The second BBC VHF radio transmitter.

[ppppenguin]

Sound in syncs didn't just guarantee audio quality in the distribution network. It also made sure that sound and vision always travelled together and never got separated. I don't think anyone ever seriously proposed it for transmission over the air as a means of adding high quality stereo. The main reason would be that most sync separators in TVs would be unable to cope with it. I also suspect it would have reduced the ability to sync accurately under poor reception conditions and it could easily have been wrecked by multipath. None of these is a problem in the distribution chain.