scdoro,
wu, wl refer tot the upper and lower passband corner frequencies. Regarding selection of a passband frequency to use for the low pass prototype, this is not so straightforward. Frequently the geometric mean between the upper and lower passband frequencies is used (Sqrt(wu X wl)). However, this results in an asymmetric response for both analog design and impulse invariant design. For the bilinear transform method, the problem is further complicated by the frequency warping. You have 3 choices:
~ Use wl for the low pass prototype, and take
~ what you get at the high end. This usually gives the
~ widest bandwidth.
~
~ Use wu for the low pass prototype, and take
~ what you get at the low end.
~
~ Use the geometric mean for the low pass prototype, and take
~ what you get at both the low end and the high end.
~
You will probably have to "cut and try" until you get acceptable results using the bilinear transform method, since you can only choose one frequency of the analog prototype at which to "pre-warp".
Regards,
Kral