[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

unruh unruh at invalid.ca
Fri Jun 7 18:02:38 UTC 2013

On 2013-06-07, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 07/06/2013 16:17, unruh wrote:
>> On 2013-06-07, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>>> On 06/06/2013 20:17, Doug Calvert wrote:
>>>> unruh has a nice page about interrupt coalescing:
>>>> http://axion.physics.ubc.ca/scatter/rt.html
>>> This is a quite interesting note, unfortunately rendered rather less
>>> useful by the graphs, which have no axis names or units marked, and
>>> which appear to have inconsistent scales (although this may simply be
>>> lack of legibility).  For example:
>>>     http://axion.physics.ubc.ca/scatter/rtdt1.gif
>>> The units on the vertical axis appear to be labelled:
>>>     -0.0001  -5x10-6  0  5x10-6  0.0001
>> Yes, supermongo, the plotting program, does tend to make 5 and 6 similar
>> at the small sizes used in superscripts.  But as you say, it is a linear
>> scale, and thus that must be a 5 rather than a 6.
>>> which I interpret as -100us, -5 us, 0,  +5us  +100us, but it appears to
>> Since that is, as you point out, an interpretation that produces silly
>> results, it is probably the interpretation that is at fault.
>>> be a linear axis.  Is the 5x10-6 supposed to be 5x10-5?  The character
>>> needs serious improvement as it looks like 5x10-6 to me.  But in any
>>> case, as microseconds are referred to in the text, would it not be a lot
>>> clearer if the graphs were also in microseconds, rather than some small
>>> fraction of seconds?
>>> Similar problems in:
>>>     http://axion.physics.ubc.ca/scatter/gps.gif
>>> where the -5 and -6 suffices are very difficult to distinguish.  I do
>>> hope that Bill can re-do these graphs some time as it would enhance the
>>> value of this work.
>> I am not at all sure that I can find the data anymore to redo the
>> graphs.
>> About all I could do is to use gimp to replace the axes labels and make
>> the 5 look a bit more like 5 rather than 6. The problem is at the small
>> scale of the superscripts, the little culicue up at the bottom of the 5
>> in the font used becomes a single pixel closing off the the loop, making
>> it look a lot like a 6. So I used gimp to open it up again, and also to
>> labels on the axes.
> Thanks, Bill.  Sounds like an update to "supermongo" is needed!  Can it 
> label the axes?  Not critical to regenerate the graphs, but perhaps if 
> you add a little to the text to clarify scales and axes it may help others.

I did use gimp to label the axes  and scales and to make the 5's look
like 5's. I
think tht the problem is that the font used for 6 pixel high text makes
5 look like 6.
Have a look again to see if it is better. 

> Cheers,
> David

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