Yo Kurt! On Thu, 19 Jan 2017 21:20:23 +0100 Kurt Roeckx <k...@roeckx.be> wrote:
> On Thu, Jan 19, 2017 at 02:30:35PM -0500, Eric S. Raymond wrote:
> > Gary E. Miller <g...@rellim.com>:
> > > > - to fuzz the low-order bits of the clock.
> > >
> > > Hmm, can you expand on this a bit? Which clock? How much fuzz?
> > > Does this degrade anything?
> >
> > Whenever ntpd polls the system clock, it fuzzes the lowest-order
> > digits of the result. The amount of fuzz to apply is bounded by
> > half the measured interval between system clock ticks.
> >
> > That shouldn't degrade anything. I presume it's a measure to foil
> > timing attacks of some sort. Daniel might be able to say more.
>
> Adding random (white) noise to a measurement is done to improve the
> resolution after averaging, it's ussually in combination with
> oversampling. Adding this white noise is done in the analog signal,
> before you convert it to digital.
Yeah, I have seen that before. It can be good, or it can be bad.
In my GR-601W experiments I can show it would be bad.
> And I guess improving the resolution was important
> when on some systems you have a resolution in the order of 1 to 50
> ms.
I'm worried about 1 micro Second or less. And one should not
confuse accuracy with resolution. A PPS signal only has a resolution
of one Second, but can eaaily have an accuracy of 10 nano seconds.
A signal on a USB 2.0 bus can only have a resolution of about 1 micro
Second, but that can be locked to a PPS to 100 micro Seconds jitter.
> But I'm currently not really sure that it either improves
> things, make things worse, or has no effect at all.
Hal does not think it is in the measurement path at all. If it is
then it will be eauy to do some testing.
RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com Tel:+1 541 382 8588
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"If you can’t measure it, you can’t improve it." - Lord Kelvin
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