[Fpga-synth] Delay feedback - strangeness

[Scott Nordlund]

> 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.

This is normal.  Consider a case where the input is an impulse and the feedback is high.  For positive feedback your output is an impulse train with all harmonics present.  For negative feedback your output is an impulse train with every other impulse inverted.  The frequency of this "alternating" impulse train is reduced by half, and only odd harmonics are present.  Plenty of commercial effect processors offer both positive and negative feedback.

> 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.

Commercial (especially old or low end) effect units generally act the same way.  Once you start thinking about interpolation, you might as well make it a modulation delay.

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