[ntp:questions] Accuracy of audio tones via VOIP

Robert Scott no-one at notreal.invalid
Wed Jul 17 14:31:50 UTC 2013

On Tue, 16 Jul 2013 15:34:58 GMT, unruh <unruh at invalid.ca> wrote:

>Now, I strongly suspect that his competition, who claim 10PPM accuracy
>on their ability to measure the frequency (against an absolute time
>standard) of a piano string, are just lying.

I never said they could measure the frequency of a piano string to
that accuracy.  I said that their basic calibration was accurate to
10ppm.  So if you used their devices to measure a stable tone from a
laboratory audio signal generator, you could measure it with that much
accuracy.  You are right that real-world piano strings are not so
stable and have too short a sustain to measure any better than about
0.2 cents (about 115ppm).  But in keeping with long-established
principles in scientific measurement, it is good to have instruments
that are at least an order of magnitude more accurate than inherent
inaccuracies of the thing you are trying to measure.  That way to
don't start unnecessarily giving up margin.

One on my competitors actually delivers a physical tone generator
along with the software purchase.  It is just a cheap commercial
musical pitch reference that they measure and document against a
laboratory standard before they send it out.

>But that is the challenge he has set himself. Note that he wants
>his clients to connect to that tone and to have that tone delivered
>reliably, for many minutes, in order to calibrate to that accuracy

I can achieve the desired 10ppm lock on the NIST tones in about 30
seconds if the audio is clear.  It is not hard.

Robert Scott
Hopkins, MN

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