Re: [Discuss-gnuradio] FFT scaling consistency

[Sylvain Munaut]

Hi,

> Then: no, QT GUI does that, see [1] (generated by [2]). The tone at
> samp_rate / fft_length * 10 has a power of 0 dB; also, Gaussian noise
> with an amplitude of 1 has an average (picture is heavily averaged)
> power of -20 dB == 1/fft_length. Perfect!

Oh, that's interesting.

I didn't see that when I did the test here. Turns out it's the window
selection, if you select Rectangular, you have 0dB and it's consistent
between WX and Qt.

If you select something else, then it's no longer 0dB and it becomes
inconsistent between the WX and Qt widgets.
Now obviously windowing will have some effect on the peak value, but:
 - I didn't expect it to be this large, I expected more like a few
tenth of dB, not precisely 3 or 6dB.
 - I certainly don't see why Wx or Qt would make a difference


> To the complex vs. real discussion: Picture [3] tells me that I see
> the noise at -23 dB; now, this will get extremely philosophical
> whether when observing a real signal this should be -20 dB instead.

Yes, it's definitely more of a philosophical question than anything else.
Honestly I don't really care either way, I just though I'd bring this
up at the same time and see if there was a "convention" that was
expected.

Cheers,

    Sylvain