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libsndfile and GNU Octave
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<META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
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<H1><B>libsndfile and GNU Octave</B></H1>
<P>
<A HREF="http://www.octave.org/">GNU Octave</A> is a high-level interactive
language for numerical computations.
There are currently two development streams, a stable 2.0.X series and a
development 2.1.X series.
Octave reads and writes data in binary formats that were originally developed
for
<A HREF="http://www.mathworks.com/">MATLAB</A>.
Version 2.0.X of Octave uses binary data files compatible with MATLAB
version 4.2 while Octave 2.1.X uses binary data files compatible
with MATLAB version 5.0 as well as being able to read the older MATLAB 4.2
format.
</P>
<P>
From version 1.0.1 of libsndfile onwards, libsndfile has the ability of reading
and writing a small subset of the binary data files used by both versions
of GNU Octave.
This gives people using GNU Octave for audio based work an easy method of
moving audio data between GNU Octave and other programs which use libsndfile.
</P>
<P>
For instance it is now possible to do the following:
</P>
<UL>
<LI> Load a WAV file into a sound file editor such as
<A HREF="http://www.metadecks.org/software/sweep/">Sweep</A>.
<LI> Save it as a MAT4 file.
<LI> Load the data into Octave for manipulation.
<LI> Save the modified data.
<LI> Reload it in Sweep.
</UL>
<P>
Another example would be using the MAT4 or MAT5 file formats as a format which
can be easily loaded into Octave for viewing/analyzing as well as a format
which can be played with command line players such as the one included with
libsndfile.
</P>
<H2><B>Details</B></H2>
<P>
Octave, like most programming languages, uses variables to store data, and
Octave variables can contain both arrays and matrices.
It is also able to store one or more of these variables in a file.
When reading Octave files, libsndfile expects a file to contain two
variables and their associated data.
The first variable should contain a variable holding the file sample rate
while the second variable contains the audio data.
</P>
<P>
For example, to generate a sine wave and store it as a binary file which
is compatible with libsndfile, do the following:
</P>
<PRE>
octave:1 > samplerate = 44100 ;
octave:2 > wavedata = sin ((0:1023)*2*pi/1024) ;
octave:3 > save sine.mat samplerate wavedata
</PRE>
<P>
The process of reading and writing files compatible with libsndfile can be
made easier by use of two Octave script files :
</P>
<PRE>
octave:4 > [data fs] = sndfile_load ("sine.mat") ;
octave:5 > sndfile_save ("sine2.mat", data, fs) ;
</PRE>
<P>
In addition, libsndfile contains a command line program which which is able
to play the correct types of Octave files.
Using this command line player <B>sndfile-play</B> and a third Octave script
file allows Octave data to be played from within Octave on any of the platforms
which <B>sndfile-play</B> supports (at the moment: Linux, MacOS X, Solaris and
Win32).
</P>
<PRE>
octave:6 > sndfile_play (data, fs) ;
</PRE>
<P>
These three Octave scripts are installed automatically in Octave's site
script directory when libsndfile is installed (except on Win32) ie when
libsndfile is being installed into /usr/local, the Octave scripts will
be installed in /usr/local/share/octave/site/m/.
</P>
<P>
There are some other Octave scripts for audio to be found
<A HREF="http://octave.sourceforge.net/audio/index.html">here</A>.
</P>
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<P>
The libsndfile home page is here :
<A HREF="http://www.mega-nerd.com/libsndfile/">
http://www.mega-nerd.com/libsndfile/</A>.
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