Well you could try the butlp with a-rate cutoff frequency to see if it  
still happens.  That code is in git now.
==John ff

Quoting Iain McCurdy <i_mccurdy@hotmail.com>:

> I've noticed that butlp produces pretty pronounced artefacts when  
> the cutoff frequency is moving in any way quickly as the example  
> below demonstrates. Obviously things are dramatically improved when  
> kr = sr but at lower control rates all of Csound's other low-pass  
> filters sound significantly better. I am wondering whether butlp is  
> missing out on some internal interpolation.
> I'm testing on 6.01 OSX10.6.
>
> <CsoundSynthesizer>
> <CsOptions>
> -odac -d
> </CsOptions>
> <CsInstruments>
>
> 0dbfs=1
> nchnls = 2
> ksmps = 16
>
> alwayson 1
>
> instr 1
> koct lfo 3,4,1
> asig vco2 0.5,400,4,0.5
>
> asig butlp asig,cpsoct(7+koct)
>
> ;asig clfilt asig,cpsoct(7+koct),0,2
> ;asig tonex asig,cpsoct(7+koct),3
> ;asig moogladder asig,cpsoct(7+koct),0
> ;asig bqrez asig/3,cpsoct(7+koct),1
> ;ahp,asig,abp,abr statevar asig, cpsoct(7+koct), 1
>
> outs asig,asig
> endin
>
>
> </CsInstruments>
>
> <CsScore>
> </CsScore>
> </CsoundSynthesizer>
>
>
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