22.3. "sunau" --- 读写 Sun AU 文件
**********************************

**源代码:** Lib/sunau.py

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

"sunau" 模拟提供了一个处理 Sun AU 声音格式的便利接口。请注意此模块与
"aifc" 和 "wave" 是兼容接口的。

音频文件由标头和数据组成。标头的字段为：

+-----------------+-------------------------------------------------+
| 域              | 目录                                            |
|=================|=================================================|
| magic word      | 四个字节 ".snd"                                 |
+-----------------+-------------------------------------------------+
| header size     | 标头的大小，包括信息，以字节为单位。            |
+-----------------+-------------------------------------------------+
| data size       | 数据的物理大小，以字节为单位。                  |
+-----------------+-------------------------------------------------+
| 编码            | 指示音频样本的编码方式。                        |
+-----------------+-------------------------------------------------+
| sample rate     | 采样率                                          |
+-----------------+-------------------------------------------------+
| # of channels   | 的通道数。                                      |
+-----------------+-------------------------------------------------+
| info            | 提供音频文件描述的ASCII字符串（用空字节填充）。 |
+-----------------+-------------------------------------------------+

Apart from the info field, all header fields are 4 bytes in size. They
are all 32-bit unsigned integers encoded in big-endian byte order.

The "sunau" module defines the following functions:

sunau.open(file, mode)

   If *file* is a string, open the file by that name, otherwise treat
   it as a seekable file-like object. *mode* can be any of

   "'r'"
      只读模式。

   "'w'"
      只写模式。

   Note that it does not allow read/write files.

   A *mode* of "'r'" returns an "AU_read" object, while a *mode* of
   "'w'" or "'wb'" returns an "AU_write" object.

sunau.openfp(file, mode)

   同 "open()"，用于向后兼容。

The "sunau" module defines the following exception:

exception sunau.Error

   An error raised when something is impossible because of Sun AU
   specs or implementation deficiency.

The "sunau" module defines the following data items:

sunau.AUDIO_FILE_MAGIC

   An integer every valid Sun AU file begins with, stored in big-
   endian form.  This is the string ".snd" interpreted as an integer.

sunau.AUDIO_FILE_ENCODING_MULAW_8
sunau.AUDIO_FILE_ENCODING_LINEAR_8
sunau.AUDIO_FILE_ENCODING_LINEAR_16
sunau.AUDIO_FILE_ENCODING_LINEAR_24
sunau.AUDIO_FILE_ENCODING_LINEAR_32
sunau.AUDIO_FILE_ENCODING_ALAW_8

   Values of the encoding field from the AU header which are supported
   by this module.

sunau.AUDIO_FILE_ENCODING_FLOAT
sunau.AUDIO_FILE_ENCODING_DOUBLE
sunau.AUDIO_FILE_ENCODING_ADPCM_G721
sunau.AUDIO_FILE_ENCODING_ADPCM_G722
sunau.AUDIO_FILE_ENCODING_ADPCM_G723_3
sunau.AUDIO_FILE_ENCODING_ADPCM_G723_5

   Additional known values of the encoding field from the AU header,
   but which are not supported by this module.


22.3.1. AU_read 对象
====================

AU_read objects, as returned by "open()" above, have the following
methods:

AU_read.close()

   Close the stream, and make the instance unusable. (This is  called
   automatically on deletion.)

AU_read.getnchannels()

   Returns number of audio channels (1 for mono, 2 for stereo).

AU_read.getsampwidth()

   返回采样字节长度。

AU_read.getframerate()

   返回采样频率。

AU_read.getnframes()

   返回音频总帧数。

AU_read.getcomptype()

   Returns compression type. Supported compression types are "'ULAW'",
   "'ALAW'" and "'NONE'".

AU_read.getcompname()

   Human-readable version of "getcomptype()".  The supported types
   have the respective names "'CCITT G.711 u-law'", "'CCITT G.711
   A-law'" and "'not compressed'".

AU_read.getparams()

   返回一个 "namedtuple()" "(nchannels, sampwidth, framerate, nframes,
   comptype, compname)"，与 "get*()" 方法的输出相同。

AU_read.readframes(n)

   Reads and returns at most *n* frames of audio, as a "bytes" object.
   The data will be returned in linear format.  If the original data
   is in u-LAW format, it will be converted.

AU_read.rewind()

   重置文件指针至音频开头.

以下两个方法都使用指针，具体实现由其底层决定。

AU_read.setpos(pos)

   Set the file pointer to the specified position.  Only values
   returned from "tell()" should be used for *pos*.

AU_read.tell()

   Return current file pointer position.  Note that the returned value
   has nothing to do with the actual position in the file.

The following two functions are defined for compatibility with the
"aifc", and don't do anything interesting.

AU_read.getmarkers()

   返回 "None"。

AU_read.getmark(id)

   引发错误异常。


22.3.2. AU_write 对象
=====================

AU_write objects, as returned by "open()" above, have the following
methods:

AU_write.setnchannels(n)

   设置声道数。

AU_write.setsampwidth(n)

   Set the sample width (in bytes.)

   3.4 版更變: Added support for 24-bit samples.

AU_write.setframerate(n)

   Set the frame rate.

AU_write.setnframes(n)

   Set the number of frames. This can be later changed, when and if
   more  frames are written.

AU_write.setcomptype(type, name)

   Set the compression type and description. Only "'NONE'" and
   "'ULAW'" are supported on output.

AU_write.setparams(tuple)

   The *tuple* should be "(nchannels, sampwidth, framerate, nframes,
   comptype, compname)", with values valid for the "set*()" methods.
   Set all parameters.

AU_write.tell()

   Return current position in the file, with the same disclaimer for
   the "AU_read.tell()" and "AU_read.setpos()" methods.

AU_write.writeframesraw(data)

   写入音频数据但不更新 *nframes*。

   3.4 版更變: 现在可接受任意 *bytes-like object*。

AU_write.writeframes(data)

   写入音频数据并更新 *nframes*。

   3.4 版更變: 现在可接受任意 *bytes-like object*。

AU_write.close()

   Make sure *nframes* is correct, and close the file.

   This method is called upon deletion.

Note that it is invalid to set any parameters after calling
"writeframes()" or "writeframesraw()".
