[Fpga-synth] Delay feedback - strangeness

[Eric Brombaugh]

OK - here's something that's not immediately intuitive.

I've spent most of the day futzing with the feedback delay on my ARM 
FPGA synth. I've updated the SRAM access logic to store an entire 24-bit 
word and I've backed off the output gain of my monosynth to give it some 
headroom. This seems to work just fine - prevents saturation and sounds 
nice. With my 4Mb SRAM and 64kHz sample rate that gives ~2 sec max delay.

For a lark, I allowed the feedback gain range to be +/-2.0. This allows 
me to push the system into infinite echo (by setting 1.0), or even 
gradually increasing volume. The thing that's weird is that negative 
gain doesn't sound like positive gain. There's a noticeable difference 
in the quality of the resulting sound. It's most dramatic at short 
delays where the comb-filtering effect is pretty strong, so I'm guessing 
it has to do with how the phase cancellation works out.

One annoying thing is the little 'pluck' sounds that are created when 
stepping the delay amount. Need some sort of controlled slewing or 
envelope gating to get around that. Or a resampling-based approach with 
continuously variable delay and resolution to less than 1 sample.

Eric