[PD] Spectrum graphing amplitude problem

Charles Henry czhenry at gmail.com
Tue Oct 23 00:33:08 CEST 2007

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)

> You need a flashing light source
> like a fluorescent tube. It's the Nyquist frequency of the light source that
> causes the aliasing, not of your eyes, unless you try it while blinking
> rapidly. The wagon wheels in western movies appear to turn backwards when
> the spokes are moving faster than the frame rate of the movie.
> Martin

Vision doesn't work exactly like a camera.  There's no sampling that
occurs--the visual signals are more/less continuous except for
momentary "refresh" periods brought about by eye motion and nystagmus.
I thought about it today, and I'm not entirely sure why it happens
(perhaps there is a certain maximum speed which can be perceived--this
is equivalent to saying there is a finite bandwidth of motion
perception).  I don't really know.  I need to defer to *real* experts,
but I agree, the description is more precise when talking about a
camera effect...


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