> you must filter the sawtooth wave with a lowpass resonant filter setting
> a relatively high Q value,

Yeah, the high Q seems to be the problem. Unfortunately, the butterlp
opcode doesn't have a Q setting. Yesterday I wanted to try the zfilter2
opcode. This custom filter has a 'damping' parameter, which to my
knowledge, is somehow inversely proportional to the Q value. However, the
example in the Manual is somehow strange:

a1 zfilter2 asig, kdamp, kfreq, 1, 2, 1, ia1, ia2
                                         ^^^^^^^^^

What are ia1/ia2 ? Is the there a table of commonly used filter
parameters, i.e. for lowpass/bandpass/highpass/bandreject in any order ?
How can I calculate them myself ? In the manual they write that you
have to obtain the parameters from an external filter design application
such as Matlab. Is there something public domain ?