[ntp:questions] Can NTP sync within 1ms

Rob nomail at example.com
Sun Apr 27 16:28:51 UTC 2014

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.

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