Have a look at the filter code in Csound, for example: https://github.com/csound/csound/blob/master/Opcodes/butter.c https://github.com/csound/csound/blob/mast

Have a look at the filter code in Csound, for example: https://github.com/csound/csound/blob/master/Opcodes/butter.c https://github.com/csound/csound/blob/master/Opcodes/butter....