[music-dsp] Re: Filter Envelopes in Polyphonic VA synths

dougall irving dougalli at hotmail.com
Tue Apr 9 23:43:28 EDT 2002


Thanks for all the advice on filter envelopes.  All very helpful.  The 
filter code I have been using has a lot of calculations for each coeffecient 
so I have been experimenting with the Chamberlin 12dB filter in the source 
code archive.  It has the advantage that the two coeffecients are not 
dependent on each other which makes using a lookup table easier.

Could anyone explain how the resonance, bandwidth, scale and q factors 
relate?  I have simply been setting the bandwidth to 1 and all the others 
are then equal but this doesn't sound that great.  Am I missing something 
here?

Perhaps someone could tell me where to get a copy of the paper this code is 
referenced from "Effect Deisgn Part 1, Jon Dattorro, J. Audio Eng. Soc., Vol 
45, No. 9, 1997 September"

Thanks again.

Code : ( from music-dsp source code archive )

cutoff = cutoff freq in Hz
fs = sampling frequency //(e.g. 44100Hz)
f = 2 sin (pi * cutoff / fs) //[approximately]
q = resonance/bandwidth [0 < q <= 1]  most res: q=1, less: q=0

scale = q

//--beginloop
low = low + f * band;
high = scale * input - low - q*band;
band = f * high + band;
notch = high + low;
//--endloop


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