22

✏️ Editing: ossaudiodev.rst.txt

:mod:`ossaudiodev` --- Access to OSS-compatible audio devices
=============================================================

.. module:: ossaudiodev
   :platform: Linux, FreeBSD
   :synopsis: Access to OSS-compatible audio devices.

.. versionadded:: 2.3

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 anyway.

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

.. seealso::

`Open Sound System Programmer's Guide <http://www.opensound.com/pguide/oss.pdf>`_
      the official documentation for the OSS C API

The module defines a large number of constants supplied by the OSS device
   driver; see ``<sys/soundcard.h>`` on either Linux or FreeBSD for a listing.

:mod:`ossaudiodev` defines the following variables and functions:

.. exception:: OSSAudioError

This exception is raised on certain errors.  The argument is a string describing
   what went wrong.

(If :mod:`ossaudiodev` receives an error from a system call such as
   :c:func:`open`, :c:func:`write`, or :c:func:`ioctl`, it raises :exc:`IOError`.
   Errors detected directly by :mod:`ossaudiodev` result in :exc:`OSSAudioError`.)

(For backwards compatibility, the exception class is also available as
   ``ossaudiodev.error``.)

.. function:: open(mode)
              open(device, mode)

Open an audio device and return an OSS audio device object.  This object
   supports many file-like methods, such as :meth:`read`, :meth:`write`, and
   :meth:`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.

*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`.

*mode* is one of ``'r'`` for read-only (record) access, ``'w'`` for
   write-only (playback) access and ``'rw'`` for both. Since many sound cards
   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
   sound cards are half-duplex: they can be opened for reading or writing, but
   not both at once.

Note the unusual calling syntax: the *first* argument is optional, and the
   second is required.  This is a historical artifact for compatibility with the
   older :mod:`linuxaudiodev` module which :mod:`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

.. function:: openmixer([device])

Open a mixer device and return an OSS mixer device object.   *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`.

.. _ossaudio-device-objects:

Audio Device Objects
--------------------

Before you can write to or read from an audio device, you must call three
methods in the correct order:

#. :meth:`setfmt` to set the output format

#. :meth:`channels` to set the number of channels

#. :meth:`speed` to set the sample rate

Alternately, you can use the :meth:`setparameters` method to set all three audio
parameters at once.  This is more convenient, but may not be as flexible in all
cases.

The audio device objects returned by :func:`.open` define the following methods
and (read-only) attributes:

.. method:: oss_audio_device.close()

Explicitly close the audio device.  When you are done writing to or reading from
   an audio device, you should explicitly close it.  A closed device cannot be used
   again.

.. method:: oss_audio_device.fileno()

Return the file descriptor associated with the device.

.. method:: oss_audio_device.read(size)

Read *size* bytes from the audio input and return them as a Python string.
   Unlike most Unix device drivers, OSS audio devices in blocking mode (the
   default) will block :func:`read` until the entire requested amount of data is
   available.

.. method:: oss_audio_device.write(data)

Write the Python string *data* to the audio device and return the number of
   bytes written.  If the audio device is in blocking mode (the default), the
   entire string is always written (again, this is different from usual Unix device
   semantics).  If the device is in non-blocking mode, some data may not be written
   ---see :meth:`writeall`.

.. method:: oss_audio_device.writeall(data)

Write the entire Python string *data* to the audio device: waits until the audio
   device is able to accept data, writes as much data as it will accept, and
   repeats until *data* has been completely written. If the device is in blocking
   mode (the default), this has the same effect as :meth:`write`; :meth:`writeall`
   is only useful in non-blocking mode.  Has no return value, since the amount of
   data written is always equal to the amount of data supplied.

The following methods each map to exactly one :c:func:`ioctl` system call.  The
correspondence is obvious: for example, :meth:`setfmt` corresponds to the
``SNDCTL_DSP_SETFMT`` ioctl, and :meth:`sync` to ``SNDCTL_DSP_SYNC`` (this can
be useful when consulting the OSS documentation).  If the underlying
:c:func:`ioctl` fails, they all raise :exc:`IOError`.

.. method:: oss_audio_device.nonblock()

Put the device into non-blocking mode.  Once in non-blocking mode, there is no
   way to return it to blocking mode.

.. method:: oss_audio_device.getfmts()

Return a bitmask of the audio output formats supported by the soundcard.  Some
   of the formats supported by OSS are:

Consult the OSS documentation for a full list of audio formats, and note that
   most devices support only a subset of these formats.  Some older devices only
   support :const:`AFMT_U8`; the most common format used today is
   :const:`AFMT_S16_LE`.

.. method:: oss_audio_device.setfmt(format)

Try to set the current audio format to *format*---see :meth:`getfmts` for a
   list.  Returns the audio format that the device was set to, which may not be the
   requested format.  May also be used to return the current audio format---do this
   by passing an "audio format" of :const:`AFMT_QUERY`.

.. method:: oss_audio_device.channels(nchannels)

Set the number of output channels to *nchannels*.  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.

.. method:: oss_audio_device.speed(samplerate)

Try to set the audio sampling rate to *samplerate* samples per second.  Returns
   the rate actually set.  Most sound devices don't support arbitrary sampling
   rates.  Common rates are:

+-------+-------------------------------------------+
   | Rate  | Description                               |
   +=======+===========================================+
   | 8000  | default rate for :file:`/dev/audio`       |
   +-------+-------------------------------------------+
   | 11025 | speech recording                          |
   +-------+-------------------------------------------+
   | 22050 |                                           |
   +-------+-------------------------------------------+
   | 44100 | CD quality audio (at 16 bits/sample and 2 |
   |       | channels)                                 |
   +-------+-------------------------------------------+
   | 96000 | DVD quality audio (at 24 bits/sample)     |
   +-------+-------------------------------------------+

.. method:: oss_audio_device.sync()

Wait until the sound device has played every byte in its buffer.  (This happens
   implicitly when the device is closed.)  The OSS documentation recommends closing
   and re-opening the device rather than using :meth:`sync`.

.. method:: oss_audio_device.reset()

Immediately stop playing or recording and return the device to a state where it
   can accept commands.  The OSS documentation recommends closing and re-opening
   the device after calling :meth:`reset`.

.. method:: oss_audio_device.post()

Tell the driver that there is likely to be a pause in the output, making it
   possible for the device to handle the pause more intelligently.  You might use
   this after playing a spot sound effect, before waiting for user input, or before
   doing disk I/O.

The following convenience methods combine several ioctls, or one ioctl and some
simple calculations.

.. method:: oss_audio_device.setparameters(format, nchannels, samplerate[, strict=False])

Set the key audio sampling parameters---sample format, number of channels, and
   sampling rate---in one method call.  *format*,  *nchannels*, and *samplerate*
   should be as specified in the :meth:`setfmt`, :meth:`channels`, and
   :meth:`speed`  methods.  If *strict* is true, :meth:`setparameters` checks to
   see if each parameter was actually set to the requested value, and raises
   :exc:`OSSAudioError` if not.  Returns a tuple (*format*, *nchannels*,
   *samplerate*) indicating the parameter values that were actually set by the
   device driver (i.e., the same as the return values of :meth:`setfmt`,
   :meth:`channels`, and :meth:`speed`).

For example,  ::

(fmt, channels, rate) = dsp.setparameters(fmt, channels, rate)

is equivalent to  ::

fmt = dsp.setfmt(fmt)
      channels = dsp.channels(channels)
      rate = dsp.rate(rate)

.. method:: oss_audio_device.bufsize()

Returns the size of the hardware buffer, in samples.

.. method:: oss_audio_device.obufcount()

Returns the number of samples that are in the hardware buffer yet to be played.

.. method:: oss_audio_device.obuffree()

Returns the number of samples that could be queued into the hardware buffer to
   be played without blocking.

Audio device objects also support several read-only attributes:

.. attribute:: oss_audio_device.closed

Boolean indicating whether the device has been closed.

.. attribute:: oss_audio_device.name

String containing the name of the device file.

.. attribute:: oss_audio_device.mode

The I/O mode for the file, either ``"r"``, ``"rw"``, or ``"w"``.

.. _mixer-device-objects:

Mixer Device Objects
--------------------

The mixer object provides two file-like methods:

.. method:: oss_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 :exc:`IOError`.

.. method:: oss_mixer_device.fileno()

Returns the file handle number of the open mixer device file.

The remaining methods are specific to audio mixing:

.. method:: oss_mixer_device.controls()

This method returns a bitmask specifying the available mixer controls ("Control"
   being a specific mixable "channel", such as :const:`SOUND_MIXER_PCM` or
   :const:`SOUND_MIXER_SYNTH`).  This bitmask indicates a subset of all available
   mixer controls---the :const:`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::

mixer=ossaudiodev.openmixer()
      if mixer.controls() & (1 << ossaudiodev.SOUND_MIXER_PCM):
          # PCM is supported
          ... code ...

For most purposes, the :const:`SOUND_MIXER_VOLUME` (master volume) and
   :const:`SOUND_MIXER_PCM` controls should suffice---but code that uses the mixer
   should be flexible when it comes to choosing mixer controls.  On the Gravis
   Ultrasound, for example, :const:`SOUND_MIXER_VOLUME` does not exist.

.. method:: oss_mixer_device.stereocontrols()

Returns a bitmask indicating stereo mixer controls.  If a bit is set, the
   corresponding control is stereo; if it is unset, the control is either
   monophonic or not supported by the mixer (use in combination with
   :meth:`controls` to determine which).

See the code example for the :meth:`controls` function for an example of getting
   data from a bitmask.

.. method:: oss_mixer_device.reccontrols()

Returns a bitmask specifying the mixer controls that may be used to record.  See
   the code example for :meth:`controls` for an example of reading from a bitmask.

.. method:: oss_mixer_device.get(control)

Returns the volume of a given mixer control.  The returned volume is a 2-tuple
   ``(left_volume,right_volume)``.  Volumes are specified as numbers from 0
   (silent) to 100 (full volume).  If the control is monophonic, a 2-tuple is still
   returned, but both volumes are the same.

Raises :exc:`OSSAudioError` if an invalid control was is specified, or
   :exc:`IOError` if an unsupported control is specified.

.. method:: oss_mixer_device.set(control, (left, right))

Sets the volume for a given mixer control to ``(left,right)``. ``left`` and
   ``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 :exc:`OSSAudioError` if an invalid mixer control was specified, or if the
   specified volumes were out-of-range.

.. method:: oss_mixer_device.get_recsrc()

This method returns a bitmask indicating which control(s) are currently being
   used as a recording source.

.. method:: oss_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 :exc:`IOError` if an
   invalid source was specified.  To set the current recording source to the
   microphone input::

mixer.setrecsrc (1 << ossaudiodev.SOUND_MIXER_MIC)

MR SKB