Matlab – Lowpass/Bandpass filter design in MATLAB

communicationlowpass-filterMATLAB

For an analog communication system design in MATLAB firstly I need to do these two design:

Design a low-pass filter [slow]=lowpassfilter(s,fcut,fs) which filters input signal s with cutoff frequency fcut and sampling frequency fs in Hertz.
Design a band-pass filter [sband]=bandpassfilter(s,fcutlow,fcuthigh,fs) which filters input signal s with cutoff frequencies fcutlow and fcuthigh and sampling frequency fs in Hertz.

Could you please help me?

Best Answer

I found this question which has so many views and still no good answer.

The following code will do what you need. Since no filter-type is specified, I used a butterworth-filter to demonstrate it. s is the input signal and x is the filtered signal. fs is the sampling rate in Hz.

% Design and apply the lowpass filter
order = 4;
fcut  = 8000;
[b,a] = butter(order,fcut/(fs/2),'low');
x     = filter(b,a,s);


% Design and apply the bandpass filter
order    = 10;
fcutlow  = 1000;
fcuthigh = 2000;
[b,a]    = butter(order,[fcutlow,fcuthigh]/(fs/2), 'bandpass');
x        = filter(b,a,s);

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Matlab filter questions

Building on Tristan's answer, here is some Octave Code which might or might not be Matlab compatible. The butter function derives the transfer function coefficients for you. alt text .

hz = 8000;
x = [1:1:hz*10];
t = x./hz;
pi = 3.1415;

% Create signal with 10 hz, 200 hz and 500 hz components
raw_signal = sin(10*2*pi*t)+sin(200*2*pi*t)+sin(500*2*pi*t);

% View Raw Signal Over .1 Second Window
plot(t, raw_signal)
title('Raw Signal with 10Hz, 200Hz, 500Hz Components')
xlabel('Time (Sec)')
ylabel('Amplitude')
set(gca,'XLim', [5, 5.1]);

% Create Band Pass Butterworth Filter
[S_numer, S_denom] = butter(5, [100/hz 350/hz]);
band_passed_signal = filter(S_numer, S_denom, raw_signal);

% View Band Pass Filtered Signal Over .1 Second Window
plot(t, band_passed_signal)
title('Band Pass Filtered Signal')
xlabel('Time (Sec)')
ylabel('Amplitude')
set(gca,'XLim', [5, 5.1]);

Python – How to write lowpass filter for sampled signal in Python

You appear to have two questions:

How do I design a lowpass filter with a 10MHz cutoff frequency and an arbitrary filter order?
How do I realise that filter in Python?

Filter design is beyond the scope of Stack Overflow - that's a DSP problem, not a programming problem. Filter design is covered by any DSP textbook - go to your library. I like Proakis and Manolakis' Digital Signal Processing. (Ifeachor and Jervis' Digital Signal Processing isn't bad either.)

If you must have teh coeds, try this blog post, which shows how to design a Butterworth lowpass filter with scipy.

As for implementation of the filter in Python, scipy has a lfilter() function which applies a FIR or IIR filter to a signal in one dimension.

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Python – How to write lowpass filter for sampled signal in Python

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