@cypnk I know the west german WDR ComputerClub TV show in the 80s had some clever system with the very metal sounding name "hard-bit-rock". It encoded BASIC programs into a couple of scan lines of the video signal. If you had the right filter you could then record them onto your Datasette. Here's a slightly hazy translation about the show:
Britain's BBC also used the spare lines in the vertical blanking interval (also used for Teletext) to transmit software, the BBC Micro's Teletext adapter (and other devices) could work with it but none of that hardware was cheap. The technique is still used today for the various digital text /graphics services other than linear TV and radio that are transmitted via digital video broadcasting in Europe..
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