Cascade of discrete filters in MATLAB

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!

Answer 1

[Star Strider]

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.

[Juan]

Thanks! But it doesn't work. This is my code:

 original_audio = 'audio.wav';
[mono Fs] = audioread(original_audio);
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);
sb = dfilt.cascade(lpf,hpf);
fvtool(sb)
filtered_audio = filter(sb,1,mono); % *HERE I HAVE THE ERROR*
f=0:Fs/n:Fs;
fn = f*(2*pi)/Fs;
n=length(filtered_audio)-1;
figure;
plot(filtered_audio)
fftfiltered_audio = abs(fftshift(fft(filtered_audio)));
figure;
plot(fn/pi,fftfiltered_audio);
axis([0 1 -inf inf]);

And I have this error: Error using dfilt.cascade/filter Dimension argument must be a positive integer scalar in the range 1 to 2^31.

And this: Error in document (line 16) filtered_audio = filter(sb,1,mono);

[Star Strider]

You are not calling filter correctly.

Try this:

filtered_audio = filter(sb,mono);

Answer 2

[CS Researcher]

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.