Cascade of discrete filters in MATLAB

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Hello everyone. I'm trying to make a band-reject filter but I can't put 2 filters in cascade.
This:
lp = fir1(100, 0.77, 'low', rectwin(101));
hp = fir1(100, 0.83, 'high', rectwin(101));
dfilt.cascade(lp,hp)
gives the following error: Error using dfilt.cascade (line 43) Numeric section in a cascade must be a nonempty scalar.
Thanks!

Accepted Answer

Star Strider
Star Strider on 8 May 2016
To use dfilt.cascade, you have to create the correct dfilt filter objects from your filters first:
lp = fir1(100, 0.77, 'low', rectwin(101));
hp = fir1(100, 0.83, 'high', rectwin(101));
lpf = dfilt.df2(lp,1);
hpf = dfilt.df2(hp,1);
ntch = dfilt.cascade(lpf,hpf);
figure(1)
freqz(ntch)
You may want to reconsider your filter designs.
  2 Comments
Star Strider
Star Strider on 8 May 2016
You are not calling filter correctly.
Try this:
filtered_audio = filter(sb,mono);

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More Answers (1)

CS Researcher
CS Researcher on 8 May 2016
You would have to convert the filters to dfilt type. Try this:
lp = fir1(100, 0.77, 'low', rectwin(101));
hp = fir1(100, 0.83, 'high', rectwin(101));
h1 = dfilt.df2t(lp,1);
h2 = dfilt.df2t(hp,1);
h = dfilt.cascade(h1,h2);
However, Mathworks documentation states:
"Note that with the DSP System Toolbox installed, one usually does not construct cascade filters explicitly. Instead, one obtains these filters as a result from a design using FDESIGN." You should look into this.

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