Polyphase IIR halfband decimator using biquad stages
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Using DSP/Signal Toolbox.
I am trying to use matlab to generate coefficients for polyphase IIR halfband decimator. Which can be achieved using :
%%Halfband filter (-3dB at 0.5), Transition band 0.1 wide and 80dB stopband attenuation
f = fdesign.decimator(2, 'halfband','TW,Ast' ,0.1,80);
h1 = design(f,'ellip');
h1.realizemdl
This generates:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/150468/image.png)
However refactoring phase 1 to see the structure it looks like a cascade of first order df1 stages. Phase 2 is just a 0.5 gain.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/150469/image.png)
The filter structure I am targeting is implemented as cascaded biquads. Phase 1 of a higher order than Phase 2.
*How do I get matlab to generate polyphase IIR filters using biquad stages?* where Phase 2 is more than a gain step. Ideally using the ellip method.
I have tried a few things like:
He3 = design(f,'ellip','FilterStructure','cascadewdfallpass');
He3.realizemdl
Which creates a Phase 2 with a real filter but not using biquad stages.
I am not aware of how to find which 'FilterStructures' are valid but found a reference to 'df2sos', which does not seem compatible with the ellip moethods.
He3 = design(f,'ellip','FilterStructure','df2sos');
Error using fdesign.decimator/ellip (line 19)
'df2sos' is an invalid structure for this design.
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