Re: AKICIF: Security cameras

""David G. Bell"" <dbell@xxxxxxxxxxxxxxx> wrote in message news:20080907.2312.109295snz@xxxxxxxxxxxxxxxxxx
On Sunday, in article
<20a880f6-f25e-419f-8580-a25e823a6e69@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
mnhcc@xxxxxxxxxxxxxxx "Marty Helgesen" wrote:

On Sep 6, 8:14B pm, mike weber <fairport...@xxxxxxxxx> wrote:
> On Sat, 6 Sep 2008 17:59:14 -0700 (PDT), cryptoguy
>
> <treifam...@xxxxxxxxx> wrote:
>
> >The IR intereference I described upthread had nothing to do
> >with the AC current. It arises from the solid-state ballast in
> >newer flourescents, and creates a flicker at much higher
> >frequency - 10kHz to 40kHz. This is close to the modulation
> >frequency of IR controls, and I speculated that it might also
> >interfer with the IR driven motion dectection circuitry.
>
> I don't think that matches the problem described - as i understood > it,
> the problem was a roll bar in the steady image.

Yes, or, more precisely, a series of bars scrolling up. The images of
the two exits involved were not affected.

One point that's not clear to me is why there are intervals during the
day's record in which there are no scrolling bars. At those times
nothing is recorded because there is no real motion in the areas and
no simulated motion from the scrolling bars.

Guessing: mains frequency isn't constant. If I remember right, frequency
and voltage both drop at times of high load. I don't know if they have
to keep the cycle count to some tight daily limit--think clocks--but
that might lead to higher frequency at night, causing rolling fast
enough to trigger the motion detection.

--
David G. Bell -- SF Fan, Filker, and Punslinger.

On the horizon, a carrier task force of the Salvation Navy was
turning into the wind, preparing to launch Zeppelins.

Today's mains power -- 60 Hz -- is phase locked to a standard
that in turn follows WWV, here in America. That is, if you track
your 60 Hz sine wave here in New England, comparing it to the
60 Hz sine wave over in California, they are right in step. It
is because you have *networks* of generating stations and they
must all run accurately in sync. If something gets out of sync
then huge power flows develop from one station to another -- I
saw this very clearly when we had the great Northeast Power
Failure back around 1965. So if you run a mechanical electric
clock off your mains, and you set it right, it stays accurate
for months, til the next power interruption/failure.

I think in today's world, if you are the Electric Man I recall
from 1940's comics, you can hop in to an electric outlet here in
New England and hop out again over in CA. So the mains must all
be in sync.

Titeotwawki -- mha [rasff 2008 Sep 08]




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