Databases Reference
In-Depth Information
Introduction
In the last decade, we have been witnessing a transformation—some call it a
revolution—in the way we communicate, and the process is still under way. This
transformation includes the ever-present, ever-growing Internet; the explosive
development of mobile communications; and the ever-increasing importance of
video communication. Data compression is one of the enabling technologies
for each of these aspects of the multimedia revolution. It would not be practical to put images,
let alone audio and video, on websites if it were not for data compression algorithms. Cellular
phones would not be able to provide communication with increasing clarity were it not for
compression. The advent of digital TV would not be possible without compression. Data
compression, which for a long time was the domain of a relatively small group of engineers and
scientists, is now ubiquitous. Make a call on your cell phone, and you are using compression.
Surf on the Internet, and you are using (or wasting) your timewith assistance fromcompression.
Listen to music on your MP3 player or watch a DVD, and you are being entertained courtesy
of compression.
So what is data compression, and why do we need it? Most of you have heard of JPEG
and MPEG, which are standards for representing images, video, and audio. Data compression
algorithms are used in these standards to reduce the number of bits required to represent
an image or a video sequence or music. In brief, data compression is the art or science of
representing information in a compact form. We create these compact representations by
identifying and using structures that exist in the data. Data can be characters in a text file,
numbers that are samples of speech or image waveforms, or sequences of numbers that are
generated by other processes. The reason we need data compression is that more and more of
the information that we generate and use is in digital form—consisting of numbers represented
by bytes of data. And the number of bytes required to represent multimedia data can be
huge. For example, in order to digitally represent 1 second of video without compression
(using the CCIR 601 format described in Chapter 18), we need more than 20 megabytes, or
160 megabits. If we consider the number of seconds in a movie, we can easily see why we
would need compression. To represent 2 minutes of uncompressed CD-quality music (44,100
