191 lines
6.7 KiB
TeX
191 lines
6.7 KiB
TeX
\section{Standard Module \sectcode{aifc}}
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\label{module-aifc}
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\stmodindex{aifc}
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This module provides support for reading and writing AIFF and AIFF-C
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files. AIFF is Audio Interchange File Format, a format for storing
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digital audio samples in a file. AIFF-C is a newer version of the
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format that includes the ability to compress the audio data.
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Audio files have a number of parameters that describe the audio data.
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The sampling rate or frame rate is the number of times per second the
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sound is sampled. The number of channels indicate if the audio is
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mono, stereo, or quadro. Each frame consists of one sample per
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channel. The sample size is the size in bytes of each sample. Thus a
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frame consists of \var{nchannels}*\var{samplesize} bytes, and a
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second's worth of audio consists of
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\var{nchannels}*\var{samplesize}*\var{framerate} bytes.
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For example, CD quality audio has a sample size of two bytes (16
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bits), uses two channels (stereo) and has a frame rate of 44,100
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frames/second. This gives a frame size of 4 bytes (2*2), and a
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second's worth occupies 2*2*44100 bytes, i.e.\ 176,400 bytes.
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Module \code{aifc} defines the following function:
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\renewcommand{\indexsubitem}{(in module aifc)}
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\begin{funcdesc}{open}{file\, mode}
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Open an AIFF or AIFF-C file and return an object instance with
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methods that are described below. The argument file is either a
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string naming a file or a file object. The mode is either the string
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\code{'r'} when the file must be opened for reading, or \code{'w'}
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when the file must be opened for writing. When used for writing, the
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file object should be seekable, unless you know ahead of time how many
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samples you are going to write in total and use
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\code{writeframesraw()} and \code{setnframes()}.
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\end{funcdesc}
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Objects returned by \code{aifc.open()} when a file is opened for
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reading have the following methods:
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\renewcommand{\indexsubitem}{(aifc object method)}
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\begin{funcdesc}{getnchannels}{}
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Return the number of audio channels (1 for mono, 2 for stereo).
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\end{funcdesc}
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\begin{funcdesc}{getsampwidth}{}
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Return the size in bytes of individual samples.
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\end{funcdesc}
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\begin{funcdesc}{getframerate}{}
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Return the sampling rate (number of audio frames per second).
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\end{funcdesc}
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\begin{funcdesc}{getnframes}{}
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Return the number of audio frames in the file.
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\end{funcdesc}
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\begin{funcdesc}{getcomptype}{}
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Return a four-character string describing the type of compression used
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in the audio file. For AIFF files, the returned value is
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\code{'NONE'}.
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\end{funcdesc}
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\begin{funcdesc}{getcompname}{}
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Return a human-readable description of the type of compression used in
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the audio file. For AIFF files, the returned value is \code{'not
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compressed'}.
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\end{funcdesc}
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\begin{funcdesc}{getparams}{}
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Return a tuple consisting of all of the above values in the above
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order.
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\end{funcdesc}
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\begin{funcdesc}{getmarkers}{}
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Return a list of markers in the audio file. A marker consists of a
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tuple of three elements. The first is the mark ID (an integer), the
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second is the mark position in frames from the beginning of the data
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(an integer), the third is the name of the mark (a string).
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\end{funcdesc}
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\begin{funcdesc}{getmark}{id}
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Return the tuple as described in \code{getmarkers} for the mark with
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the given id.
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\end{funcdesc}
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\begin{funcdesc}{readframes}{nframes}
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Read and return the next \var{nframes} frames from the audio file. The
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returned data is a string containing for each frame the uncompressed
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samples of all channels.
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\end{funcdesc}
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\begin{funcdesc}{rewind}{}
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Rewind the read pointer. The next \code{readframes} will start from
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the beginning.
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\end{funcdesc}
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\begin{funcdesc}{setpos}{pos}
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Seek to the specified frame number.
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\end{funcdesc}
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\begin{funcdesc}{tell}{}
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Return the current frame number.
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\end{funcdesc}
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\begin{funcdesc}{close}{}
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Close the AIFF file. After calling this method, the object can no
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longer be used.
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\end{funcdesc}
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Objects returned by \code{aifc.open()} when a file is opened for
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writing have all the above methods, except for \code{readframes} and
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\code{setpos}. In addition the following methods exist. The
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\code{get} methods can only be called after the corresponding
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\code{set} methods have been called. Before the first
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\code{writeframes} or \code{writeframesraw}, all parameters except for
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the number of frames must be filled in.
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\begin{funcdesc}{aiff}{}
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Create an AIFF file. The default is that an AIFF-C file is created,
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unless the name of the file ends in '.aiff' in which case the default
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is an AIFF file.
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\end{funcdesc}
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\begin{funcdesc}{aifc}{}
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Create an AIFF-C file. The default is that an AIFF-C file is created,
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unless the name of the file ends in '.aiff' in which case the default
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is an AIFF file.
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\end{funcdesc}
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\begin{funcdesc}{setnchannels}{nchannels}
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Specify the number of channels in the audio file.
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\end{funcdesc}
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\begin{funcdesc}{setsampwidth}{width}
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Specify the size in bytes of audio samples.
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\end{funcdesc}
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\begin{funcdesc}{setframerate}{rate}
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Specify the sampling frequency in frames per second.
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\end{funcdesc}
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\begin{funcdesc}{setnframes}{nframes}
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Specify the number of frames that are to be written to the audio file.
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If this parameter is not set, or not set correctly, the file needs to
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support seeking.
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\end{funcdesc}
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\begin{funcdesc}{setcomptype}{type\, name}
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Specify the compression type. If not specified, the audio data will
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not be compressed. In AIFF files, compression is not possible. The
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name parameter should be a human-readable description of the
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compression type, the type parameter should be a four-character
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string. Currently the following compression types are supported:
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NONE, ULAW, ALAW, G722.
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\end{funcdesc}
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\begin{funcdesc}{setparams}{nchannels\, sampwidth\, framerate\, comptype\, compname}
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Set all the above parameters at once. The argument is a tuple
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consisting of the various parameters. This means that it is possible
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to use the result of a \code{getparams} call as argument to
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\code{setparams}.
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\end{funcdesc}
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\begin{funcdesc}{setmark}{id\, pos\, name}
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Add a mark with the given id (larger than 0), and the given name at
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the given position. This method can be called at any time before
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\code{close}.
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\end{funcdesc}
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\begin{funcdesc}{tell}{}
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Return the current write position in the output file. Useful in
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combination with \code{setmark}.
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\end{funcdesc}
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\begin{funcdesc}{writeframes}{data}
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Write data to the output file. This method can only be called after
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the audio file parameters have been set.
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\end{funcdesc}
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\begin{funcdesc}{writeframesraw}{data}
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Like \code{writeframes}, except that the header of the audio file is
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not updated.
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\end{funcdesc}
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\begin{funcdesc}{close}{}
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Close the AIFF file. The header of the file is updated to reflect the
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actual size of the audio data. After calling this method, the object
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can no longer be used.
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\end{funcdesc}
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