EE-597 Matlab Pointers

Speed Improvements:

Avoid "for" loops whenever possible by vectorizing your code. Most Matlab commands are capable of operating on vector arguments.  For example, rather than pointwise multplying two vectors via

for i=1:M

    z(i) = x(i)*y(i);

end;

take advantage of the pointwise multiplication command ".*"

z = x.*y;

 

If you have to use a "for" loop, your code will run much faster if you initialize vectors before entering the loop. If this is not done, Matlab may need to allocate memory at every iteration of the loop, slowing things down tremendously.  As an example, the "for" loop above should have been initialized with

z = zeros(1,M);

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Importing, Exporting, and Playing Sound:

• The commands "wavread" and "wavwrite" can be used to import and export .wav files.  Note: for "wavwrite" to work properly, the input file must not have entries with absolute value one or greater.
• The command "sound" can be used to play a data vector at a specified sampling rate.  On Windows 95/98/NT machines, the command "wavplay" may also be used.
• When comparing the sound of two or more files, be sure that all files have the same "loudness";  the ear/brain often mistakes "louder" as "better".  Matching signal variances is a rough way to accomplish this.

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Miscellaneous:

Make sure that you do not create a variable whose name is used for a Matlab function -- this leads to strange behavior that is often hard to trace.  When in doubt, type "help varname".

 

Sometimes it is convenient to know to what extent Matlab has finished processing your sound file.  The "printf" line below displays the percentage of loop completion.

cnt=0;

for i=1:M,

    cnt_cur = floor(100*(i/M));

    if cnt_cur>cnt, cnt=cnt_cur; fprintf('%3d%%\b\b\b\b',cnt); end;

    % main processing goes here

end;

Matlab is capable of interpreting infinity and uses "inf" for this purpose.  This might be useful in conjunction with, for example, the "erfc" function.

Getting Help:

• To find instructions on how to use a particular command, type "help command_name".

 
• If you are looking for the name for a Matlab command, make a guess and type "lookfor guess".  If this doesnt work, type "help" to look for likely subtopics, then type "help subtopic".

 
• Some relatively good Matlab tutorials can be found at the Univ. of British Columbia, and Vanderbilt.

Anything Else?

• Do you have suggestions for this page?  Are there "tricks" you have learned or stumbling blocks that deserve mentioning?  If so, email me.