[music-dsp] allpass fine tuning of sweeping comb filter (karplusstrong etc)

[Andrew Simper]

Hi Ross,

Still in Melbourne? I'm over in London now working with FXpansion. I'm
pretty sure that in physical modelling you don't "sweep" the frequency since
this introduces Doppler pitch changes which don't happen on strings. I think
most people perform many crossfades instead with an allpass filter with
fixed coefficients for each crossfade to a new delay line length:

http://www-ccrma.stanford.edu/~jos/smith-nam/Large_Delay_Changes.html


All the best,

Andrew. 


-----Original Message-----
From: music-dsp-bounces at ceait.calarts.edu
[mailto:music-dsp-bounces at ceait.calarts.edu] On Behalf Of Ross Bencina
Sent: 25 November 2005 09:51
To: music-dsp at shoko.calarts.edu
Subject: [music-dsp] allpass fine tuning of sweeping comb filter
(karplusstrong etc)

Hi Peoples

I've implemented the first-order allpass tuning method for Karplus Strong
described by Jaffe and Smith in their CMJ article (and probably well known
in the PM literature I guess). It works as intended, however, when I sweep
the cutoff frequency (recomputing the allpass coefficient every sample) I
get some nasty stuff being injected into the delay line... I think this is a
well known problem with the allpass tuning method.

My question: what are the common options for implementing an accurately
tuned comb filter which can be swept dynamically? Can I reduce the negative
effects of the allpass somehow?

Previously I was using linear interpolation for sub-sample delays but in the
resonant comb filter case this causes frequency dependent damping.

Any and all suggestions gratefully accepted...

Best wishes

Ross. 

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