## Correct transfer function with 1st order IIR filter

Started by 7 years ago2 replieslatest reply 7 years ago180 views

What diffreneces it makes in magnitude and/or phase response when using

H(z) = (b0 + b1*z^-1 + b2*z^-2)/(a0 + a1*z^-1 + a2*z^-2) by adding b2=0, a2=0

H(z) = (b0 + b1*z^-1)/(a0 + a1*z^-1)

for 1st order filter?

All software I'm using for to build VST plug-ins (SynthEdit, FlowStone, etc.) implements a biquad (BLT) module for coefficient input and now it looks like by using it I can't get 90° phase response with a filter designed using Octave (by Octave plot phase is 90°). I have tried with single filter, series and parallel connections. When two 1st order filters (given below) are connected in serie it shows 0dB/180° and with parallel connection ~+303.5dB/0°.

Here are the Octave plots (both filters has the same phase response):

and filter coefficients I'm playing with:

AP1 = [ -1.50119987579016e+015 1.50119987579016e+015 1.0 0.999999999999999 ]
AP2 = [  6.66133814775094e-016 6.66133814775094e-016 1.000000000000001 -1.0 ]

EDIT: Ok, digged the web a bit more and found these three (1 2 3) articles published at analog.com.

EDIT2: Here are responses SynthEdit gives (I have the other filter in Left cahennel and the other in Right channel of plug-in):

Magnitude

Phase

Phase inverted

As seen in plots, something happens for the magnitudes and phases when filters are implemented as VST plug-in?

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