On Fri, 09 Mar 2018 00:14:28 +0000, Dave Plowman (News) wrote:

In article ,
Johnny B Good wrote:
Hum bars would have only become apparent with the advent of colour TV
broadcasting which started mid 1967 in the UK. Prior to that, the
monochrome broadcasts (both 405 and 625 line) were locked to the
national grid.

Not so. They went off mains lock some time before colour. I think it may
have been not long after BBC2 started.

Interesting, I wasn't aware that they'd divorced the system from the
mains frequency ahead of time. Prior to the start of actual colour
broadcasting, the 625 line system could have remained locked to the
national grid frequency. It was only when the PAL colour system came into
use that they were obliged to synchronise everything to the colour burst
frequency and run independently of the national grid reference which was
no longer stable enough in the short term to be of any use in the PAL
system.

Presumably, this change was implemented early on in readiness for colour
transmissions, perhaps to ease the final transition to a system that
could no longer be locked to the mains frequency.

--
Johnny B Good