mode the following character sequences are interpreted specially, affecting all characters that follow them :

The colours within teletext mode are based on the primary and secondary colours created by the binary addition of the red (bit 0), green (bit 1) and blue (bit 2) components. The following colours are therefore available :

Double height characters are usually represented by the same character being present in both the upper and lower cells of the double height character. If the two characters are not the same, or the colours of those characters differ, the display will have undefined results. Graphics characters cannot be double height.

Differences in character set

The text character codes differ in their representation from the more commonly used ASCII character set. The following changes should be noted :

Characters with bit 7 set are almost identical to those with bit 7 clear (with the exception of the codes above which will be treated specially). The following changes should be noted for the related characters with bit 7 set (differences from the above) :

Graphics characters

Graphics characters are composed of parts of the character set. Each graphic symbol is a 2x3 grid, constructed from the bits of the characters :

+---+---+

| 0 | 1 |

+---+---+

| 2 | 3 |

+---+---+

| 4 | 6 |

+---+---+

Graphics character processing is only applied to codes in the range &20-&3F and &60-&7F. This means that the block of characters in the range &40-&5F, which includes the upper case text characters, are rendered as text. By default, graphics characters are non-separated and are rendered as solid, joining to the character beside them. After the 'separated graphics' have been selected, the cells of the character will be separated from the surrounding cells by the background colour of the character.

The control characters described above are usually rendered as if they contained a space character. In 'hold mode', however, the last graphic character rendered will be used in place of the control character.

Selecting a teletext mode

Only a single teletext mode is defined by default - mode 7. This provides the standard 40x25 character teletext mode as expected by most applications. However, specialist applications may select any mode they wish to use as a teletext mode through a mode specifier block with the mode flags set to 6 (gap mode and teletext). If a particular number of characters is required within the teletext mode these can also be passed within the mode specifier to as mode variables 1 (ScrLCol) and 2 (ScrBRow). If the number of characters specified cannot be met, the closest number which can will be selected. Whilst any colour depth is possible for teletext modes, it is not necessary to select more than 256 colours. Usually only 16 colours would be necessary, but the restrictions on most modern graphic hardware means that 16 colour modes are unsuitable. By default, mode 7 now uses 640x480 in 256 colours (under earlier versions of the operating system it used 320x250 in 16 colours).

The mode string parser used by OS components has been extended to allow teletext modes to be selected (although these are obviously not usable within the Desktop). The new TX and TY parameters can be used to change the number of characters in the display. The T parameter selects a teletext mode with a given number of colours. Consult the OS_ScreenMode changes document for more details.

Controlling the teletext mode

The features of teletext mode can be controlled by VDU commands. Only a limited feature set is currently controllable, although a number have been reserved for future expansion. VDU 23,18 is reserved for teletext control operations. The following operations are available, and are described in the VDU Codes section :

VDU VDU 23,18,1, VDU VDU 23,18,2 and VDU VDU 23,18,16 are described in the VDU Codes section below.

Limitations

The only limitation imposed by teletext mode is that RISC OS may no longer be configured to start in such modes. If MODE 7 is configured for startup the system will default to a suitable mode for the configured monitor type.