[ntp:questions] Confused about pool rotation

Robert Scott no-one at notreal.invalid
Thu Feb 7 01:33:32 UTC 2013


On Wed, 06 Feb 2013 16:10:44 -0800, E-Mail Sent to this address will
be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid>
wrote:


>(Shrug)
>
>Considering that iPhones, and android phones can sync with
> NTP servers just fine without an extra APP,
>  and the SDKs have monotonic nanosecond timers already?
>

As I explained in another thread last week, my app is not a time app.
It is a precision audio frequency measuring app.  The users of this
app expect frequency accuracy on the order of 12 ppm.  To achieve this
accuracy I cannot rely on the nominal 100 ppm accuracy of the quartz
crystal that drives the device's audio sampling rate.  So I need to
perform an initial calibration to determine each device's actual audio
sample rate, which is quite stable, and which is totally independent
of the system clock.  For the past two years my app has been doing
this by having the user call the NIST telephone service and letting
the device listen to standard 500 Hz or 600 Hz tones to determine the
calibration factor.  But this is a hassle.  So I am trying to achieve
12 ppm frequency calibration by developing a temporary local time
stamp that is disciplined by SNTP.  I figure two SNTP samples about 2
to 4 hours apart will give the required frequency accuracy, assuming
typical SNTP jitter.  This only needs to be done once since ongoing
time accuracy is not my goal.  And I hope to automate the process so
the user can just start the calibration just before retiring for the
night.  The next day my app will be calibrated on his device and the
user will not need to do that again. 

> I don't see how $300 can be considered low cost.

It is quite simple.  There are no comparable apps to mine that cost
less.  There are two comparable apps to mine that cost quite a bit
more.


> Not to mention phones with GPS, can get stratum 0 time
>  right from the satellite, without the asymmetrical
>  network latency.
>
>
> Cell towers are already synchronized with stratum one
>  servers and / or GPS,  and usually contain stable oscillators
>   e.g. rubidium, cesium, ...
>
>  Cell towers can't do seamless handoffs for moving cell phones
>   to adjacent towers without close synchronization.

My app also has to run on iPads without cell tower access.



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