convert transfer functions between s domain and z domain

s->z: use function "c2d"

z->s: use function "d2c"

resampling: "d2d"

VHDL on mac

http://epicentertech.biz/services/mac_eda/index.html

http://www.csee.umbc.edu/help/VHDL/index.shtml#cadence

http://ghdl.free.fr/