Before you can write to or read from an audio device, you must call
three methods in the correct order:
setfmt() to set the output format
channels() to set the number of channels
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:
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.
fileno(
)
Return the file descriptor associated with the 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.
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().
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.
nonblock(
)
Put the device into non-blocking mode. Once in non-blocking mode, there
is no way to return it to blocking mode.
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.
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.
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.
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)
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().
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().
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.
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()).
