[hpsdr] frequency-specific frequency domain AGC

[Warren C. Pratt]

Hi Roger,

First of all, I wish your wife and you the very best as she recovers.  I 
certainly hope that she has a speedy and complete recovery.  It sounds 
like you got her to the hospital right way, which is a very important 
factor.

The 'FAGC' sounds very interesting.  If there is sufficient interest, 
I'd like to put this on my list of future enhancements to the WDSP 
library.  Interestingly, it's very similar in concept to the CFC 
transmit compressor that I recently added.  This compressor looks at the 
magnitude of the signal in each frequency bin and compresses the bin 
accordingly.

You stated:

"I do this by applying such correction using the equations given in my
first post for each individual FFT bin, adding this step between the
filter convolution step and the reverse FFT step to return to the time
domain."

One thing that I should mention (perhaps for discussion at some future 
time) is that inserting instantaneous non-linear gain changes within a 
normal FFT-Convolution filter creates some level of 
artifacts/distortion.  The extent of it will depend upon the relative 
amplitudes and frequencies of the signals and I don't know if it 
"bothers" your decoder algorithm.  The more common way of dealing with 
this situation is using Weighted Overlap Add (WOLA).  Here's a link to a 
discussion/description:

<https://www.dsprelated.com/freebooks/sasp/Weighted_Overlap_Add.html>

73,

Warren  NR0V