[Discuss-gnuradio] FFT scaling consistency

[Sylvain Munaut]

Hi,


Yesterday someone in the chat raised an interesing issue. The absolute
scale of the various FFT GUI sinks.

If you take:

[signal source ampl=1.0] -> [gui fft sink]

(use a source freq that fits exactly on 1 fft bin)

With complex + WXGUI it peaks at -3dB
With complex + QTSink it peaks at -9dB
With float + WXGUI it peaks at -9dB
With float + QTSink it peaks at -15dB


So basically:

 - The WXGui + Complex should be 0dB IMHO.
   A complex cosine of ampl 1.0 should be our 'absolute' reference for 0dB

 - QTSink is always 6dB lower
   -> That doesn't sound logical, they should just match. GUI
selection should have no influence on the absolute reading when
working with purely synthetic data.

 - Float is always 6dB lower than complex
   -> I'm torn on this one ... can't decide :p
      If you integrate the energy (x^2) over one cycle between a
complex cosine and a float cosine, you get half the energy. So this
should be -3 dB right ? And I guess the other 3dB comes from the fact
that half of this energy is in the positive spectrum and half of it in
the negative one (so if you take the total energy in the whole
spectral domain, it does indeed match the one in the time domain) ?
      The question is then, when displaying a float fft, should we
just raise by 3dB ?


Anyway, I hope this make sense :p

Thoughts ?


Cheers,

    Sylvain