libao Drivers

Libao supports both live output drivers and file 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.

Driver options may be passed to the drivers via the *options argument to ao_open_live() and ao_open_file(), or they may be set in the configuration file as name=value pairs.

Options Understood by All Drivers

• "debug" - (value not required) Requests driver print detailed debugging information.
• "matrix" - Set an output channel mapping similar to the use of the matrix field in ao_sample_format. 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.
• "quiet" - (value not required) Requests the driver print no output whatsoever, even in the event of error.
• "verbose" - (value not required) Requests that the driver print more detailed information concerning normal operation.

Standard Driver Options

• "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'.
• "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').
• "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.
• "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.

Live Output Drivers

aixs

Option keys:

• "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.
• "id" - (see 'Standard Driver Options' above).

alsa

Option keys:

• "buffer_time" - Override the default hardware buffer size (in milliseconds).
• "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.
• "id" - (see 'Standard Driver Options' above).
• "period_time" - Override the default hardware period size (in microseconds).
• "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.

arts

Option keys:

• "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".

esd

Option keys:

• "host" - Deprecated synonym for 'server' below.
• "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".

irix

macosx

Option keys:

• "buffer_time" - Set the hardware buffer size to the equivalent of value milliseconds.

nas

Option keys:

• "buf_size" - Set size of audio buffer on server in bytes.
• "host" - Deprecated synonym for 'server' below.
• "server" - (see 'Standard Driver Options' above). Set location of NAS server; See nas(1) for format.

null

oss

Option keys:

• "dev" - (see 'Standard Driver Options' above). By default, the driver tries "/dev/sound/dsp", followed by "/dev/dsp".
• "dsp" - Deprecated synonym for "dev".
• "id" - (see 'Standard Driver Options' above).

pulse

Option keys:

• "dev" - (see 'Standard Driver Options' above). This maps to a specific Pulse sink; it may be specified by Pulse sink name, or by number.
• "id" - (see 'Standard Driver Options' above). Maps to a specific pulse sink number.
• "server" - Specifies Pulseaudio server to use.
• "sink" - Deprecated synonym for "dev".

roar

Option keys:

• "server" - Specifies Roar server to use.
• "host" - Specifies Roar server to use (legacy synonym for 'server').
• "id" - Selects audio device to use for playback by number. For a roar server, this is mapped to a given output mixer.
• "dev" - Selects audio device to use for playback by name. For a roar server, this is a number and thus synonymous with 'id' above.
• "client_name" - descriptive name for this client (eg, the application name).
• "role" - stream type as enumerated by Roar; see RoarAudio documentation.

sndio

Option keys:

• "dev" - (see 'Standard Driver Options' above).
• "id" - (see 'Standard Driver Options' above).

sun

Option keys:

• "dev" - (see 'Standard Driver Options' above). By default, the driver tries "/dev/sound/0"
• "id" - (see 'Standard Driver Options' above).

wmm

Option keys:

• "dev" - (see 'Standard Driver Options' above).
• "id" - (see 'Standard Driver Options' above).

File Output Drivers

au

raw

Option keys:

• "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".

wav

The ranking system currently used is:

*arts priority depends on whether or not the arts install was built with HAVE_ARTS_SUSPENDED; when present, the default aRts priority is 45, else it is 15.

Clearly, any ranking scheme will fail to make everybody happy. For such cases, the configuration files can be easily used to define an appropriate default output device.

Adding debug to the libao configuration file 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.