[ntp:questions] Using NTP to calibrate sound app

no-one at no-place.org no-one at no-place.org
Tue Jan 29 17:22:15 UTC 2013

> Regarding: 
>> Smartphone manufactures, for whatever reason,
>> sometimes design a sound system that delivers 22,150 samples per
>> second when my app requests that more standard 22,050.  By the way,
>> this has not been a problem with Apple devices (iPhone/iPad), which
>> all seem to be close enough to nominal without custom calibration.
>> But Android devices and Windows laptops are a different story.

On Tue, 29 Jan 2013 10:23:56 +0100, Uwe Klein
<uwe at klein-habertwedt.de> wrote:

>Can you modify your software to be rate agnostic/flexible?
>the samling rate error you show is 4000ppm while crystal deviation
>should not go beyond 50 .. 100 ppm ( 1 "cent" should be ~416ppm !?)

My software is flexible with regard to audio sampling rate, provided I
know what that rate is.  And that is exactly what my calibration does
- discover the actual sampling rate by comparison to an external
standard.  When I said that some devices give me 22,150 samples per
second instead of 22,050 that was something I discovered through
calibration.  There is no way for me to tell internally that the
sampling rate is 22,150.  These devices do have an API that returns
what they think the actual sample rate is based on what I requested.
But even on the devices that are giving me 22,150 samples that API
still claims the sample rate is the requested 22,050.  That API is
mostly for use when a non-standard sampling rate is requested and the
system has to find the nearest standard sampling rate that it can

That 4000 ppm deviation is probably systematic and intentional, but
you will have to ask the OEMs who designed the devices exactly what
their intentions were in doing so.  I merely repond to what observe.

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