[ntp:questions] Accuracy of audio tones via VOIP

unruh unruh at invalid.ca
Tue Jul 9 20:42:00 UTC 2013

On 2013-07-09, Robert Scott <no-one at notreal.invalid> wrote:
> On Tue, 09 Jul 2013 18:59:05 +0100, David Woolley
><david at ex.djwhome.demon.invalid> wrote:
>>On 09/07/13 15:35, Robert Scott wrote:
>>> as much as the local quartz oscillator frequencies?  Does anyone have
>>> any insight on how Skype and other VOIP systems manage record/playback
>>> rate synchronization?
>>The workings of Skype are a trade secret.  Other ones will have a jitter 
>>buffer, and will typically dump or repeat frames if that gets too full 
>>or risks underflowing.  RTP allows the source to mark good places to do 
> That does not sound too encouraging for the pitch accuracy of tones
> transmitted over Skype.  Of course you would never notice it in normal
> speech, which is, I guess, all that Skype is targetting.  But for
> calibrating to the standard frequency tones from NIST (500 Hz and 600
> Hz), it probably is not trustworthy, right?

I certainly would not trust it, and have no idea why anyone would. Skype
is not the place to get timing accuracy from. Since you have internet
access, you can calibrate your system clock using ntpd/chrony/... and
then have a good referece to calibrate other sound sources from. Why are
you asking?


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