How to get the a and b coefficients out of the DSP digital filter function?

Specific Domains Signal and Image Processing
1

Hello, i want to get the a and b coefficients of a digital Butterworth filter in Julia.

I have to iteratively compute the results for live-sound purpose, so I cannot use a filter() function.

Here is where my code is at
"Wn = 1/120

lowpass_filter = digitalfilter(Lowpass(Wn),Butterworth(4))

filter_tf = tf(zpk(lowpass_filter.z, lowpass_filter.p, lowpass_filter.k))"

I know that the coefficients a re in the polynomial field of filter_tf, but don’t know yet how to get them out.

If you know how (how a simpler solution) I would really appreciate.

2

Have you looked at DSP.jl 's functions coefa() and coefb() ?
E.g:

coefa(lowpass_filter)
coefb(lowpass_filter)
3

Thank you.