[ntp:questions] Accuracy of audio tones via VOIP

Robert Scott no-one at notreal.invalid
Tue Jul 16 00:19:34 UTC 2013

On Mon, 15 Jul 2013 19:27:12 GMT, unruh <unruh at invalid.ca> wrote:

>By the way, there will be a rate difference between the clock on the
>soundcard and the clock in the computer as well. It is the computer that
>determines the frequency, which it is the soundcard that digitizes it.
>So what do you do about that mismatch in your software?

It is my experience working with audio applications that the only
oscillator that affects reproduced audio frequency is the one in the
soundcard.  The software merely builds blocks of data samples and
delivers them to the soundcard drivers which play them out at a rate
determined by the soundcard clock oscillator.  The "system" clock has
no affect on this process.

>Have you tested your sound cards with an "over the phone" tone to see
>how accurate it is?

Yes.  The frequency of NIST tones delivered by landline are as
accurate as those received by a very strong WWV shortwave signal.

>> As for your last question, I have measured sound boards that are 11
>> cents off from their nominal playback rate (22200 sps instead of the
>> nominal 22050 sps).  Most soundcards are less than 1 cent off.  But
>> the standard among my competitors in the field of electronic aids to
>> piano tuning is 0.02 cents.  So if I want to compete then my app must
>> be able to calibrate to that accuracy too.
>We have discussed this in the past. It is lunacy, and furthermore, I do
>not believe that they can actually measure the frequency of the string
>accurately enough, especially since the "harmonics" of a piano string
>are out of tune anyway, so there is no periodicity in the sound from a
>piano.  And each mode of a multi-strig note is mistuned as well due to
>coupling between the strings and the soundboard. 
>Ie, their numbers are completely made up. 

This is quite a bit off topic, but if you research the various
professional piano tuning aids you will see that they all measure the
harmonics individually (actually called "partials" since, as you
pointed out, they are not exactly harmonically related).  By proper
filtering a single partial can be measured with to within 0.2 cents
quite reliably.

Robert Scott
Hopkins, MN

More information about the questions mailing list