Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
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documented standards and can be licensed for use on a variety of platforms. As an alternative,
there exist some free, open source audio file formats, including OGG and FLAC.
Platform-restricted files can be used only under certain operating systems. For
example, WMA files run under Windows, AIFF files run under Apple OS, and AU files run
under Unix and Linux. The MP3 format is cross-platform. AAC is a cross-platform format that
has become widely popular from its use on phones, pad computers, digital radio, and video game
consoles.
The basic format for uncompressed audio
data is called PCM (pulse code modulation).
The term pulse code modulation is derived from
the way in which raw audio data is generated and
communicated. That is, it is generated by the
process of sampling and quantization described in
Section 5.1 and communicated as binary data by
electronic pulses representing 0s and 1s. WAV, AIFF, AU, RAW, and PCM files can store
uncompressed audio data. RAW files contain only the audio data, without even a header on the
file.
You can’t tell from the file extension whether or not a file is compressed, and if it is
compressed, you can’t necessarily tell what compression algorithm (called a codec) was used.
There are both compressed and uncompressed versions of WAV, AIFF, and AU files. When you
save an audio file, you can choose which type you want.
One basic reason that WAV and AIFF files come in compressed and uncompressed
versions is that, in reality, these are container file formats rather than simple audio files. A
container file wraps a simple audio file in a meta-format which specifies blocks of information
that should be included in the header along with the size and position of chunks of data following
the header. The container file may allow options for the format of the actual audio data,
including whether or not it is compressed. If the audio is compressed, the system that tries to
open and play the container file must have the appropriate codec in order to decompress and play
it. AIFF files are container files based on a standardized format called IFF. WAV files are based
on the RIFF format. MP3 is a container format that is part of the more general MPEG standard
for audio and video. WMA is a Windows container format. OGG is an open source, cross-
platform alternative.
In addition to audio data, container files can include information like the names of songs,
artists, song genres, album names, copyrights, and other annotations. The metadata may itself be
in a standardized format. For example, MP3 files use the ID3 format for metadata.
Compression is inherent in some container file types and optional in others. MP3, AAC,
and WMA files are always compressed. Compression is important if one of your main
considerations is the ability to store lots of files. Consider the size of a CD quality audio file,
which consists of two channels of 44,100 samples per second with two bytes per sample. This
gives
Aside: Pulse code modulation was
introduced by British scientist A. Reeves in
the 1930s. Reeves patented PCM as a way
of transmitting messages in “amplitude-
dichotomized, time-quantized” form what
we now call “digital.”
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