I had one of those but it was built into our TV, a Magnavox. It relied on a small black frame that local broadcasters used to insert their ads. National broadca

I had one of those but it was built into our TV, a Magnavox. It relied on a small black frame that local broadcasters used to insert their ads. National broadcaster like NBC would stream to their local affiliates. And the stream would have blank spaces to an insert their own ads. It’s very fast ,0.1s, that your eye would see briefly but to smart enough electronics was very perceptible. Once the commercial block was detected volume goes down. And the subsequent black frame turns it back up.