[linux-audio-dev] [ann] unmatched - a LADSPA amp tone

[Steve Harris]

On Tue, Oct 22, 2002 at 09:38:26 +0200, Tim Goetze wrote:
> the problem with converging IIR is that either the IIR or the 
> converging algorithm or both don't like impulses that are too
> lively. the converger will simply fall dead when it sees an 
> exponentially decaying white noise impulse. it makes sense,

I don't understand why. Is it just beacuse the kernle is a bit too long to
be stable, or is there someting more to it than that?

> what i don't get about the valve (the rect influence seems
> negligible) is that it only shapes the negative portion of a 
> sine. without a LP, there's faint aliasing. it doesn't sound 
> so bad though, in fact i like it.

Thats because its only odd harmonics (or is it even?). It should be
follwed by a DC offset remover. This is pretty normal for a tube
simulation. The aliasing is because I never got round to oversampling the
valve. It needs some more optimisation to make that a good idea, and LP
filters are so much easier ;)

> however i'm into harsh distortion now, and have fed the fender 
> three sines from zero to full gain (three octaves of 'c'). 
> the shaping it does has some important properties that i'm only
> beginning to understand, but it really does look fundamentally 
> different. it seems to self-oscillate at the zero-crossings, 
> and otherwise compresses the sine much like your valve does,
> only the output signal is not a straight line where compressed, 
> but rather another irregular but smooth oscillation about a DC 
> value. a (realtime) electric circuit simulator would come in 

Hmmm... I'm quite out of my depth here, but could this be a class B effect
I'm not correctly modelling? I suspect a class B crossover discontinuity,
effected by the mostion and mass of the speaker cones (pretty high for a
guitar amp I'd guess) could look like that. Any chance of a short sample,
to make sure I'm thinking of the right shape.

Infact, this should just happen, if you use the crossover plugin and
put the output through your speaker cone IIR you should see a similar
shape, as the LP characteristic of the IIR damps the abrupt 0 sticking
point from the crossover.

- Steve