Teletext

Teletext may mean one of the following:

• The screen format, character set and control codes implemented by the Mullard SAA 5050 family and emulations thereof;
• A standard for transmitting and interpreting text information pages within a video signal;
• "Teletext", ITV's public teletext service (formerly Oracle).

This article deals exclusively with the screen format and the applications of Teletext adaptors on the BBC Micro.

Screen format

Teletext pages are displayed on a 40 × 25 character text-only screen. The display is normally driven from a memory buffer with one byte allocated to each character. Teletext uses a 7 bit character set based on ASCII, with extra control codes to select colours, graphics and display modes. These control codes occupy a character cell and take effect until the end of the text row.

Receiving Teletext on the BBC Micro

Acorn offered Teletext receivers that plugged into the 1 MHz bus. Acorn's device was installed in the iconic 'cheese wedge' case and could be tuned to four channels. The Morley Electronics teletext adapter connected to the user port. Both systems were accompanied by a ROM that converted the micro to a Teletext viewer.

Until the withdrawal of analogue television broadcasting in the UK, a TV licence fee was payable as the devices were capable of receiving television broadcasts.

Telesoftware

This was a system of transmitting free software over TV channels. A similar concept for radio, Basicode, ran on BBC Radio 1 and the Dutch station NOS for much of the 1980s. Special pages containing text listings of the software were grabbed by the receiver and saved to cassette or disc. Acorn's Teletext ROM required a patch to receive some programs correctly, and the patch program "PATCH2" was a permanent member of the telesoftware menus.

Acorn reserved the command *TELESOFT to select the Teletext filing system.

-- beardo 22:39, 7 November 2007 (UTC)