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Documentation for the ossaudiodev module. 

Initial revision supplied by Nicholas FitzRoy-Dale <wzdd@lardcave.net>
(emailed to me [gward@python.net] 2003-03-08 23:37 +1100).
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Greg Ward 2003-03-09 23:34:52 +00:00
parent 1fdb633530
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\section{\module{ossaudiodev} ---
Access to Open Sound System-compatible audio hardware}
\declaremodule{builtin}{ossaudiodev}
\platform{OSS}
\modulesynopsis{Access to OSS-compatible audio hardware.}
% I know FreeBSD uses OSS - what about Net- and Open-?
This module allows you to access the Open Sound System audio interface.
The Open Sound System interface is present on Linux and FreeBSD.
This module provides a very "bare bones" wrapper over the IOCTLs used to
access the audio hardware. The best - albeit rather daunting - way to
get a feel for the interface is from the Open Sound System official
documentation:
\url{http://www.opensound.com/pguide/oss.pdf}
The module defines a number of constants which may be used to program the
device. These constants are the same as those defined in the C include file
\code{<sys/soundcard.h>}.
\code{ossaudiodev} defines the following variables and functions:
\begin{excdesc}{error}
This exception is raised on errors. The argument is a string
describing what went wrong.
\end{excdesc}
\begin{funcdesc}{open}{\optional{device, }mode}
This function opens the audio device and returns an OSS audio device
object. This object can then be used to do I/O on. The \var{device}
parameter is the audio device filename to use. If it is not specified, this
module first looks in the environment variable \code{AUDIODEV} for a device
to use. If not found, it falls back to \file{/dev/dsp}.
The \var{mode} parameter is one of \code{'r'} for record-only access,
\code{'w'} for play-only 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\optional{, mode}}} This function
opens the mixer device and returns an OSS mixer device object. The \var{device}
parameter is the mixer device filename to use. If it is not specified, this
module first looks in the environment variable \code{MIXERDEV} for a device to
use. If not found, it falls back to \file{/dev/mixer}. You may specify
\code{'r'}, \code{'rw'} or \code{'w'} for \var{mode}; the default is \code{'r'}.
\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:
\code{setfmt} to set the output format,
\code{channels} to set the number of channels, and
\code{speed} to set the sample rate.
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 \code{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 \code{write}; in nonblocking
mode, waits until the device becomes available before feeding it more data.
Returns 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:
AFMT_MU_LAW - a logarithmic encoding. This is the default format on /dev/audio
and is the format used by Sun .au files.
AFMT_A_LAW - a logarithmic encoding
AFMT_IMA_ADPCM - a 4:1 compressed format defined by the Interactive Multimedia
Association.
AFMT_U8 - Unsigned, 8-bit audio.
AFMT_S16_LE - Unsigned, 16-bit audio, little-endian byte order (as used by Intel
processors)
AFMT_S16_BE - Unsigned, 16-bit audio, big-endian byte order (as used by 68k,
PowerPC, Sparc)
AFMT_S8 - Signed, 8 bit audio.
AFMT_U16_LE - Signed, 16-bit little-endian audio
AFMT_U16_BE - Signed, 16-bit big-endian audio
Most systems support only a subset of these formats. Many devices only support
AFTM_U8; the most common format used today is AFMT_S16_LE.
\end{methoddesc}
\begin{methoddesc}[audio device]{setfmt}{format}
Used to set the current audio format to \var{format} - see \code{getfmts} for a
list. May also be used to return the current audio format - do this by passing
an ``audio format'' of \code{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
\code{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 \code{reset}.
\end{methoddesc}
\begin{methoddesc}[audio device]{post}
To be used like a lightweight \code{sync}, the \code{post} IOCTL informs the
audio device that there is a likely to be a pause in the audio output - ie,
after playing a spot sound effect, before waiting for user input, or before
doing disk IO.
\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 \code{speed}, \code{channels} and
\code{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
\code{SOUND_MIXER_PCM} or \code{SOUND_MIXER_SYNTH}). This
bitmask indicates a subset of all available mixer channels - the
\code{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 \code{SOUND_MIXER_VOLUME} (Master volume) and
\code{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, \code{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
\function{channels} to determine which).
See the code example for the \function{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 \function{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 of \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}