[ntp:questions] Accuracy of audio tones via VOIP

unruh unruh at invalid.ca
Wed Jul 10 17:57:40 UTC 2013

On 2013-07-10, Rob <nomail at example.com> wrote:
> Robert Scott <no-one at notreal.invalid> wrote:
>> No, I am not interested in time.  I am talking about frequency.  If
>> you call 1-303-499-7111 you will hear the audio that is transmitted on
>> WWV, which includes 500 Hz and 600 Hz tones.  As transmitted these
>> tones are as accurate as NIST can make them.  But as received they
>> might appear at a different frequency.
> Then don't use that as a reference, I would say.
> Even over the normal phone network, while the tones may be very accurate
> there is not so much you can use them for, because the local determination
> of their frequency will always be far less accurate for tones that short.

Not far less. While signal to noise means that the determination will
scale roughly as the square root of the time, for a sufficiently high
amplitude of signal, the prefactor can be very large. Even a single
cycle is enough to get the frequency to very high accuracy. 
(Obvously you do not do it by doing a discrete fourier transform.)
Ie you should be able to determine  the frequency to millihz. 
Assuming that the signal at your phone is accurate. If it is not (VOIP)
you obviously cannot determine the frequency very well. 

But again. why does the OP not just measure it? I guess he is hoping
that someone already had done his work for him.

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