[time-nuts] Mains and TV 50Hz

[Murray Greenman]

Hal and others,

I am sure that if one could accumulate mains cycles long enough, a
reasonable level of accuracy could be obtained, but you need to ask
yourself (1) to what standard is this 'reference' traceable, and (2)
what happens if there is a power failure?

On this latter point, let's say that a clock driven by the mains is
correct to within 10 seconds/day (about 1e-4), but in the longer term
accuracy is improved. To reach even 1ppm level, you need to accumulate
time for 100 days. What chance is there of accumulating for 100 days
without a power failure?

About the TV system, in New Zealand (and no doubt other countries using
the PAL standard) the field frequency is specified to be 50Hz (frame
frequency 25Hz). Unlike NTSC, everything except the colour burst
frequency (4433618.75Hz) can be derived directly from 1MHz by binary
division, which is really handy.

Whether the transmitted frequency is accurately 50Hz depends on the
network. Some of them are quite poor (i.e. worse than 1ppm), but I know
that TVNZ operate their North Island network from a Trimble Thunderbolt
in each studio, which controls a digital mixing desk. While there is
delay managed within the network, the transmitted line and frame
frequency is constant and also related to the GPS second. For many years
the government labs in Wellington measured and published monthly figures
of the error in the TV reference.

Perversely, some of the smaller networks have highly accurate vision
carrier frequencies (allows for channel reuse), but poorly controlled
sync!

Regards,
Murray ZL1BPU