[PD] Spectrum graphing amplitude problem

[Mathieu Bouchard]

On Mon, 22 Oct 2007, Martin Peach 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. 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.

Ok, I'm sorry, that was a really bad example. I don't quite know how the 
eye works, but there is some amount of low-pass filtering at work, that 
tends to cancel the Nyquist effect most of the time.

Most other artificial lights flash too. Most tungsten lightbulbs and most 
infamously CRT light. It depends on whether the combination of all 
low-pass effects of all components turns out to be smoothing the light 
emission enough so that it looks more like the spokes are blurring rather 
than going backwards.

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