Growing up, I was fascinated by all the different ITV channel identifications, by which I mean just before a programme started, the originating companies’ logo would be run.
These station idents had a couple of uses, apart from obviously indicating the originating production company, they also disguised the inevitable non-sync cut (vertical frame roll) (these were analogue days) that would occur as your local television incumbent switched to the originating company.
The theory is that this momentary disturbance would not disrupt the start of the actual programme titles.
For instance, if you were in the London area, about to watch Coronation Street, the cut to Manchester would be a brief roll on the Granada ident settle, then the programme titles would start.
In these early days the transition, or junction made by the station Transmission Controller would normally be a cut or fade to black, followed by the inevitable non-sync cut.
The majority of viewers were totally oblivious to this changeover.
Now a day this doesn't occur due to the advent of digital technology.
It can be seen on old VHS off air recordings.
Here's a recorded example, 1985, the TVAM hand over to the start of Thames.
It’s worth noting that the BBC had a much better synchronization system in place. As most of their programmes were originating from Studios and VTR’s in Television Centre(TVC), you would rarely see a non-synchronous cut on BBC television.