In article 3319@cmkrnl, jeh@cmkrnl.com (Jamie Hanrahan, Kernel Mode Systems) writes:
> In article <2g7p56$9s9@crl2.crl.com>, lreeves@crl.com (Les Reeves) writes:
> > The colorburst frequency is not only cast in stone-it is extremely accurate.
> > It is more accurate as a frequency reference than WWV. This is provided
> > that you are tuned to a network-supplied program.
> Also, at that time it was stated that the networks used rubidium-clock
> frequency standards, which are secondary standards: They're awfully good but
> they still have to be calibrated against something better. NIST (the folks who
> run WWV) uses cesium-beam clocks, which are primary standards, needing no
> calibration for frequency. Have the networks since upgraded to cesium-beam
> clocks? And, given that the local stations probably haven't, does it matter
> anyway? Even if they have, they're still "only" as good as NIST's clocks, so
> why should one over-the-air signal be better than another? (propagation
> changes on shortwave, maybe?)
>
Colorburst transmit frequency is required to be +/-10Hz (5.5873E-6 or 5.5873ppm).
PPM means parts per million.
This requires a good ovenized oscillator(that isn't cheap). Rubidium Oscillators
go for about $20,000 I think. Cesium Beam clocks are > $200,000.
The clocks that NIST uses are the best in the world. They have about 10 of them
that are all averaged together.
WWV, however loses a lot in its method of transmission and to propagation effects.
Received accuracy (if you have a stable enough PLL to track it without further
loss of accuracy) is about 1E-7 (0.1ppm) frequency accuracy and 1ms for timing.
Even to keep this accuracy would cost you at least $1000. Stratum 3 oscillators used in non-central office telephone equipment are 4.7ppm and cost at least $2000.
Since the colorburst crystal in your TV is > 100ppm, any PLL that uses that
crystal to lock onto an external source cannot be any better than 100ppm.
WWV is not the problem. Unless you are in TV broadcasting I don't think you
would need better than WWV accuracy.
Incidently, NIST was working on a computer system where you could request time
and frequency by modem. It would figure out the delay of the telephone path
and compensate for it. Some manufacturers (True Time was one of them) was working
on clock sources that are locked to the GPS (Global Positioning System) system
with Stratum 3 accuracy and better timing accuracy.
