Clayton Smith

I noticed a bug: the frequency axis on the audio FFT seems to be incorrect when lower audio sample rates are used.

Another problem: filter widths are not restricted, and so aliasing can now occur when the audio rate is decreased. For instance, when I widen the USB filter, a tone that...

This is because the demod ranges defined in `selectDemod` are built under the assumption that the audio rate will be 48000. For instance: https://github.com/gqrx-sdr/gqrx/blob/7b736c41f150a2741f6a30d81d190c076e5e5003/src/applications/gqrx/mainwindow.cpp#L1157

It looks like this will need some careful consideration to account for all the places in the UI and DSP that are making assumptions about the audio rate.

> That's just filter leakage. Ah, I think you're right. After looking closer, I see that nbrx and wfmrx have audio resamplers late in their receive chains, which should prevent...

I see now why I was confused by the behaviour when I expanded the filter bandwidth. The filter's transition width depends on the low & high filter cutoffs, so it...

Another issue. While in 8 kHz mode, switching from USB to FM (stereo) leaves the audio FFT in a weird configuration where negative frequencies are displayed: 

Maybe that happens because the requested range exceeds what is possible given the current audio sample rate. There's even a FIXME about that: https://github.com/gqrx-sdr/gqrx/blob/4eaeb163ac470d187d928a71622fb5dbdaedaa9e/src/applications/gqrx/mainwindow.cpp#L1132 Many other modes similarly request FFT...

I guess all the ranges should be capped to a maximum of half the audio rate.

Sorry, it looks like I caused a merge conflict when I merged #1066. I just had a quick look over this code and it's looking decent. I think I'll try...