[PD] biquad~ with elementary filters [was: Re: dinosaurs ...]

[martin brinkmann]

Frank Barknecht wrote:
>> that is true, but it looks like no one has made a (usual lp,hp,etc.)
>> filter with these objects until now.
> 
> Except Miller. [1]

you are right, there are some "usual filters" made with
pole/zero objects, the shelving and peaking ones, a nonresonant lp
and a bp, iirr. and the examples in millers book have been very helpfull
with my own attemps to make a resonant lowpass.
but the "standard" filters (resonant 2,4 pole lp,hp,bp,notch,...)
for sound synthesis do not exist (yet) in "vanilla-pd".

> Though I agree that some more of these would be handy. I tried to make
> a biquad~ clone with the elementary filters, but failed so far. I
> assume from [2] that the transfer function of two rzero~ and two
> rpole~ in series should be the same as a biquad~, however I don't know
> how to convert the ff1, ff2, ff3, fb1 and fb2 coefficients of biquad~
> into the coefficients to use at the second inlets of the elemetary
> filters. Can anyone help?

i had the same problem, and did not find a solution. (it might be quite
complicated anyway), so i tried to build a filter by "directly"
placing the pole and zero. it sounds not bad, but i still have a problem
with scaling the output correctly. if i use this filter in a synth i
have to adjust the volume quite often.


bis denn!
   martin