Congratulations on making it to the semifinals of the Hackaday prize! It wasn't much of a surprise for me, lol. I'm looking forward to seeing the progression of this project, it has already been coming along nicely. Great job and good hacking to you!!

Thanks! I'm very excited to have made it this far!

I noticed when the signals appeared on the screen, the rest of the spectrum suddenly got darker, and then when the signals ended, it slowly got lighter again. Is that due to AGC?

Yes, that's a visual AGC (not related to audio AGC) basically I put that in there so I wouldn't have to know beforehand what values to use for the waterfall display. I'm not actually that happy with it, I think it serves its purpose, but it also makes it harder to really get a feel for what's happening. I'm considering a few options:

1. Figure out optimal values and lock them in.
2. Make it user adjustable (don't love that idea)
3. Keep the AGC, but make it so it brightens the whole waterfall, not just the more recent lines, if that makes sense. I can't do this yet due to limitations on the display (driver). If I do this, I'll probably have some bar that shows display gain, so it's clear.

So much to to!

Just kidding about the S-Meter breaking the audio, that's totally not what happened. I forgot I was in the middle of testing some audio filtering stuff when I added the S-Meter. When I turn the old filter back on, everything sounds like it did before. Still need to get that filter figured out! If there are any DSP guys reading this. Let me know, I have questions!