23 Oct
2002
23 Oct
'02
10:41 a.m.
Steve Harris wrote:
yep. :) they should build hw lagrange- or sinc-interpolation
into those fantastic 'multimedia' cpus. would save us a lot
of work.
still i'm not convinced that a sine should only be compressed
if < 0 by valve_1209.so.
i've copied (950 + 50 silent) samples of each into
http://quitte.de/fender-sine.wave -- the quitte.de quota is
measured in k, not M :(. the two oscillations mentioned above
seem to be one actually, their period of about 12 samples is
independent from the sine frequency that is fed to the amp, i
double-checked with a 'f#' sine (that got lost accidentally,
it does not look substantially different however). actually
i should record the output from the circuit not the speaker.
On Tue, Oct 22, 2002 at 09:38:26 +0200, Tim Goetze
wrote:
yes, the filter quickly becomes unstable during convergence.
i think it's simply against the nature of the iir, which i take
to prefer smooth oscillation. even if there existed some
'magical' set of coefficients that could emulate the noise
impulse, finding it (i can only argument on the basis of my
feeling for the algorithms involved) will require a different
method than the one employed (foreach coefficient a small
step, then combine what looked reasonable).
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.
yup, it definitely looks more like it on the scope. but the
aliasing the crossover introduces is simply unbearable. i think
i'll go for a ride and show some waves to the guy who fixed my
fender amp recently, he knows a lot about the circuitry.
tim
ps: i've updated the 'unmatched.html' page to show graphs of both
the original and the plugin response.