[time-nuts] Mains and TV 50Hz
sar10538 at gmail.com
Wed Oct 8 10:40:10 UTC 2008
Having somewhat started this topic, I'd like to say thank you for this
interesting post. Indeed, I believe that it is probably possible to
gain some form of freq standard from the mains, although it would have
to be over a long term period and it does not appear that there is any
statutory requirement to keep the frequency tight enough to be of any
real practical use. On the other hand, your observations on the field
freq accuracy of TVNZ looks like a possible candidate for this.
This could be sampled on one channel of a sound card with the suitably
divided signal under test fed to the other card and it should be
fairly easy to determine the freq with a good deal of accuracy within
a relatively short period of time. The hardware for this would be
fairly simple to build and possibly give results accurate to a GPS
stabilised freq counter.
Steve - ZL3TUV
2008/10/6 Murray Greenman <Murray.Greenman at rakon.com>:
> 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
> Murray ZL1BPU
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Steve Rooke - ZL3TUV & G8KVD
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