[PD] Spectrum graphing amplitude problem

Roman Haefeli reduzierer at yahoo.de
Tue Oct 23 01:29:11 CEST 2007


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)
> 
> > 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.  

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.

roman


	
		
___________________________________________________________ 
Der frühe Vogel fängt den Wurm. Hier gelangen Sie zum neuen Yahoo! Mail: http://mail.yahoo.de





More information about the Pd-list mailing list