[ntp:questions] Can NTP sync within 1ms

William Unruh unruh at invalid.ca
Mon Apr 28 15:19:03 UTC 2014

On 2014-04-28, Rob <nomail at example.com> wrote:
> Jochen Bern <Jochen.Bern at LINworks.de> wrote:
>> However, that *does* leave me wondering where the 12 us figure comes
>> into play. With the typical distances between 2m and even 70cm
>> repeaters, the mobile transceivers will see shifts *far* beyond that
>> between different repeaters' signals.
> The figure comes from two different experts in the field.  They have
> built such systems for the emergency services (police, fire brigade,
> ambulance) that have been operating in the country for a few decades
> and have been replaced by a digital (TETRA) system in the meantime.
> But their knowledge and experience remains valid.
> In those days we could not afford such an experiment, they used fixed
> analog leased lines to transfer audio with a fixed and known delay,
> but today we have internet and GPS and we can achieve the same thing
> much easier.
>> FWIW, will you have the audio
>> cards output AM (that will then get modulated onto an otherwise
>> unsynchronized HF), or do you plan to have the card generate HF directly
>> into the PA?
> The existing system mentioned above uses SDR techniques to synthesize
> the FM signal directly from the digital samples, but we like to use
> existing repeater hardware that already has FM modulation, so we want
> to use soundcards to produce analog audio that is fed into the repeaters.
> The HF is not unsynchronized, it is locked to a GPSDO.  But I agree
> that it would be much more predictable to do it the digital way.  Now
> we have extra variables like the exact deviation setting and the
> characteristics of the modulator.  Fortunately all repeaters run the
> same hardware (Tait TB8100).
> I am still investigating how to output digital samples to a soundcard
> in Linux at exactly determined time (versus just writing sample blocks
> to the soundcard driver at a predermined moment.  they will still be
> buffered after that)
> All in all it is funny to read all the "that cannot be done"-like comments
> by several persons on a ntp newsgroup while systems like this have been in
> use since the seventies, and in fact have already been build by amateurs
> and are in operation today.  So I prefer to go by the experience of
> the people who built those networks, rather than the armchair experts.

Not, "it cannnot be done", but "it is silly to try". I simply have a
really really hard time figuring out why you want to do that. 
For voice, the max frequency is something like 4KHz (eg telephone
quality) which is 250us (125us sampling) . It is really really hard for me to figure out
what bad things would happen if that were the resolution you used
instead of that 12us. It just seems wildly over-specified. And then you
make statements like it is the transmission time that needs to be 12us
not the reception time, which also do not make sense. And a phase jump
even of 120usec I doubt would be audible, maybe 12 ms would be. (are you
sure you did not misunderstand your "experts"?)

If it really is 12us I would be really interested in knowing why. I
might even learn something.

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