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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: how to analyze clock drift
References: <87bno4gnuz.fsf@debian.uxu> <m4gsfv$i7k$1@speranza.aioe.org>
 	<87k32s3ppt.fsf@debian.uxu>
 	<87sihf32kr.fsf@adaheads.sparre-andersen.dk>
 	<87fvdeet0q.fsf@debian.uxu>
Date: Thu, 20 Nov 2014 10:27:34 -0600
Message-ID: <85a93lon0p.fsf@stephe-leake.org>
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Date: 2014-11-20T10:27:34-06:00
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Emanuel Berg <embe8573@student.uu.se> writes:

> Jacob Sparre Andersen <jacob@jacob-sparre.dk> writes:
>
>> Try to subtract a linear fit and plot the result.
>> There may be a short-term systematic pattern, but
>> over longer time, it looks like you have a slowly,
>> randomly drifting function.
>
> You mean, I should make a figure (graph) and then
> learn from it by inspecting it?
>
> By "subtract a linear fit", do you mean I should
> broadly visualize this as a linear function by some
> smoothing-out filter even though the data of course
> isn't a straight line?

Just use a standard "linear fit" algorithm
(http://en.wikipedia.org/wiki/Linear_regression,
http://stephe-leake.org/ada/sal.html sal-math_double-linear_fit.ads) to
fit the data to y_fit = mx + b, then plot y - y_fit.

That will show the short term sawtooth, plus any longer term drift.

The short term sawtooth is probably due to the way the software is using
a hardware clock; the clock period doesn't quite divide the desired
software period.

-- 
-- Stephe