[Fpga-synth] SVF working

[Scott Gravenhorst]

Hey Gang,

I've just gotten a digital state variable filter working.

As I did this, I realized many of the caveats, this is a tricky filter to
use compared with the simple single stage IIR I did.  Resonance opens up a
whole new can of worms.  I can see that a wider data path would help.  This
filter uses signed 18 bits.  I want to see how far I can push Q before I
widen the path.  I'll bet that even with limited Q, like 5.0 max, this
filter might be a better synth filter than the single stage IIR and tuning
the SVF is as easy as the IIR.

-- ScottG

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-- Scott Gravenhorst
-- GateMan I - Xilinx Spartan-3E Based MIDI Synthesizer
-- PolyDaWG/8 - 8 Voice FPGA Polyphonic MIDI Synthesizer
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.