Re: Magnitudes drop when adding more


overgaard wrote:
> Hi,
>
> I've just "restarted" my interest in DSP and mainly FFT.
> I'm using FFTW which I think is absolutely amazingly good! :)
>
> I was playing around with FFT a few years back and I think I
> remember stumbled on this problem back then too. The sad
> part is - I don't remember if I solved it or not back then.
> (what good memory, eh? hehe)
>
> Anyway, I'm troubled with something I quite can't figure out.
> I've done a test application that generates sinewaves and stores them
> to a WAV-file. When I open the WAV-file and run FFTW on it and plot
> a spectrum of it, it looks fine on frequency. BUT the magnitudes are
> behaving strangely. If I have a single tone, lets say at 1000 Hz
> it show up at full scale. If I add one more tone, they BOTH drop in
> magnitude. To doublecheck I opened the WAV file i Audacity and looked att
> the spectrum there. It looks fine there even if I add three tones or
> more.
> But in my application, the more tones I add, the lower they ALL get.
> Why is this? (ripping the hair of my head)

This COULD have to do with the .wav format. A few months ago I made
some recordings in the .wav format. Somebody who helped me set
up the experiment asked me if I wanted volume information or just the
waveforms. I must admit that I did not understand exactly what he
asked,
but I interpreted the question as if the .wav format separates the
waveforms
and the volume/gain information.

If that is correct, each individual tone must necessarily become weaker
if the volume is to be kept constant.

Don't take my word for this, though! Ask somebody who knows the
..wav format.

Rune

.

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