[Fpga-synth] Delay feedback - strangeness

[Eric Brombaugh]

Scott Nordlund wrote:

> 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.

Yeah - I went back through Chamberlin's book and looked at some of his 
material on reverb & delay effects. The structure of the all-pass filter 
networks gives some interesting insight into this.

> 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.

That's one of the things on the to-do list. I've only used about 20% of 
the FPGA, so there's plenty of room for more stuff.

Eric