Re: [Discuss-gnuradio] FFT scaling consistency

[Matt Ettus]

I normalized the wx complex display a long time ago.  As you have discovered, windowing affects this, but in two separate ways.  First, windowing reduces total signal power because most bins are less than 1.  We normalize for that, so the total energy remains constant.  You will see that if you run with awgn.

The second issue is spreading energy into adjacent bins. This will affect anything you expected to remain in one bin, like a sine wave.  There isn't any way to fix this as its not broken.  To find the power in a sine you'll need to add the power of several adjacent bins.

Matt

On Nov 26, 2014 3:08 AM, "Sylvain Munaut" <address@hidden> wrote:

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

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