26 Feb
2012
26 Feb
'12
1:10 p.m.
Dave wrote:
It used to be the case that TVs synced to the mains
frequency but I
don't think that?s been the case since we went 625 line.
Despite some claims,
I've never seen credible evidence of the existence
of a TV that "synced to the mains frequency", even for the 405-line
system. I think it's an urban legend. TVs sync to the recovered vertical
sync pulse of the incoming video signal.
It's certainly possible that a TV designed for 60 Hz mains might have a
power supply that has problems with 50 Hz, and rather less likely the
other way around, but that has nothing to do with the field rate of the
video signal, which has no inherent synchronization to the mains.
There are, on the other hand, several reasons why the broadcast field
rate iwas chosen to be the same as the mains frequency. However, even
those reasons are questionable, since NTSC color broadcasting has always
been done with a field rate that is 0.1% lower than the mains frequency,
yet the slow beat between the mains frequency and the field rate has
never caused a problem.