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<H1> 32. Audio Processing </H1>
<!--docid::SEC207::-->
<P>
Octave provides a few functions for dealing with audio data. An audio
`sample' is a single output value from an A/D converter, i.e., a small
integer number (usually 8 or 16 bits), and audio data is just a series
of such samples. It can be characterized by three parameters: the
sampling rate (measured in samples per second or Hz, e.g. 8000 or
44100), the number of bits per sample (e.g. 8 or 16), and the number of
channels (1 for mono, 2 for stereo, etc.).
</P>
<P>
There are many different formats for representing such data. Currently,
only the two most popular, <EM>linear encoding</EM> and <EM>mu-law
encoding</EM>, are supported by Octave. There is an excellent FAQ on audio
formats by Guido van Rossum <guido@cwi.nl> which can be found at any
FAQ ftp site, in particular in the directory
`<TT>/pub/usenet/news.answers/audio-fmts</TT>' of the archive site
<CODE>rtfm.mit.edu</CODE>.
</P>
<P>
Octave simply treats audio data as vectors of samples (non-mono data are
not supported yet). It is assumed that audio files using linear
encoding have one of the extensions `<TT>lin</TT>' or `<TT>raw</TT>', and that
files holding data in mu-law encoding end in `<TT>au</TT>', `<TT>mu</TT>', or
`<TT>snd</TT>'.
</P>
<P>
<A NAME="doc-lin2mu"></A>
<A NAME="IDX1183"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>lin2mu</B> <I>(<VAR>x</VAR>, <VAR>n</VAR>)</I>
<DD>Converts audio data from linear to mu-law. Mu-law values use 8-bit
unsigned integers. Linear values use <VAR>n</VAR>-bit signed integers or
floating point values in the range -1<=<VAR>x</VAR><=1 if <VAR>n</VAR> is 0.
If <VAR>n</VAR> is not specified it defaults to 0, 8 or 16 depending on
the range values in <VAR>x</VAR>.
</DL>
<P>
<A NAME="doc-mu2lin"></A>
<A NAME="IDX1184"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>mu2lin</B> <I>(<VAR>x</VAR>, <VAR>bps</VAR>)</I>
<DD>Converts audio data from linear to mu-law. Mu-law values are 8-bit
unsigned integers. Linear values use <VAR>n</VAR>-bit signed integers
or floating point values in the range -1<=y<=1 if <VAR>n</VAR> is 0. If
<VAR>n</VAR> is not specified it defaults to 8.
</DL>
<P>
<A NAME="doc-loadaudio"></A>
<A NAME="IDX1185"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>loadaudio</B> <I>(<VAR>name</VAR>, <VAR>ext</VAR>, <VAR>bps</VAR>)</I>
<DD>Loads audio data from the file `<TT><VAR>name</VAR>.<VAR>ext</VAR></TT>' into the
vector <VAR>x</VAR>.
<P>
The extension <VAR>ext</VAR> determines how the data in the audio file is
interpreted; the extensions `<TT>lin</TT>' (default) and `<TT>raw</TT>'
correspond to linear, the extensions `<TT>au</TT>', `<TT>mu</TT>', or `<TT>snd</TT>'
to mu-law encoding.
</P>
<P>
The argument <VAR>bps</VAR> can be either 8 (default) or 16, and specifies
the number of bits per sample used in the audio file.
</P>
</DL>
<P>
<A NAME="doc-saveaudio"></A>
<A NAME="IDX1186"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>saveaudio</B> <I>(<VAR>name</VAR>, <VAR>x</VAR>, <VAR>ext</VAR>, <VAR>bps</VAR>)</I>
<DD>Saves a vector <VAR>x</VAR> of audio data to the file
`<TT><VAR>name</VAR>.<VAR>ext</VAR></TT>'. The optional parameters <VAR>ext</VAR> and
<VAR>bps</VAR> determine the encoding and the number of bits per sample used
in the audio file (see <CODE>loadaudio</CODE>); defaults are `<TT>lin</TT>' and
8, respectively.
</DL>
<P>
The following functions for audio I/O require special A/D hardware and
operating system support. It is assumed that audio data in linear
encoding can be played and recorded by reading from and writing to
`<TT>/dev/dsp</TT>', and that similarly `<TT>/dev/audio</TT>' is used for mu-law
encoding. These file names are system-dependent. Improvements so that
these functions will work without modification on a wide variety of
hardware are welcome.
</P>
<P>
<A NAME="doc-playaudio"></A>
<A NAME="IDX1187"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>playaudio</B> <I>(<VAR>name</VAR>, <VAR>ext</VAR>)</I>
<DD><A NAME="IDX1188"></A>
<DT><U>Function File:</U> <B>playaudio</B> <I>(<VAR>x</VAR>)</I>
<DD>Plays the audio file `<TT><VAR>name</VAR>.<VAR>ext</VAR></TT>' or the audio data
stored in the vector <VAR>x</VAR>.
</DL>
<P>
<A NAME="doc-record"></A>
<A NAME="IDX1189"></A>
</P>
<DL>
<DT><U>Function File:</U> <B>record</B> <I>(<VAR>sec</VAR>, <VAR>sampling_rate</VAR>)</I>
<DD>Records <VAR>sec</VAR> seconds of audio input into the vector <VAR>x</VAR>. The
default value for <VAR>sampling_rate</VAR> is 8000 samples per second, or
8kHz. The program waits until the user types <KBD>RET</KBD> and then
immediately starts to record.
</DL>
<P>
<A NAME="doc-setaudio"></A>
<A NAME="IDX1190"></A>
</P>
<DL>
<DT><U>Function File:</U> setaudio <B>([<VAR>w_type</VAR></B> <I>[, <VAR>value</VAR>]])</I>
<DD>Execute the shell command `<SAMP>mixer [<VAR>w_type</VAR> [, <VAR>value</VAR>]]</SAMP>'
</DL>
<P>
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