dB values seam to be incorrect on both Periodogram & Spectrogram

[ashley-b]

When display a single test tone of amplitude 1.0 in the Periodogram or Spectrogram display the amplitude seams down from were it should be. 3dB down for complex and 9dB down for real signals.
It does not seam to be significantly affected by the windowing mode.
But adjusting the "Full Scale" parm fixes the the issue. eg 1.0 *sqrt(1/2) for complex bring the displayed amplitude to were i would expect.

Complex normal

Complex adjusted

Real normal

Real adjusted

I see this bug under the Pothos v0.7.0-g42d23cb1

[guruofquality]

If you send in a DC tone at 1.0 amplitude and set the window to rectangular you should see exactly 0 dBfs (if the full scale is 0). One you change the frequency or window, you start to spread out that power among multiple fft bins. The total cumulative power of those bins should add up to 0 dBfs. In the example above, you have two powerful bins which are down about 3dB, so the cumulative power is correct.