31 Oct
2002
31 Oct
'02
6:11 p.m.
On Thu, Oct 31, 2002 at 10:05:12 +0100, Tim Goetze wrote:
and we need quite a few. i've created an image of
the spectral
evolution of an ~500 Hz sine from inaudible to full distortion:
http://quitte.de/spectral-evolution.gif
Excellent. Can you either stick the data somewhere, or build a
signal-amplitude x harmonic-amplitude table from it?
What windowing function did you use? Do you know if the shape varies with
the frequency of the fundamental? It would be a pain if it did.
looking at the above plot, i more and more think that
the chebyshev
may well be the way to go. i don't think any combination of filtering,
exp()- and sine()-based shaping will produce anything like this; if
this can be modeled without employing the chebyshev, my feeling is
that a suitable shaper would need to be based on a more accurate
model of what's happening inside the amp.
I think its certainly worth a go, the way the harmonics come and go
through the amplitude stages is very interesting.
the table idea sounds convincing, however there's
still the problem of
how to blend between polynomials, maybe a listening test will prove
My guess is that we could get away with recalculating the cheby
polynomials every few samples (its pretty cheap anyway). The envelope
tracker needs to be pretty quick, but I still think it will be OK.
Blending between the polynomial coefficents will be pretty expensive and
may be like blending between filter coefficents. :(
so how many harmonics need to be generated?
My harmonic gen uses 10 (its a sweet spot for the PIII's cache IIRC), but
we could go up much higher if we have to. 10 is significantly cheaper than
a single valve.
I have no real idea how many we would need. One possibility is a slight
valve effect before (or parralel to) the cheby to generate some low level
harmonics, just to make sure there are some there. It might sound wierd if
they abruptly stop.
FT'ing some high guitar notes shows that note
attacks briefly contain
frequency content higher than 10 kHz, but the 'body' usually is far
below; the root of the highest note on the guitar is about 2.5 kHz.
OK, well we can worry about that when we come to it.
- Steve