In Vision Decoder
A Teletext In-Vision Decoder is a decoder which outputs the decoded page to another device for display. This is in contrast to a decoder built into a television, which can be viewed only on that television's display and (usually) not output to any external devices. The "in-vision" refers to the page being in the visible part of the video signal as opposed to "out-of-vision" where the teletext signal usually resides.
The first teletext decoders where all "in-vision" in a sense, as they were optional extras which could be plugged into a TV to see the decoded page. As teletext became more popular, TVs started appearing with the decoders built in, with obvious advantages, and in-vision boxes gradually become less common, now generally only found in studios and enthusiasts' homes.
Use in Broadcast
In-Vision decoders were used to broadcast specific teletext pages (usually carousels) over the air at times TV stations would normally be off-air, for example during the afternoon and late at night. In the early days this was essentially a marketing tool to show those without their own decoders or teletext-capable sets what they were missing out on, but in later years it was used only to fill up dead air during the night. Probably the most well-known use of In-Vision teletext was the BBC's Pages From Ceefax, which broadcast a carousel of news, sport and weather information taken directly from the Ceefax service. This officially ended on BBC2 on the 22nd of October 2012, just before the final analogue region in the UK (Northern Ireland) was switched off the following day. Other services of note include ITV's Jobfinder and 4Tel on view, although this was likely not generated by an in-vision decoder in the true sense.