[hpsdr] new architecture - FFT front-end

[David Bridgham]

I've been mulling over this idea of using the Jetson board to do an FFT
of the entire (mostly) sample stream to then make the frequency-domain
signal available to applications.  Seems very intriguing and I love
hearing about entirely new ways of approaching problems.

I'm curious though.  When designing a codec, one chooses the parameters
for the FFT very carefully.  The sampling rate is given to you by the
sample stream coming in (though you may decimate to change that) but you
pick the size of the FFT, the overlap between blocks, and the window
function all to make the FFT bins show up just how you want them.

With this new architecture, all those choices are made up front and all
the codecs running behind it have to use them.  So my question is, is
there a way in the frequency domain to convert from one to another?  Can
you simply average a bunch of FFT bins together to get larger bin sizes
and interpolate between bins if you want smaller?  Or do you have to do
an IFFT back to the time domain and then FFT again to change anything?

I usually get about halfway through a DSP book before I get bogged down
so maybe this is all covered in later chapters.

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