Doc/lib/libossaudiodev.tex

\section{\module{ossaudiodev} ---
         Access to OSS-compatible audio devices}

\declaremodule{builtin}{ossaudiodev}
\platform{Linux, FreeBSD, possibly other \UNIX-like systems}
\modulesynopsis{Access to OSS-compatible audio devices.}

This module allows you to access the OSS (Open Sound System) audio
interface.  OSS is available for a wide range of open-source and
commercial Unices, and is the standard audio interface for Linux and
recent versions of FreeBSD.

% Things will get more complicated for future Linux versions, since
% ALSA is in the standard kernel as of 2.5.x.  Presumably if you
% use ALSA, you'll have to make sure its OSS compatibility layer
% is active to use ossaudiodev, but you're gonna need it for the vast
% majority of Linux audio apps anyways.  
%
% Sounds like things are also complicated for other BSDs.  In response
% to my python-dev query, Thomas Wouters said:
%
% > Likewise, googling shows OpenBSD also uses OSS/Free -- the commercial
% > OSS installation manual tells you to remove references to OSS/Free from the
% > kernel :)
%
% but Aleksander Piotrowsk actually has an OpenBSD box, and he quotes
% from its <soundcard.h>:
% >  * WARNING!  WARNING!
% >  * This is an OSS (Linux) audio emulator.
% >  * Use the Native NetBSD API for developing new code, and this
% >  * only for compiling Linux programs.
%
% There's also an ossaudio manpage on OpenBSD that explains things
% further.  Presumably NetBSD and OpenBSD have a different standard
% audio interface.  That's the great thing about standards, there are so
% many to choose from ... ;-)  
%
% This probably all warrants a footnote or two, but I don't understand
% things well enough right now to write it!   --GPW

\begin{seealso}
\seetitle[http://www.opensound.com/pguide/oss.pdf]
         {Open Sound System Programmer's Guide} {the official
         documentation for the OSS C API}
\seetext{The module defines a large number of constants supplied by
         the OSS device driver; see \file{<sys/soundcard.h>} on either
         Linux or FreeBSD for a listing .}
\end{seealso}

\module{ossaudiodev} defines the following variables and functions:

\begin{excdesc}{error}
This exception is raised on certain errors.  The argument is a string
describing what went wrong.

(If \module{ossaudiodev} receives an error from a system call such as
\cfunction{open()}, \cfunction{write()}, or \cfunction{ioctl()}, it
raises \exception{IOError}.  Errors detected directly by
\module{ossaudiodev} result in \exception{ossaudiodev.error}.)
\end{excdesc}

\begin{funcdesc}{open}{\optional{device, }mode}
Open an audio device and return an OSS audio device object.  This
object supports many file-like methods, such as \method{read()},
\method{write()}, and \method{fileno()} (although there are subtle
differences between conventional Unix read/write semantics and those of
OSS audio devices).  It also supports a number of audio-specific
methods; see below for the complete list of methods.

Note the unusual calling syntax: the \emph{first} argument is optional,
and the second is required.  This is a historical artifact for
compatibility with the older \module{linuxaudiodev} module which
\module{ossaudiodev} supersedes.  % XXX it might also be motivated
% by my unfounded-but-still-possibly-true belief that the default
% audio device varies unpredictably across operating systems.  -GW

\var{device} is the audio device filename to use.  If it is not
specified, this module first looks in the environment variable
\envvar{AUDIODEV} for a device to use.  If not found, it falls back to
\file{/dev/dsp}.

\var{mode} is one of \code{'r'} for read-only (record) access,
\code{'w'} for write-only (playback) access and \code{'rw'} for both.
Since many soundcards only allow one process to have the recorder or
player open at a time it is a good idea to open the device only for the
activity needed.  Further, some soundcards are half-duplex: they can be
opened for reading or writing, but not both at once.
\end{funcdesc}

\begin{funcdesc}{openmixer}{\optional{device}}
Open a mixer device and return an OSS mixer device object.  
\var{device} is the mixer device filename to use.  If it is
not specified, this module first looks in the environment variable
\envvar{MIXERDEV} for a device to use.  If not found, it falls back to
\file{/dev/mixer}.

\end{funcdesc}

\subsection{Audio Device Objects \label{ossaudio-device-objects}}

Setting up the device

To set up the device, three functions must be called in the correct
sequence:
\begin{enumerate}
\item \method{setfmt()} to set the output format,
\item \method{channels()} to set the number of channels, and
\item \method{speed()} to set the sample rate.
\end{enumerate}

The audio device objects are returned by \function{open()} define the
following methods:

\begin{methoddesc}[audio device]{close}{}
This method explicitly closes the device.  It is useful in situations
where deleting the object does not immediately close it since there are
other references to it.  A closed device should not be used again.
\end{methoddesc}

\begin{methoddesc}[audio device]{fileno}{}
Returns the file descriptor associated with the device.
\end{methoddesc}

\begin{methoddesc}[audio device]{read}{size}
Reads \var{size} samples from the audio input and returns them as a
Python string.  The function blocks until enough data is available.
\end{methoddesc}

\begin{methoddesc}[audio device]{write}{data}
Writes Python string \var{data} to the audio device and returns the
number of bytes written.  If the audio device is opened in blocking
mode, the entire string is always written.  If the device is opened in
nonblocking mode, some data may not be written---see
\method{writeall()}.
\end{methoddesc}

\begin{methoddesc}[audio device]{writeall}{data}
Writes the entire Python string \var{data} to the audio device.  If the
device is opened in blocking mode, behaves identially to
\method{write()}; in nonblocking mode, waits until the device becomes
available before feeding it more data.  Returns \code{None}, since the
amount of data written is always equal to the amount of data supplied.
\end{methoddesc}

Simple IOCTLs:

\begin{methoddesc}[audio device]{nonblock}{}
Attempts to put the device into nonblocking mode.  Once in nonblocking
mode there is no way to return to blocking mode.

Raises \exception{IOError} if the IOCTL failed.
\end{methoddesc}

\begin{methoddesc}[audio device]{getfmts}{}
Returns a bitmask of the audio output formats supported by the
soundcard.  On a typical Linux system, these formats are:

\begin{tableii}{l|l}{constant}{Format}{Description}
\lineii{AFMT_MU_LAW}
       {a logarithmic encoding.  This is the default format on
        \file{/dev/audio} and is the format used by Sun .au files.}
\lineii{AFMT_A_LAW}
       {a logarithmic encoding}
\lineii{AFMT_IMA_ADPCM}
       {a 4:1 compressed format defined by the Interactive Multimedia
        Association.} 
\lineii{AFMT_U8}
       {Unsigned, 8-bit audio.}
\lineii{AFMT_S16_LE}
       {Unsigned, 16-bit audio, little-endian byte order (as used by
        Intel processors)}
\lineii{AFMT_S16_BE}
       {Unsigned, 16-bit audio, big-endian byte order (as used by 68k,
        PowerPC, Sparc)}
\lineii{AFMT_S8}
       {Signed, 8 bit audio.}
\lineii{AFMT_U16_LE}
       {Signed, 16-bit little-endian audio}
\lineii{AFMT_U16_BE}
       {Signed, 16-bit big-endian audio}
\end{tableii}
Most systems support only a subset of these formats.  Many devices only
support \constant{AFMT_U8}; the most common format used today is
\constant{AFMT_S16_LE}.
\end{methoddesc}

\begin{methoddesc}[audio device]{setfmt}{format}
Used to set the current audio format to \var{format}---see
\method{getfmts()} for a list.  May also be used to return the current
audio format---do this by passing an ``audio format'' of
\constant{AFMT_QUERY}. Returns the audio format that the device was set
to, which may not be the requested format.
\end{methoddesc}

\begin{methoddesc}[audio device]{channels}{num_channels}
Sets the number of output channels to \var{num_channels}.  A value of 1
indicates monophonic sound, 2 stereophonic.  Some devices may have more
than 2 channels, and some high-end devices may not support mono.
Returns the number of channels the device was set to.
\end{methoddesc}

\begin{methoddesc}[audio device]{speed}{samplerate}
Sets the samplerate to \var{samplerate} samples per second and returns
the rate actually set.  Most sound devices don't support arbitrary
sample rates.  Common rates are:

8000---default rate
11025---speech recording
22050
44100---Audio CD-quality (at 16 bits/sample and 2 channels)
96000---DVD-quality
\end{methoddesc}

\begin{methoddesc}[audio device]{sync}
Waits until the sound device has played every byte in its buffer and
returns.  This also occurs when the sound device is closed.  The OSS
documentation recommends simply closing and re-opening the device rather
than using \method{sync()}.
\end{methoddesc}

\begin{methoddesc}[audio device]{reset}
Immediately stops and playing or recording and returns the device to a
state where it can accept commands.  The OSS documentation recommends
closing and re-opening the device after calling \method{reset()}.
\end{methoddesc}

\begin{methoddesc}[audio device]{post}
To be used like a lightweight \method{sync()}, the \method{post()}
IOCTL informs the audio device that there is a likely to be a pause in
the audio output---i.e., after playing a spot sound effect, before
waiting for user input, or before doing disk I/O.
\end{methoddesc}

Convenience methods

\begin{methoddesc}[audio device]{setparameters}{samplerate,num_channels,format,emulate}
Initialise the sound device in one method.  \var{samplerate},
\var{channels} and \var{format} should be as specified in the
\method{speed()}, \method{channels()} and \method{setfmt()}
methods.  If \var{emulate} is true, attempt to find the closest matching
format instead, otherwise raise ValueError if the device does not
support the format.  The default is to raise ValueError on unsupported
formats.
\end{methoddesc}

\begin{methoddesc}[audio device]{bufsize}{}
Returns the size of the hardware buffer, in samples.
\end{methoddesc}

\begin{methoddesc}[audio device]{obufcount}{}
Returns the number of samples that are in the hardware buffer yet to be
played.
\end{methoddesc}

\begin{methoddesc}[audio device]{obuffree}{}
Returns the number of samples that could be queued into the hardware
buffer to be played without blocking.
\end{methoddesc}

\subsection{Mixer Device Objects \label{mixer-device-objects}}

File-like interface

\begin{methoddesc}[mixer device]{close}{}
This method closes the open mixer device file.  Any further attempts to
use the mixer after this file is closed will raise an IOError.
\end{methoddesc}

\begin{methoddesc}[mixer device]{fileno}{}
Returns the file handle number of the open mixer device file.
\end{methoddesc}

Mixer interface

\begin{methoddesc}[mixer device]{controls}{}
This method returns a bitmask specifying the available mixer controls
(``Control'' being a specific mixable ``channel'', such as
\constant{SOUND_MIXER_PCM} or \constant{SOUND_MIXER_SYNTH}).  This
bitmask indicates a subset of all available mixer channels---the
\constant{SOUND_MIXER_*} constants defined at module level.  To determine if,
for example, the current mixer object supports a PCM mixer, use the
following Python code:

\begin{verbatim}
mixer=ossaudiodev.openmixer()
if mixer.channels() & (1 << ossaudiodev.SOUND_MIXER_PCM):
	# PCM is supported
	<code>
\end{verbatim}

For most purposes, the \constant{SOUND_MIXER_VOLUME} (Master volume) and
\constant{SOUND_MIXER_PCM} channels should suffice---but code that uses the
mixer should be flexible when it comes to choosing sound channels.  On
the Gravis Ultrasound, for example, \constant{SOUND_MIXER_VOLUME} does not
exist.
\end{methoddesc}

\begin{methoddesc}[mixer device]{stereocontrols}{}
Returns a bitmask indicating stereo mixer channels.  If a bit is set,
the corresponding channel is stereo; if it is unset, the channel is
either monophonic or not supported by the mixer (use in combination with
\method{channels()} to determine which).

See the code example for the \method{channels()} function for an example
of getting data from a bitmask.
\end{methoddesc}

\begin{methoddesc}[mixer device]{reccontrols}{}
Returns a bitmask specifying the mixer controls that may be used to
record.  See the code example for \method{controls()} for an example of
reading from a bitmask.
\end{methoddesc}

\begin{methoddesc}[mixer device]{get}{channel}
Returns the volume of a given mixer channel.  The returned volume is a
2-tuple \code{(left_volume,right_volume)}.  Volumes are specified as
numbers from 0 (silent) to 100 (full volume).  If the channel is
monophonic, a 2-tuple is still returned, but both channel volumes are
the same.

If an unknown channel is specified, \exception{error} is raised.
\end{methoddesc}

\begin{methoddesc}[mixer device]{set}{channel, (left, right)}
Sets the volume for a given mixer channel to \code{(left,right)}.
\code{left} and \code{right} must be ints and between 0 (silent) and 100
(full volume).  On success, the new volume is returned as a 2-tuple.
Note that this may not be exactly the same as the volume specified,
because of the limited resolution of some soundcard's mixers.

Raises \exception{IOError} if an invalid mixer channel was specified;
\exception{TypeError} if the argument format was incorrect, and
\exception{error} if the specified volumes were out-of-range.
\end{methoddesc}

\begin{methoddesc}[mixer device]{get_recsrc}{}
This method returns a bitmask indicating which channel or channels are
currently being used as a recording source.
\end{methoddesc}

\begin{methoddesc}[mixer device]{set_recsrc}{bitmask}
Call this function to specify a recording source.  Returns a bitmask
indicating the new recording source (or sources) if successful; raises
\exception{IOError} if an invalid source was specified.  To set the current
recording source to the microphone input:

\begin{verbatim}
mixer.setrecsrc (1 << ossaudiodev.SOUND_MIXER_MIC)
\end{verbatim}
\end{methoddesc}