[PD] Spectrum graphing amplitude problem

Mathieu Bouchard matju at artengine.ca
Mon Oct 22 20:05:30 CEST 2007


On Sun, 21 Oct 2007, Jason Plumb wrote:
> Mathieu Bouchard wrote:
>> The most rapid change you can have in a signal is an alternance of two 
>> values: e.g. +1, -1, +1, -1, +1, -1, ... which has S/2 frequency.
> Woah, that's a *super* good way to remember that.  Thanks.  I love examples, 
> and that's great!

I don't know why you would be taught that in any other way, as a first way 
of learning it. It's perfectly good to learn it in other ways, to 
consolidate your knowledge, but the first way to learn it ought to be the 
most obvious.

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's cool, makes sense.  Since I now understand that I'm dealing with 
> a graph/display issue, maybe I need to do some heavier lifting?  That 
> is, unless somebody can suggest a better way, I guess I'll try and do 
> block-synchronized snapshots, somehow walk/traverse the fft results 
> myself and look for local discretized maximums.  Doesn't seem like much 
> fun...

When scaling down a graph, you often don't want to decimate it (ignore 
values). Instead you might want to know the minima and maxima, so you can 
plot two curves at once: if you reduce your graph by 10, you will replace 
every chunk of 10 values by the max of that chunk of 10 values in one 
graph, and by the min of that chunk in the other graph. (If you want to 
see the exact position of the peak then you can't use that method at all.)

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada


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