Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
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A five minute piece of music, uncompressed, takes up over 50 MB of memory. MP3 and AAC
compression can reduce this to less than a tenth
of the original size. Thus, MP3 files are
popular for portable music players, since
compression makes it possible to store many
more songs.
Compression algorithms are of two
basic types: lossless or lossy. In the case of a
lossless compression algorithm, no audio
information is lost from compression to
decompression. The audio information is
compressed, making the file smaller for storage
and transmission. When it is played or
processed, it is decompressed, and the exact
data that was originally recorded is restored. In
the case of a lossy compression algorithm, it’s
impossible to get back exactly the original data
upon decompression. Examples of lossy
compression formats are MP3, AAC, Ogg
Vorbis, and the -law and A-law compression
used in AU files. Examples of lossless
compression algorithms include FLAC (Free
Lossless Audio Codec), Apple Lossless,
MPEG-4 ALS (Audio Lossless Coding),
Monkey’s Audio, and TTA (True Audio).
More details of audio codecs are given in
Section 5.2.1.
With the introduction of portable music players, copy-protected audio files became more
prevalent. Apple introduced iTunes in 2001, allowing users to purchase and download music
from their online store. The audio files, encoded in a proprietary version of the AAC format and
using the.m4p file extension, were protected with Apple’s FairPlay DRM system. DRM enforces
limits on where the file can be played and whether it can be shared or copied. In 2009, Apple
lifted restrictions on music sold from its iTunes store, offering an unprotected .m4a file as an
alternative to .m4p. Copy-protection is generally embedded within container file formats like
MP3. WMA (Windows Media Audio) files are another example, based on the Advanced
Systems Format (ASF) and providing DRM support.
Common audio file types are summarized in Table 5.1.
Aside:
Why are 52,920,000 bytes equal to about
50.5 MB? You might expect a megabyte
to be 1,000,000 bytes, but in the realm
of computers, things are generally done
in powers of 2. Thus, we use the
following definitions:
kilo = 210 = 1024
mega =
220
= 1,048,576
You should become familiar with the
following abbreviations:
kilobits kb
210
bits
kilobytes kB
210
bytes
megabits Mb
220
bits
megabytes MB
220
bytes
Based on these definitions, 52,920,000
bytes is converted to megabytes by
dividing by 1,048,576 bytes.
Unfortunately, usage is not entirely
consistent. You’ll sometimes see “kilo”
assumed to be 1000 and “mega”
assumed to be 1,000,000, e.g., in the
specification of the storage capacity of a
CD.
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