21.4. "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。

An audio file consists of a header followed by the data.  The fields
of the header are:

+-----------------+-------------------------------------------------+
| Field           | 内容                                            |
+=================+=================================================+
| magic word      | The four bytes ".snd".                          |
+-----------------+-------------------------------------------------+
| header size     | Size of the header, including info, in bytes.   |
+-----------------+-------------------------------------------------+
| data size       | Physical size of the data, in bytes.            |
+-----------------+-------------------------------------------------+
| encoding        | Indicates how the audio samples are encoded.    |
+-----------------+-------------------------------------------------+
| sample rate     | The sampling rate.                              |
+-----------------+-------------------------------------------------+
| # of channels   | The number of channels in the samples.          |
+-----------------+-------------------------------------------------+
| info            | ASCII string giving a description of the audio  |
|                 | file (padded with null bytes).                  |
+-----------------+-------------------------------------------------+

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.


21.4.1. AU_read Objects
=======================

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()

   Returns a tuple "(nchannels, sampwidth, framerate, nframes,
   comptype, compname)", equivalent to output of the "get*()" methods.

AU_read.readframes(n)

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

AU_read.rewind()

   设置当前文件指针位置。

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。


21.4.2. AU_write Objects
========================

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.)

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*。

AU_write.writeframes(data)

   写入音频数据并更新 *nframes*。

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()".
