[ntp:questions] Can NTP sync within 1ms

jimp at specsol.spam.sux.com jimp at specsol.spam.sux.com
Sun Apr 27 16:49:28 UTC 2014

Rob <nomail at example.com> wrote:
> William Unruh <unruh at invalid.ca> wrote:
>>> The next problem is to send output to a soundcard and making it send
>>> a sample at the sampling clock edge closest to a specified time.
>>> (48kHz sampling rate corresponds to a sampling clock period of 20.8us)
>> It will certainly depend on the sound card. AFAIK most have their own
>> internal oscillators, that are not adjustable. I suspect you will have
>> to build your ownsoundcard to accomplish this. 
> There are soundcards for professional use that support locking of the
> sampling clock to an external source.
> There is also a circuit from a French guy that generates a S/PDIF signal
> from an external reference and he claims many soundcards with S/PDIF
> can lock to that.  I need to further investigate that.
>>> When that has been achieved, it of course is easy to wire up two of
>>> those systems, place them closely together, and check on a scope.
>>> It could be further improved when we can somehow lock the sampling
>>> clock to the PPS signal, so another +/- 10uS uncertainty/jitter is
>>> removed.
>> 48KHz is 20us. Why are you worried about a few us? What are you trying
>> to do?
> We are setting up a co-channel diversity network.  That means multiple
> FM transmitters that are transmitting the same signal on the same
> frequency on different sites, where the receive areas partly overlap.
> The listeners should enjoy a smooth reception while driving around.
> So of course there should be no time lag between the modulation signals
> of the different transmitters.  Experts in the field tell us we should
> be within 12us.

Unless I fat fingered the calculator, that means the difference in distance
between transmitters relative to the receiver can be no more than 3.6 km.

300 meters per microsecond; it is the law...

Jim Pennino

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