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Discussion Groups | Comp.DSP | Aliasing - A new perspective out of Box

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Re: Aliasing - A new perspective out of Box - raymund hofmann - 2003-07-19 06:30:00

"Craig" <c...@umkc.edu> schrieb im Newsbeitrag
news:8...@posting.google.com...
> "Glen Herrmannsfeldt" <g...@ugcs.caltech.edu> wrote in message
news:<Tr6Ra.74831$ye4.51585@sccrnsc01>...
> > "Bhanu Prakash Reddy" <i...@yahoo.com> wrote in message
> > news:2...@posting.google.com...
> > > HI all,
> > >
> > > Thank you very much for the  participants in " Negative Frequencies"
> > > Thread. Here is another question from me.
> > >
> > > Yesterday I was watching a movie. In that, hero was chasing villain in
> > > a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly
> > > and suddenly it appeared that the tyres of the car are rotating at
> > > only 8 miles/hr(I didnt
> > > measure..felt that its RPM is very less).The reason i feel is because
> > > of Aliasing. My eyes are sampling the scene at less than the Nyquist
> > > rate. Our CPU (Brain) samples through the sensor called Eye at a
> > > particular rate..because it has to spare some time for other 4 sensors
> > > also( In some cases 5 sensors ;)
> >
> > It isn't because the brain is sampling, but because the movie or TV
camera
> > is sampling.
> However, you can also observe the same thing when you stare at a
> ceiling fan, or a car driving down the road with spokes wheels, etc.
> So in that instance, it can be argued that your brain is "sampling".

I guess our Visual System is not a perfect low-pass without aliasing. As i
think most of our nerves are operating in some kind of "pulse frequency
modulation". Meaning: Information is not processed continuously "analog" but
somehow "quantized".
This includes the first nerve cells in the eye detecting light and continues
with the nerve cells feed by them and the whole brain.

Maybe the details of this are already worked out ...

But something else is dangerous here:

Our Consciousness !

As already discovered our Consciousness is only a very small fraction of our
"Brain Power". Only a fraction of our "sensoric stimulation" gets into our
consciousness already "symbolized" by our "cognitive filters" which are
inherited/trained since we live.

And also other things may play important roles here:

- Mechanical resonances of our eyes and the related body.
- Periodical Muscle movement of our eyes

It's a very complex and fed back signal processing system ...

Raymund Hofmann


Re: Aliasing - A new perspective out of Box - Eric Jacobsen - 2003-07-19 12:31:00

On Fri, 18 Jul 2003 18:54:46 -0400, Jerry Avins <j...@ieee.org> wrote:

>Eric Jacobsen wrote:
>> 
>> On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <j...@ieee.org> wrote:
>> 
>> >Bhanu Prakash Reddy wrote:
>> >>
>> >> >
>> >> > It isn't because the brain is sampling, but because the movie or TV camera
>> >> > is sampling.
>> >> >
>> >> > Yes, it is aliasing just like is often described here.
>> >> >
>> >> > Also note that the aliased motion may appear to be reversed.
>> >> >
>> >> > -- glen
>> >>
>> >> Even if u see really (not in a movie) a fast moving vehicle, u observe
>> >> the same situation with its tyres.In that case who is sampling...brain
>> >> only right???
>> >>
>> >> -bp$
>> >
>> >No. You won't see the effect in uniform light. At night, reflections
>> >from parts of the wheel when they assume specific ankles can act like a
>> >synchronized stroboscope, making the wheel appear stationary.
>> >
>> >Jerry
>> 
>> At night artificial lighting creates a shutter effect since the lights
>> turn off 120 times a second with 60Hz power and 100 times a second
>> with 50 Hz power.   This creates a visual sampling system because you
>> only get to sample the object when the lights are actually on.
>> Although one doesn't perceive the flicker, one does perceive the
>> aliasing effects that can happen.
>> 
>> I've personally witnessed this visual aliasing many times, understood
>> it, and therefore never gave it much more thought beyond initial
>> interest.  Once a few years ago while driving home in the daylight I
>> observed the same visual aliasing in the wheel of the car in my side
>> rear-view mirror.   It took me a while to wrap my brain around that
>> one since it was natural daylight illumination.   I was beginning to
>> worry that I was on "The Truman Show" for a while until I realized
>> that the mechanical vibration in the rear-view mirror was producing
>> the same shuttering effect achieved with AC electric artificial
>> lighting at night.
>> 
>> Eric Jacobsen
>> Minister of Algorithms, Intel Corp.
>> My opinions may not be Intel's opinions.
>> http://www.ericjacobsen.org
>
>I see that too, but people claim to see the effect with illumination by
>headlights! Then it's reflection at specific orientations. The
>explanation I rejected was that the eye itself sampled. Tain't so. If it
>were, we would observe beats.
>
>Jerry

Agreed, and the phenomena would be as repeatable in natural daylight
as it is at night, which it isn't.


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org


Re: Aliasing - A new perspective out of Box - Eric Jacobsen - 2003-07-19 12:47:00

On Sat, 19 Jul 2003 12:30:56 +0200, "raymund hofmann"
<f...@desinformation.de> wrote:

>> > It isn't because the brain is sampling, but because the movie or TV
>camera
>> > is sampling.
>> However, you can also observe the same thing when you stare at a
>> ceiling fan, or a car driving down the road with spokes wheels, etc.
>> So in that instance, it can be argued that your brain is "sampling".

I'd join in disagreeing with this notion.  I've yet to see an instance
where this effect can't be otherwise explained.  I've seen no evidence
from this effect or others to indicate that the brain samples as
revealed by any aliasing effects.

...

>And also other things may play important roles here:
>
>- Mechanical resonances of our eyes and the related body.

Not only resonance but the effects of external vibration sources.  I
think IEEE Spectrum or some related publication had a lot of
discussion a few years ago about the well-known "Frito effect" which
is experienced by examining an oscilloscope trace while munching on
Fritos.   The mechanical vibration transmitted from your teeth to your
eye sockets will do all kinds of wacky things to your perception of
the oscilloscope trace.

Note:  The young whipper-snappers in labs equipped with only the
latest equipment may miss out on this since it won't work unless the
scope trace is swept.  Some of the new all-digital machines don't do
this.

An old boss of mine (the founder of the company, actually) used to
tease new technicians when they had difficulty with lab measurements.
He would very  carefully explain to them with the usual air of
authority and knowledge that they could sometimes improve their
measurements by rapping the top of their head with their knuckles
while observing oscilloscope traces (this works with quickly swept
spectrum analyser traces as well).  We'd always know if he'd been
around if we saw a confused tech sitting at a bench bonking himself on
the head while staring at a scope.

The effect is the same as Frito chewing, it jitters your eyeballs and
screws up your perception.  I've always been amazed at how much
deviation in the scope trace you can observe but the rest of your
field of view is stable.  Human vision must have some kickass image
stabilization to do that, but the fact that you can see a continuous
(but very wiggly and sometimes highly deviated) scope trace is further
evidence to me that the brain doesn't sample.  If it does it's at some
incredibly unbelievably high rate, which I don't think is the case.

>- Periodical Muscle movement of our eyes
>
>It's a very complex and fed back signal processing system ...
>
>Raymund Hofmann
>
>

Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org


Re: Aliasing - A new perspective out of Box - 2003-08-13 03:57:00

Hello,
a bit late to comment on this thread, but still...

The aliasing phenomena observer with the human vision
is not only depending on the light sources.

It is well known that the brain operates at a few diffrent
frequencies, where (approx) 16 Hz is one of the major
fundamentals. 

Also as mentioned in an earlier post, the central area of the
visual field is optimised for detail but less for movements
while the outer part is optimised to catch movements.

so, the aliasing effect, with wheels spinning backwards
or standing still can be observed in fine sunlight.

-j


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