[ntp:questions] Jitter versus polling interval

David J Taylor david-taylor at blueyonder.neither-this-bit.nor-this.co.uk
Sun Mar 1 19:15:27 UTC 2009

Unruh wrote:
> Mill's assumptions about the behaviour of clocks is almost certainly
> wrong.
> Not least because computer clocks are in a highly variable thermal
> environment. This means that the drift rate varies with time of day
> and day
> of week and is NOT 1/f or any other time invariant spectrum. Tests
> have
> been run where the temperature is measured and an additional
> correction due
> to temperature is factored into the ntp algorithm, and vastly improved
> results are obtained. Chrony gets better results, primarily I believe
> because its use results in a varying Allan intercept, and the clock
> rate adjustment benefits from that in the noisy thermal environment
> of computers
> which are used for things other than timekeeping.

I don't run chrony and it has nothing to do with my question.

>> The book is on my Amazon wanted list, but I'm not sure how much I
>> will appreciate if it's mainly maths.  One of my criteria for
>> judging image processing papers was "does it contain images?"!
> And a book on time should contain sounds of a clock ticking?

What do /you/ think?  Was there a smiley missing?

> Mathematical analysis is crucial to proper operation of the system.
> But it does have lots of graphs if that helps.

As I indicated, I find a visual representation very helpful, and I was for 
ever trying to get people who were applying for expensive but obscure 
pieces of capital equipment  to include some graphs showing the benefits 
of buying such.


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