The system I'd love to have a better understanding of is the one TiVo deployed for the transmission of data to their boxes in the early 2000s.
They would buy a 30 minute paid programming in the early hours on cable TV networks under the name "Teleworld Paid Programming". The box would tune the channel, record the show, and then decode the data from the recording.
I always thought there was something interesting there, as the process would need to survive the MPEG encode/decode process on the TiVo itself in addition to whatever they needed to do the broadcast.
At one point Teletext on some channels had a two way system of sorts - a chat service where people could send messages over SMS to a paid for service, which would then be broadcast over teletext.
