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

.

Relevant Pages

  • Re: Magnitudes drop when adding more
    ... If I have a single tone, ... >This COULD have to do with the .wav format. ... Since I use PCM mode the samples are just "stored", ... But thanks for the insight friend! ...
    (comp.dsp)
  • Re: Lowest power tune detector.
    ... One way is to attach my laptop to my keyring, run a FFT over the output ... Your're pretty much looking at a multi tone telemetry system. ...
    (sci.electronics.design)
  • Re: About FFT bins
    ... I apply the FFT of 512 points and i read 16 K-values of FFT, ... My question is if it is possible syncronize the frecuency only computing a ... Do you have a single tone or mutiple tones present at the same time? ...
    (comp.dsp)
  • Frequency spacing needs to be embedded in data stream
    ... I am acquiring analog signal and I am using 'filtering', 'tone ... I realized that fft found a peak frequency of a value which is 1000 ... different signal using 'tone measurements' express vi which currently ...
    (comp.lang.labview)
  • Re: About FFT bins
    ... I apply the FFT of 512 points and i read 16 K-values of FFT, ... Then, with 15.625Hz of tone separations, i can tolerate an offset of 1 or ... Hz, but an offset of 4 Hz, for example, will degrade my estimation too ... My question is if it is possible syncronize the frecuency only computing a ...
    (comp.dsp)