bilinear transfermation

Hai! every one.
This question is basic for DSP experts.I try to design digital butter
worth filter(lowpass) in C.First I converted given digital specs to analog
specs.after that i caluculated prewarped analog specs.I found order, cutoff
frequency of analog butterworth filter.After that I used butterworth filter
tables to find normalized denominator coefficients of analog filter.Now i
tried to convert these analog filter coefficients to digital filter
coefficients, using bilinear transfermation.I know that we need to replace
's' with 2/T*(z-1)/(z+1) to get equivalent digital filter.
Here I have two problems.
i>It's difficult to write C code, to get polynomial coefficients in z
domain.So instead of like this,is there any other way of getting digital
filter coefficients?(using poles....etc)
ii>In analog filters, I used butterworth filter tables, instead of that
can we caluculate directly protype denominator coefficients(cutoff
frequency=1)?
I don't have depth knowledge of DSP concepts.If any body knows answers to
my questions, please let me know.
Thank you.
vyshali

vyshali wrote:
> Hai! every one.
> This question is basic for DSP experts.I try to design digital butter
> worth filter(lowpass) in C.First I converted given digital specs to analog
> specs.after that i caluculated prewarped analog specs.I found order, cutoff
> frequency of analog butterworth filter.After that I used butterworth filter
> tables to find normalized denominator coefficients of analog filter.Now i
> tried to convert these analog filter coefficients to digital filter
> coefficients, using bilinear transfermation.I know that we need to replace
> 's' with 2/T*(z-1)/(z+1) to get equivalent digital filter.
> Here I have two problems.
> i>It's difficult to write C code, to get polynomial coefficients in z
> domain.So instead of like this,is there any other way of getting digital
> filter coefficients?(using poles....etc)

Divide the filter into a casrcade of biquads, and transforme each
biquad analytically.

> ii>In analog filters, I used butterworth filter tables, instead of that
> can we caluculate directly protype denominator coefficients(cutoff
> frequency=1)?

Yes. There is the book by Antoniou that takes you through all
the steps.

> I don't have depth knowledge of DSP concepts.If any body knows answers to
> my questions, please let me know.

You have done a lot already. Find the book by Antoniou and you'll
reach your goal.

Rune