MATLAB: Are only the filter coefficients of the first section of a filter returned when I try to export all the filter coefficients in FDATool in Filter Design Toolbox 2.5 (R13SP1)

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Why are only the filter coefficients of the first section of a filter returned when I try to export all the filter coefficients in FDATool in Filter Design Toolbox 2.5 (R13SP1)?

I designed an IIR Elliptic Bandpass filter using FDATool. The filter has a Direct Form II, second-order-sections structure.

I then turned the quantization on.

Finally I go to "File->Export" to export the filter coefficients to a text file. In the text file, I only see the filter coefficients of the first section.

Best Answer

This is a bug in Filter Design Toolbox 2.5 (R13SP1) in the way that FDATool handles exporting filter coefficients. It was fixed in Filter Design Toolbox 3.0 (R14).

To work around this issue in Filter Design Toolbox 2.5 (R13SP1) and before, convert the filter structure to Single Section before exporting filter coefficients.

Related Solutions

MATLAB: Fir and iir filtering

You have to know your filter specifications. If you do, you can do both with fdesign.lowpass and fdesign.highpass

fdesign.lowpass and fdesign.highpass have different specification strings which support different filter designs.

The default string 'Fp,Fst,Ap,Ast' supports both FIR and IIR designs.

Here is an example:

Design a lowpass filter with a passband frequency of 1 kHz, a stopband frequency of 1.5 kHz, passband ripple of 0.5 dB, and stopband attenuation of 40 dB. The sampling frequency is 20 kHz

   d = fdesign.lowpass('Fp,Fst,Ap,Ast',1e3,1.5e3,0.5,40,20e3);
   designmethods(d)  %shows which designs are valid for this spec
   Hdeq = design(d,'equiripple'); % FIR filter
   Hbutt = design(d,'butter'); %IIR design

MATLAB: Filter coefficients

That depends. The filter coefficients are the coefficients of the difference equation. If your filter is an FIR filter, then the filter coefficients are the values of the impulse response.

If you have an IIR filter, then the filter coefficients are not the same as the impulse response. Remember in that case the impulse response is infinite.

For example:

b = fir1(10,0.2);
stem(b)
h = impz(b);
stem(h)
isequal(b',h)

but

   [b,a] = butter(10,0.2);
   h = impz(b,a);

But the ratio of Z-transforms of the numerator coefficients to denominator coefficiens is equal to the Z-transform of the impulse response.

For example - consider the IIR system with the following difference equation

y(n)-0.8*y(n-1) = x(n)

So the filter coefficients are:

   A = [1 -0.8]; B =1;

The impulse response is:

h(n) = 0.8^n*u(n) where u(n) is the unit step. But compare:

   h1 = impz(B,A);
   subplot(211)
   stem(h1,'color',[1 0 0]);
   subplot(212)
   n = 0:length(h1)-1;
   h = 0.8.^n;
   hold on;
   stem(h,'color',[0 0 1]);

Best Answer

Related Solutions

MATLAB: Fir and iir filtering

MATLAB: Filter coefficients

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