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<TITLE>GNU Octave - Audio Processing</TITLE>
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<P><HR><P>
<H1><A NAME="SEC164" HREF="octave_toc.html#TOC164">Audio Processing</A></H1>
<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>
<DL>
<DT><U>Function File:</U> <B>lin2mu</B> <I>(<VAR>x</VAR>)</I>
<DD><A NAME="IDX833"></A>
If the vector <VAR>x</VAR> represents mono audio data in 8- or 16-bit
linear encoding, <CODE>lin2mu (<VAR>x</VAR>)</CODE> is the corresponding mu-law
encoding.
</DL>
</P>
<P>
<DL>
<DT><U>Function File:</U> <B>mu2lin</B> <I>(<VAR>x</VAR>, <VAR>bps</VAR>)</I>
<DD><A NAME="IDX834"></A>
If the vector <VAR>x</VAR> represents mono audio data in mu-law encoding,
<CODE>mu2lin</CODE> converts it to linear encoding. The optional argument
<VAR>bps</VAR> specifies whether the input data uses 8 bit per sample
(default) or 16 bit.
</DL>
</P>
<P>
<DL>
<DT><U>Function File:</U> <B>loadaudio</B> <I>(<VAR>name</VAR>, <VAR>ext</VAR>, <VAR>bps</VAR>)</I>
<DD><A NAME="IDX835"></A>
Loads audio data from the file <TT>`<VAR>name</VAR>.<VAR>ext</VAR>'</TT> into the
vector <VAR>x</VAR>.
</P>
<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.
</DL>
</P>
<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><A NAME="IDX836"></A>
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>
<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>
<DL>
<DT><U>Function File:</U> <B>playaudio</B> <I>(<VAR>name</VAR>, <VAR>ext</VAR>)</I>
<DD><A NAME="IDX837"></A>
<DT><U>Function File:</U> <B>playaudio</B> <I>(<VAR>x</VAR>)</I>
<DD><A NAME="IDX838"></A>
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>
<P>
<DL>
<DT><U>Function File:</U> <B>record</B> <I>(<VAR>sec</VAR>, <VAR>sampling_rate</VAR>)</I>
<DD><A NAME="IDX839"></A>
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>
<P>
<DL>
<DT><U>Function File:</U> <B>setaudio</B> <I>(<VAR>type</VAR>)</I>
<DD><A NAME="IDX840"></A>
<DT><U>Function File:</U> <B>setaudio</B> <I>(<VAR>type</VAR>, <VAR>value</VAR>)</I>
<DD><A NAME="IDX841"></A>
Set or display various properties of your mixer hardware.
</P>
<P>
For example, if <CODE>vol</CODE> corresponds to the volume property, you can
set it to 50 (percent) by <CODE>setaudio ("vol", 50)</CODE>.
</P>
<P>
This is an simple experimental program to control the audio hardware
settings. It assumes that there is a <CODE>mixer</CODE> program which can be
used as <CODE>mixer <VAR>type</VAR> <VAR>value</VAR></CODE>, and simply executes
<CODE>system ("mixer <VAR>type</VAR> <VAR>value</VAR>")</CODE>. Future releases might
get rid of this assumption by using the <CODE>fcntl</CODE> interface.
</DL>
</P>
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