% ECE 1250 Lecture 5

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% Advanced Indexing, finish remaining topics.  (See handout on website.)
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% Writing script files.
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% Change directory to where script files are located so Matlab¨ can find them,
%  or use the Set Path command under the File menu to add the folder with
%  the script files to where Matlab¨ looks for commands.

% Use .m extension on file name.  Put in commands just as you would at the
%  >> prompt in the command window.  Here's a simple example of a script file
%  we shall call my_magic.m


% my_magic.m     Script to print a 3 x 3 magic square.
A = magic(3)
display('That''s a magic square!')
return


% To execute the script file my_magic.m, type the filename without the .m
>> my_magic
A =
     8     1     6
     3     5     7
     4     9     2
That's a magic square!

% Be careful not to create files with the same name as a Matlab¨ command.
% For example, say you create a file called plot.m.  When you use the plot
%  command, Matlab¨ will execute the first plot.m file it finds in the
%  search path.  If your plot.m was found first, you would no longer have
%  access to Matlab¨'s plot command.

% How to get rid of your plot.m command once it gets into Matlab¨ (and gets
%  compiled and stored in memory so the file is no longer accessed directly)?
% 1) Remove or rename your file.
% 2) Use the clear command to clear the filename, e.g., >> clear plot
% The next time you type "plot", Matlab¨ will go through the search path
%  again and find the correct file.

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% Fourier Analysis  (finding what  sinusoids are in a signal)
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% The Fast Fourier Transform, or FFT, computes what sinusoids are in a waveform.
>> y_spectrum = fft(y);

% No matter how many points are in y, the fft analyzes frequencies from 0 to the
%  sampling rate of y.  For sounds we have used, the sampling rate is 8000/sec.
% y_spectrum has as many points as y, but those points represent frequencies
%  from 0 to 8000 Hz.

% The fft command produces complex numbers that encode the amount of cosine and
%  sine at each frequency.  We use abs() to find the magnitude of the cosine and
%  sine combined at each frequency.
>> mag_y_spectrum = abs(y_spectrum);

% The plot of the fft shows us what frequencies are in a signal.
>> plot(abs(fft(y)))

% We can go the other direction, too, to create a signal from its frequencies.
>> y = ifft(y_spectrum);   % The "i" in ifft stands for "inverse".

% We can alter a sound by changing its spectrum.  A filter reduces certain
%  frequencies, for example.  We can make digital filters to alter a sound,
%  as we will see in the lab later on.

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% Example script file creating sound effect: see my_script.m
% Example script file to analyze students' test scores: see my_script2.m
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