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# 21.10. `ossaudiodev` — Access to OSS-compatible audio devices

New in version 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.

See also

Open Sound System Programmer’s Guide

  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.

`ossaudiodev` defines the following variables and functions:

exception `ossaudiodev.``OSSAudioError`

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

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

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

`ossaudiodev.``open`(mode)
`ossaudiodev.``open`(device, mode)

  Open an audio device and return an OSS audio device object. This object supports many file-like methods, such as `read()`, `write()`, and `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 `AUDIODEV` for a device to use. If not found, it falls back to `/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 `linuxaudiodev` module which `ossaudiodev` supersedes.

`ossaudiodev.``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 `MIXERDEV` for a device to use. If not found, it falls back to `/dev/mixer`.

## 21.10.1. Audio Device Objects

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

1. `setfmt()` to set the output format

2. `channels()` to set the number of channels

3. `speed()` to set the sample rate

Alternately, you can use the `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 `open()` define the following methods and (read-only) attributes:

`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.

`oss_audio_device.``fileno`()

  Return the file descriptor associated with the device.

`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 `read()` until the entire requested amount of data is available.

`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 `writeall()`.

`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 `write()`; `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 `ioctl()` system call. The correspondence is obvious: for example, `setfmt()` corresponds to the `SNDCTL_DSP_SETFMT` ioctl, and `sync()` to `SNDCTL_DSP_SYNC` (this can be useful when consulting the OSS documentation). If the underlying `ioctl()` fails, they all raise `IOError`.

`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.

`oss_audio_device.``getfmts`()

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

  Format | Description
  --- | ---
  `AFMT_MU_LAW` | a logarithmic encoding (used by Sun `.au` files and `/dev/audio`)
  `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` | Signed, 16-bit audio, little-endian byte order (as used by Intel processors)
  `AFMT_S16_BE` | Signed, 16-bit audio, big-endian byte order (as used by 68k, PowerPC, Sparc)
  `AFMT_S8` | Signed, 8 bit audio
  `AFMT_U16_LE` | Unsigned, 16-bit little-endian audio
  `AFMT_U16_BE` | Unsigned, 16-bit big-endian audio

  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 `AFMT_U8`; the most common format used today is `AFMT_S16_LE`.

`oss_audio_device.``setfmt`(format)

  Try to set the current audio format to format—see `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 `AFMT_QUERY`.

`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.

`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 `/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)

`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 `sync()`.

`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 `reset()`.

`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.

`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 `setfmt()`, `channels()`, and `speed()` methods. If strict is true, `setparameters()` checks to see if each parameter was actually set to the requested value, and raises `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 `setfmt()`, `channels()`, and `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)

`oss_audio_device.``bufsize`()

  Returns the size of the hardware buffer, in samples.

`oss_audio_device.``obufcount`()

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

`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:

`oss_audio_device.``closed`

  Boolean indicating whether the device has been closed.

`oss_audio_device.``name`

  String containing the name of the device file.

`oss_audio_device.``mode`

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

## 21.10.2. Mixer Device Objects

The mixer object provides two file-like methods:

`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 `IOError`.

`oss_mixer_device.``fileno`()

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

The remaining methods are specific to audio mixing:

`oss_mixer_device.``controls`()

  This method returns a bitmask specifying the available mixer controls (“Control” being a specific mixable “channel”, such as `SOUND_MIXER_PCM` or `SOUND_MIXER_SYNTH`). This bitmask indicates a subset of all available mixer controls—the `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 `SOUND_MIXER_VOLUME` (master volume) and `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, `SOUND_MIXER_VOLUME` does not exist.

`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 `controls()` to determine which).

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

`oss_mixer_device.``reccontrols`()

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

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

`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 `OSSAudioError` if an invalid mixer control was specified, or if the specified volumes were out-of-range.

`oss_mixer_device.``get_recsrc`()

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

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

      mixer.setrecsrc (1 << ossaudiodev.SOUND_MIXER_MIC)