[PD] Spectrum graphing amplitude problem

Charles Henry czhenry at gmail.com
Tue Oct 23 05:48:32 CEST 2007


On 10/22/07, Roman Haefeli <reduzierer at yahoo.de> wrote:
> On Mon, 2007-10-22 at 17:33 -0500, Charles Henry wrote:
> > On 10/22/07, Martin Peach <martin.peach at sympatico.ca> wrote:
> > > Mathieu Bouchard wrote:
> > > >A very simple way to explain aliased frequencies would be: spin a bicycle
> > > >wheel. When you accelerate it beyond a certain point, it will begin to look
> > > >like it's going backwards instead. This is because the wheel speed,
> > > >together with the repetitiveness of the wheel's appearance, have crossed
> > > >the Nyquist frequency of your eye.
> > >
> > > That won't work in sunlight for example.
> >
> > Haven't you ever seen it?  (in sunlight that is)

As implied, I'm 99% positive I've seen it before.  You might still be
able to convince me that I haven't ;)

> i think, that is the point that martin is trying to make.
>
> i am not an expert either, but when i encountered this effect (watching
> the frontwheel, while cycling at night, for instance), there was always
> artificial light involved. to be more precise, there was always kind of
> a fluorescence or any other kind of 'gas discharge lamp' (is that the
> correct expression?) involved. it doesn't seem to work with common light
> bulbs, since their 'afterglow time' is too long.
>
> for me, it seems, that it is rather related to the light source and its
> frequency and not to a property of the eye.

hey, what do you know, there's even a wiki page on it:
http://en.wikipedia.org/wiki/Wagon-wheel_effect

Vision is in fact not purely analog.  Although I don't necessarily
think the "discrete frame" theory mentioned in the page is true, there
is a counterexample.  When your eyes make a saccade (fast movement),
there is no blurring.  There is an inhibition of the movement in
between the two visual scenes.
There is also nystagmus.  Even when images appear perfectly clear, the
eye moves in random oscillations.  When researchers (I forget the
citation/can't find it) used an eye camera to track random
oscillations and corrected for the movement (creating a static
retinotopic image), the perceived image disappears.  There may be
something involved in correcting for nystagmus that renders an image
in a series of apparently still frames.  I don't know for sure.

Chuck




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