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<title>libao - Documentation</title>
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<td><p class=tiny>libao documentation</p></td>
<td align=right><p class=tiny>libao version 1.1.0 - 20110221</p></td>
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<h1>libao Drivers</h1>
<p>
Libao supports both <i>live</i> output drivers and <i>file</i> output
drivers. Live output drivers send audio data to sound cards and sound
daemons. File output drivers write audio to disk using a particular
file format (such as WAV, AU, etc.). You must invoke the
ao_open_????() function that corresponds to the type of driver you are
using, but otherwise live and file drivers are treated identically in
libao.
<p>Driver options may be passed to the drivers via the
<tt>*options</tt> argument to <a
href="ao_open_live.html">ao_open_live()</a> and <a
href="ao_open_file.html">ao_open_file()</a>, or they may be set in the
<a href="config.html">configuration file</a> as <tt>name=value</tt>
pairs.
<h2>Options Understood by All Drivers</h2>
<ul>
<li>"debug" - (value not required) Requests driver print detailed
debugging information.
<li>"matrix" - Set an output channel mapping similar to the use of the
<i>matrix</i> field in <a
href="ao_sample_format.html">ao_sample_format</a>. The specified
matrix overrides the backend's native channel ordering/numbering. The
channel numbering used by the driver does not necessarily reflect the
physical ordering; for example, the fourth PulseAudio channel is
always 'Center' by default whether the channel physically exists or not.
<li>"quiet" - (value not required) Requests the driver print no output
whatsoever, even in the event of error.
<li>"verbose" - (value not required) Requests that the driver print
more detailed information concerning normal operation.
</ul>
<h2>Standard Driver Options</h2>
These are options that have the same use in each driver, but may not
be supported by all drivers (due to the option being meaningless or
unimplemented for a given audio backend).
<ul>
<li>"id" - Request a specific playback device/sink/output by number.
In most audio backends, this will correspond to a specific output
device, but for all devices, this means 'give me the Nth output'.
<li>"dev" - Request a specific playback device/sink/output by name.
This name will be in a format determined by the specific driver
backend (eg, the first hardware device in ALSA format would be 'hw:0',
and for OSS it would be '/dev/dsp').
<li>"server" - Daemon-based sound subsystems often support connecting
to non-default local or remote servers. The "server" option allows
specifying the connection for a given driver in a format specific to
that system.
<li>"client_name" - specify a descriptive name for the application;
this is often used by sound backends to display status information
about which applications are currently making use of playback.
</ul>
<h2>Live Output Drivers</h2>
<h3>aixs</h3>
IBM AIX sound driver. According to the author, "tested on AIX 5.1 with
the Crystal chipsets only (found as internal audio in the 7043-140 and
on the MCA adapter DFE5 (7-6)), but it should work with other AIX
releases and the ACPA also."
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above). By default, the
driver tries "/dev/baud0/1" (device for MCA machines with the Crystal
chipset). Other possible devices are "/dev/paud0/1" for PCI machines
with the Crystal chipset, and "/dev/acpa0/1" for MCA machines with the
ACPA.
<li>"id" - (see 'Standard Driver Options' above).
</ul>
<p>
<hr width="50%">
<h3>alsa</h3>
Advanced Linux Sound Architecture (API versions 0.9.x/1.x.x; earlier
API versions are now deprecated).
<p>
<b>Option keys:</b>
<ul>
<li>"buffer_time" - Override the default hardware buffer size (in
milliseconds).
<li>"dev" - (see 'Standard Driver Options' above). ALSA device label
to use. Examples include "hw:0" for the first soundcard and "hw:1" for
the second. The alsa driver normally chooses one of "surround71",
"surround51", "surround40", "front", or "default" automatically
depending on number of output channels.
<li>"id" - (see 'Standard Driver Options' above).
<li>"period_time" - Override the default hardware period size (in
microseconds).
<li>"use_mmap" - set to "yes" or "no" to override the compiled-in
default to use or not use mmap device access. In
the past, some buggy alsa drivers have behaved
better when not using mmap access at the penalty
of slightly higher CPU usage.
</ul>
<p>
<hr width="50%">
<h3>arts</h3>
aRts Sound Daemon live output driver.
<p>
<b>Option keys:</b>
<ul>
<li>"multi" - set to "yes" to allow opening the aRts playback device
for multiply concurrent playback. Although the driver works properly
in multi mode, it is known to occasionally crash the aRts server
itself. Default behavior is "no".
</ul>
<p>
<hr width="50%">
<h3>esd</h3>
ESounD audio driver. Although declining in poularity, this sound
daemon is still used on some Linux systems. It permits multiple
programs to play sound simultaneously and sound to be sent to
networked computers.
<p>
<b>Option keys:</b>
<ul>
<li>"host" - Deprecated synonym for 'server' below.
<li>"server" - (see 'Standard Driver Options' above). The hostname
where esd is running. By default sound is played on the local host. A
port number can be specified after a colon, as in "whizbang.com:555".
</ul>
<p>
<hr width="50%">
<h3>irix</h3>
IRIX audio driver. This was inherited from the original
libao, but has not been tested. Use at your own risk. (Better yet,
fix it! I don't have access to an IRIX system.)
<p>
<hr width="50%">
<h3>macosx</h3>
MacOS X AUHAL live output driver. This driver supports MacOS X 10.5
and later (10.4 and earlier uses an earlier, incompatable interface).
<p>
<b>Option keys:</b>
<ul>
<li>"buffer_time" - Set the hardware buffer size to the equivalent of
value milliseconds.
</ul>
<p>
<hr width="50%">
<h3>nas</h3>
Network Audio System live output driver.
<p>
<b>Option keys:</b>
<ul>
<li>"buf_size" - Set size of audio buffer on server in bytes.
<li>"host" - Deprecated synonym for 'server' below.
<li>"server" - (see 'Standard Driver Options' above). Set location of
NAS server; See nas(1) for format.
</ul>
<p>
<hr width="50%">
<h3>null</h3>
Null driver. This is just a test device which does not write the
audio data anywhere.
<p>
<hr width="50%">
<h3>oss</h3>
Open Sound System driver. This is the audio system for
older Linux and FreeBSD as well as some other UNIX-like systems.
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above). By default, the driver tries
"/dev/sound/dsp", followed by "/dev/dsp".
<li>"dsp" - Deprecated synonym for "dev".
<li>"id" - (see 'Standard Driver Options' above).
</ul>
<p>
<hr width="50%">
<h3>pulse</h3>
PulseAudio live audio sound driver. Pulse is a sound server daemon
used by the modern Gnome desktop on UNIX-like systems.
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above). This maps to a specific Pulse sink; it may be specified by Pulse sink name, or by number.
<li>"id" - (see 'Standard Driver Options' above). Maps to a specific pulse sink number.
<li>"server" - Specifies Pulseaudio server to use.
<li>"sink" - Deprecated synonym for "dev".
</ul>
<p>
<hr width="50%">
<h3>roar</h3>
Roar Audio live audio sound driver.
<p>
<b>Option keys:</b>
<ul>
<li>"server" - Specifies Roar server to use.
<li>"host" - Specifies Roar server to use (legacy synonym for 'server').
<li>"id" - Selects audio device to use for playback by number. For a roar server, this is mapped to a given output mixer.
<li>"dev" - Selects audio device to use for playback by name. For a roar server, this is a number and thus synonymous with 'id' above.
<li>"client_name" - descriptive name for this client (eg, the application name).
<li>"role" - stream type as enumerated by Roar; see RoarAudio documentation.
</ul>
<p>
<hr width="50%">
<h3>sndio</h3>
SNDIO is the modern audio interface used by OpenBSD.
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above).
<li>"id" - (see 'Standard Driver Options' above).
</ul>
<p>
<hr width="50%">
<h3>sun</h3>
Sun audio driver. This is the audio system for NetBSD, OpenBSD, and
Solaris.
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above). By default, the
driver tries "/dev/sound/0"
<li>"id" - (see 'Standard Driver Options' above).
</ul>
<p>
<hr width="50%">
<h3>wmm</h3>
Windows MMSound output driver for Win98 and later.
<p>
<b>Option keys:</b>
<ul>
<li>"dev" - (see 'Standard Driver Options' above).
<li>"id" - (see 'Standard Driver Options' above).
</ul>
<p>
<hr>
<h2>File Output Drivers</h2>
<h3>au</h3>
Sun audio file driver. Writes a .au file from audio output. This
driver can write usable data to unseekable files (like standard out),
which the wav driver cannot do.
<p>
<hr width="50%">
<h3>raw</h3>
Raw sample driver. Writes the sound to disk in uncompressed,
headerless form using the byte order specified.
<p>
<b>Option keys:</b>
<ul>
<li>"byteorder" - Byte order used in the output. Use "native" for
native machine byte order, "big" for big-endian order, and "little"
for little-endian order. By default this is "native".
</ul>
<p>
<hr width="50%">
<h3>wav</h3>
Windows 'WAV' sound file output. Because of the way WAV files are
structured, this driver cannot correct files unless the target file is
seekable. Writing WAVs to stdout will result in broken files. Use
either the raw or the au driver instead.
<p>
<hr>
<a name="default_driver">
<h2>Default Driver Detection</h2>
In the absence of <a href="config.html">configuration files</a> to
explicit identify a default driver, the library will try to detect a
suitable default driver. It does this by testing every available live
output driver (using <a
href="ao_plugin_test.html">ao_plugin_test()</a>) and finding the
driver with the highest priority (see the <a
href="ao_info.html">ao_info</a> struct) that works. Drivers with
priority 0, such as the null and file output drivers, are never
selected as the default.
<p>
The ranking system currently used is:
<center>
<table border="1">
<tr><th>Priority</th><th>Drivers</th> </tr>
<tr><td>50</td> <td>pulse, roar</td> </tr>
<tr><td>45</td> <td>arts*</td> </tr>
<tr><td>40</td> <td>esd</td> </tr>
<tr><td>35</td> <td>alsa</td> </tr>
<tr><td>30</td> <td>macosx, sndio </td> </tr>
<tr><td>20</td> <td>aixs, oss, irix, sun, wmm</td> </tr>
<tr><td>15</td> <td>arts*</td> </tr>
<tr><td>10</td> <td>nas</td> </tr>
<tr bgcolor="#888888"><td> 0</td> <td>null, all file output</td></tr>
</table>
</center>
<p>
<i>*arts priority depends on whether or not the arts install was built
with <tt>HAVE_ARTS_SUSPENDED</tt>; when present, the default aRts
priority is 45, else it is 15.</i>
<p>Clearly, any ranking scheme will fail to make everybody happy. For
such cases, the <a href="config.html">configuration files</a> can be
easily used to define an appropriate default output device.
<p>Adding <tt>debug</tt> to the libao <a
href="config.html">configuration file</a> on a line by itself will
cause libao to print what static and dynamic drivers are available for
use, as well as print the testing order.
<br><br>
<hr noshade>
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<tr valign=top>
<td><p class=tiny>copyright © 2001-2003 Stan Seibert, 2010-2011 Monty</p></td>
<td align=right><p class=tiny><a href="http://www.xiph.org/">xiph.org</a><br><a href="mailto:monty@xiph.org">monty@xiph.org</a></p></td>
</tr><tr>
<td><p class=tiny>libao documentation</p></td>
<td align=right><p class=tiny>libao version 1.1.0 - 20110221</p></td>
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