MATLAB: How is the time varying IIR filter implemented when using the Digital Filter design block in Signal Processing Blockset 6.4 (R2006b)

Question

I'm using the Digital Filter block to implement a time-varying IIR filter (poles and zeros), but it seems to be producing incorrect output.

Answer 1

Documentation on the algorithm used to implement time varying IIR filter is missing. Here is additional information on the algorithm for time varying IIR filter:

At each time step the following calculations are performed:

% calculate output
y(n) = (b(1)x(n) + z1(n-1))/a(1);
% update states
% code for 3rd order filter
z1(n) = b(2)x(n) + z2(n-1) - a(2)y(n);
z2(n) = b(3)x(n) + z3(n-1) - a(3)y(n);
z3(n) = b(4)x(n)  - a(4)y(n);