[hpsdr] FIR filtering and HPSDR

[Erik Anderson]

A random article I ran into yesterday seems like it would have value here,
it is focused on "zero-latency convolution" and uses a hybrid of
convolution and FFT calculations.  It talks about overlap-keep, but if you
ignore all references to ho it also is creating an arbitrary-latency FFT
computation by disassembling a "decimate-by-frequency" FFT formula and
precomputing/reusing as many of the calculations as possible.

I haven't fully parsed everything in here yet, but I do have two questions:

(*) Would this approach have any value in say the Jetson board people have
been talking about?  It could be used to create a heck of a lot of
overlapped FFT results.

(*) How come there is no mention of window functions in here?  They seem
rather important for getting good outputs, but as soon as we start
tinkering with the inputs they no longer exist as even something to
struggle with.

Erik KM2G
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