Clock ident on TV

[Yellowtriumph]

I remember countdown clocks for BBC schools programs in the early 1960s.

I don't think TV digital clocks were popular, if you mean digits. I don't remember any.  At the last the clocks were digitally generated. The combination of mobile phones and Digital TV since the late 1990s & early 2000s meant the end of the TV clock. There used to be a telephone Speaking clock, first in 1933, just a couple of years before electronic TV. Automated from 1937. UK service started in 1936. I'm not sure if it's still running. The Speaking Clock in Ireland ran from 1970 (Local TV only started in reality in 1962, though technically 1961) to 2011, allegedly due to lack of use (by then Eircom was privatised).

The seconds are useless on Digital TV and Digital Radio. However originally clock towers were the only common mechanical clocks and had only an hour hand (no minutes), partially because accuracy was poor and partially people didn't need higher resolution. GB and Ireland mostly had local solar based time before the rise of the railways. GMT (now UTC) was originally referred to as "railway time" by most people.

The Roman Empire bureaucrats invented the day changing at midnight. Most people used dusk-to-dusk, as some religions still do.

Southern TV had a 'digital' clock, in that it consisted of digits rather than the traditional sweeping hands type. It started off in the days of monochrome but persisted into the early days of colour. It was replaced by a traditional clock in the end. That was placed at one end of a 'coffin box' arrangement that contained a set of clear glass panes - at one end you had the clock and at the other end you had a set of captions (programme titles or end of parts etc). A push button lit up one end or the other of the 'coffin' so the output was either the clock or a caption card. Clever. The output was monochrome (Vidicon camera I think) and that passed through a COX colouriser to colourise it. (I think the clock was physically upside down due to the optical path)

The clock had another function in ITV which as you probably know consisted of a network of independent studio centres interconnected by GPO/BT circuits but not 'locked' to one another in a technical sense. A camera is one of the few devices in the old analogue days that would allow the syncs driving it to 'move' or be less than 405 or 625 as the local station genlocked their station pulses to another incoming source of programming. You would never genlock 'underneath' a telecine machine as there was a strong possibility the machine would get upset and the pictures would bounce around a bit or catastrophically the EHT protection circuits would kick in and switch the EHT off! Blank screen.

Genlocking 'underneath' an operational VTR was also a complete no-no as the servo systems would go bananas. So, cameras were about the only thing you could genlock 'underneath'. 

A typical ITV break would be:
End of incoming programme.
Cut to a Commercial break from a telecine machine or ACR/TCR/VTR
During which: BT circuit switch from one programme company to another.
Commercial breaks ends and cut to announcer in vision.
Genlock to the new incoming programme whilst the announcers in vision.
or
Cut from the end of the commercial break to the station clock in vision and genlock whilst its on air.
Cut to next programme.
Seamless.