[ntp:questions] Using NTP to calibrate sound app

Rob nomail at example.com
Mon Jan 28 17:56:10 UTC 2013


no-one at no-place.org <no-one at no-place.org> wrote:
> On 28 Jan 2013 17:18:27 GMT, Rob <nomail at example.com> wrote:
>
>
>>WiFi has the same problem on a slightly smaller scale.
>>With WiFi the pingtime jitters by 50 ms or so.
>
> OK, but if I understand this time protocol properly, the time-stamp is
> adjusted by me (the client) by half the round trip time.  So what
> really counts is not so much the jitter in the round-trip ping time
> but the jitter in the symmetry (or lack of it) between request transit
> time and response transit time.  Assuming the worst case asymmetry
> between these two times we are back to the jitter you mentioned.  But
> do people really assume the worst-case asymmetry?

In my opinion, when you think a crystal is not accurate enough, relying
on single-shot time measurements via radio internet connection will
*certainly* not be accurate enough.  The protocols used on radio
channels are not symmetric, because the topology is not symmetric.
(there is a single base station communicating with a number of clients)

Typical crystal accuracy is in the same ballpark as what you require.
When you see significant differences, it is more likely that the actually
used crystal frequency is different from what you are reading from
some system information call.



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