Minimal Digital Filter Example

% this is a minimal example of 

% of digital filtering using MATLAB

% ---

%

% we are going to create a mixture of

% two sounds and filter one of them out

% ---

%

% first lets state the basics for this

% example

fs = 16000;         % sample rate for sounds

fHz1 = 220;         % 220Hz is a low sound

fHz2 = fHz1*2^3;    % three octaves higher 

% first make a 1 second sound that  

% consists of a mixture of a high note

% and a low note

low_note = cos([1:fs]*2*pi*fHz1/fs);

high_note = cos([1:fs]*2*pi*fHz2/fs);

two_notes = (low_note + high_note);

% design and apply a filter to

% remove the high note

[b a] = butter(1, [0.01 0.1]);

two_notes_filtered = filter(b,a,two_notes);

% join the filtered and unfiltered versions

% together and listen to the result one second

% of un-filtered followed by one second of

% filtered

soundsc([two_notes two_notes_filtered],fs);