strange idea....

[Carlos Vila Deutschbein]

On Wed, 22 Dec 1999, Budweiser [FTS] wrote:

> Hmmmm, this is prolly really eccentric and weird, but i thought i'd make a comment
> to synth-diy about it anyway.
> 
> I just got up and opened the curtains to my bedroom, and i looked through the fly
> screen. About 2 feet away from the fly screen is another screen / blind (shade cloth,
> you can see through it) Anyway, you see this "beating" effect of the 2 grids cancelling
> and reinforcing difference patterns (if i remember my yr 11 physics, Newton's rings?!).

AFAIK this effect is called aliasing. I happens everytime when a high
frequency phenomenon is being observed by a system which cannot cope with
these HF. There are many real-life examples of this: Look at a spinning
car wheel: As the speed increases, there comes a point when the wheel
seems to stop and begins to turn backwards. That's because the human eye
has certain limitations regardind respose speed, resolution, etc...

When we talk about audio, aliasing appears when you try to sample a high
frequency signal with a low sample rate. Famous scientist Nyquist said
that in order to sample a signal with fmax=N Hz you must take samples at
least at twice the speed (2N Hz). Thats why standart CD quality has a
sample rate of 44100 Hz, which is 2 x 22050 Hz, which happens to be about
the maximum frequency the human ear can hear. 

If you have a Sound Balaster kind of thing you can experiment with this,
but the result of decreasing the sample rate is usually not nice at all...

BTW Moire is somewhat different. It happens on TV screens when for example
a guy wears a fine-checked jacket. these quick black-white transitions
(HF) interfere with the colour processing and produce strange patterns.


 ==============================================================       
    Carlos Vila Deutschbein
    e-mail: si04697 at salleURL.edu
    www: http://www.salleURL.edu/%7esi04697
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