1st order filter output Two 1st order filters :

--[ b0, b1, a0, a1]
1 [ -1.40374673978609e+008 1.40374675978609e+008 1 1.000000235752411 ]
2 [ 2.22044577169459e-016 2.22044632680604e-016 1 -1 ]


gives these responses in Octave:



When using those same coefficients (in parallel order or even just one set of them) in another software I get different results. Example:



Magnitude response isn't a problem (unless not 'flat' in range 20Hz-20kHz) but the phase response is 180° even when implement only one of those filters. Shouldn't phase be 90° as it is in Octave plot.

Q1: is it just because of the transfer function (1st order (Octave) vs biquad (other software)) or is there something else beind this difference?
Q2: is it possible to plot these two filters as parallel system in Octave/MatLab
Matlab results.

The filters coefficients have a big dynamic variation. So you have those results. For second filter, coefficients b are about zeros.

The first filter

b = [-1.40374673978609e+008 1.40374675978609e+008];
a = [1 1.000000235752411];
freqz(b, a)




The second filter

b = [2.22044577169459e-016 2.22044632680604e-016];
a = [1 -1];
freqz(b, a)