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In everyday life, we often come in contact with compressed signals: when using
mobile telephones, mp3 players, digital cameras, or DVD players. The signals in each
of these applications, telephone-band speech, high fidelity audio signal, and still or
video images are not only sampled and quantized to put them into a form suitable for
saving in mass storage devices or to send them across networks, but also compressed.
The first operation is very basic and is presented in all courses and introductory books
on signal processing. The second operation is more specific and is the subject of
this book: here, the standard tools for signal compression are presented, followed
by examples of how these tools are applied in compressing speech and musical audio
signals. In the first part of this book, we focus on a problem which is theoretical in
nature: minimizing the mean squared error. The second part is more concrete and
qualifies the previous steps in seeking to minimize the bit rate while respecting the
psychoacoustic constraints. We will see that signal compression consists of seeking
not only to eliminate all redundant parts of the original signal but also to attempt the
elimination of inaudible parts of the signal.
The compression techniques presented in this book are not new. They are explained
in theoretical framework, information theory, and source coding, aiming to formalize
the first (and the last) element in a digital communication channel: the encoding
of an analog signal (with continuous times and continuous values) to a digital
signal (at discrete times and discrete values). The techniques come from the work
by C. Shannon, published at the beginning of the 1950s. However, except for the
development of speech encodings in the 1970s to promote an entirely digitally
switched telephone network, these techniques really came into use toward the end of
the 1980s under the influence of working groups, for example, “Group Special Mobile
(GSM)”, “Joint Photographic Experts Group (JPEG)”, and “Moving Picture Experts
Group (MPEG)”. |