How teletext works! In Europe, it is still used. There is weather, classifieds, LOTS of "seeking partner" ads usually for the older generation, etc...
Really neat technology/"hack" when you think about it
Wow. Your comment prompted me to ask my dad whether he still uses teletext. I assumed the answer would be 'no', because he can read and watch news on his iPad.
But apparently he still uses teletext for quick access to news headlines and share prices.
The channel guide and semi-interactive text content (i.e. "modern" teletext) are special packets in the MPEG stream, and are cached by the receiver.
I believe the multiplayer adapter was piloted in a few select regions, but I cannot say for certain.
Anyway, I did have a friend with Sega Channel growing up. It was revolutionary. I would see nothing like it again until well over a decade later.
An interesting detail of the SEGA Channel is that it was a one-way modem. There was no way for the cartridge to communicate anything back to the headend. Instead, there was a constant broadcast of
Game 1, Menu program, Game 2, Menu program, Game 3, ...
looping forever. When you selected a game, it would just wait for it to come around on the loop and store it locally.