[LAD] Software sound

Fons Adriaensen fons at linuxaudio.org
Sun Aug 31 19:42:48 UTC 2014

On Sun, Aug 31, 2014 at 08:13:01PM +0100, W.Boeke wrote:

> Sorry Fons, if you google "moog filter schematic" you will
> see 3 stages, where a stage is a capacitor in series with 2
> transistor emitters (which have a diode V/I characteristic).

All versions I've every seen have four stages, and with three
the feedback wouldn't have the right phase. If you find a
schematic with three stages please post a link.

Each stage is a capacitor in parallel with the series connection
of two emitter impedances, and driven by a current source, the
collector currents of the previous stage.

> About that complicated code: I saw implentations with a lot of
> code lines, and preventing oscillations originating from the
> non-linear tanh functions needed a lot of care.

I analysed this thing to dead around ten years ago, when I wrote
the MCP plugins. There is no instability problem resulting from
the non-linearity. The main problem with a straightforward digital 
implementation is that it becomes a bad approximation at higher
frequencies, this requires some attention to get right. The easy
solution is to run it at a higher sample rate.



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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