Platforms: Linux, FreeBSD
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
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:
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.)
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.
Before you can write to or read from an audio device, you must call three methods in the correct order:
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:
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.
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.
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) |
The following convenience methods combine several ioctls, or one ioctl and some simple calculations.
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(channels)
Audio device objects also support several read-only attributes:
The mixer object provides two file-like methods:
The remaining methods are specific to audio mixing:
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.
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.
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.
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.
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)