22.9. "ossaudiodev"  --- 访问兼容OSS的音频设备
**********************************************

======================================================================

该模块允许您访问 OSS（开放式音响系统）音频接口。 OSS 可用于广泛的开源
和商业 Unices，并且是 Linux 和最新版本的 FreeBSD 的标准音频接口。

在 3.3 版更改: 此模块中过去会引发 "IOError" 的操作现在将引发 "OSError"
。

参见:

  开放之声系统程序员手册
     OSS C API 的官方文档

  该模块定义了大量由OSS设备驱动提供的常量； 请参阅
  ``<sys/soundcard.h>`` Linux 或 FreeBSD 上的列表。

"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 "OSError". 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".


22.9.1. 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 a *bytes-like object* *data* to the audio device and return
   the number of bytes written.  If the audio device is in blocking
   mode (the default), the entire data 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()".

   在 3.5 版更改: 现在支持可写的 *字节类对象*。

oss_audio_device.writeall(data)

   Write a *bytes-like object* *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.

   在 3.5 版更改: 现在支持可写的 *字节类对象*。

在 3.2 版更改: Audio device objects also support the context
management protocol, i.e. they can be used in a "with" statement.

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 "OSError".

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:

   +---------------------------+-----------------------------------------------+
   | 格式                      | 描述                                          |
   |===========================|===============================================|
   | "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:

   +---------+---------------------------------------------+
   | 采样率  | 描述                                        |
   |=========|=============================================|
   | 8000    | "/dev/audio" 的默认采样率                   |
   +---------+---------------------------------------------+
   | 11025   | 语音录音                                    |
   +---------+---------------------------------------------+
   | 22050   |                                             |
   +---------+---------------------------------------------+
   | 44100   | CD质量的音频（16位采样和2通道）             |
   +---------+---------------------------------------------+
   | 96000   | DVD质量的音频（24位采样）                   |
   +---------+---------------------------------------------+

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"".


22.9.2. 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
   "OSError".

oss_mixer_device.fileno()

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

在 3.2 版更改: Mixer objects also support the context management
protocol.

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 is specified, or
   "OSError" 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 "OSError" if an invalid source was specified.
   To set the current recording source to the microphone input:

      mixer.setrecsrc (1 << ossaudiodev.SOUND_MIXER_MIC)
