On Thu, 08 Mar 2018 14:49:19 +0000, Terry Casey wrote:

In article , briang1 @blueyonder.co.uk
says...

Yes well, I whenever convinced it was that accurate since I saw hum
bars rolling down tv screens in the 1960s.

Ah yes, I remember them well - but our mains driven Smiths Sectric clock
was always right to the second at 8am the following day!

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.

I remember admiring the frequent adjustments being made (about every 15
seconds or so) by the national grid when watching the barely perceivable
hum bars roll up and down the mid grey, "after hours" raster display on
our TV set in the late 70s when it was obvious the master power station
(s) was (were) being controlled from a crystal/atomic clock reference
source to retain the grid's state of hard won resynchronisation with
'real time' during the wee small hours.

--
Johnny B Good