Media Compression - Scalable Continuous Media Streaming Systems - page 25

2

Media Compression

In multimedia streaming systems media are almost always compressed to reduce their data
rate. Thus, the choice and properties of the media codec will have a significant impact
on the media streaming process. In this chapter we first introduce the basic concepts in
media coding and compression, and then discuss the issues related to media streaming. In
particular, we investigate the issue of matching the media bit-rate to the network bandwidth
available, and study in some detail video transcoding techniques that can be used to reshape
the media bit-rate to fit within the varying available network bandwidth. Readers interested
in a more general introduction to the area of media coding and compression are referred to
the many excellent texts available [1-5].

2.1 Introduction
Compression of media data is a highly developed research area with many successful applica-
tions. The prime examples are the development of the various MPEG compression standards
for audio and video compression - MPEG1 [6-10], MPEG2 [11-19], and MPEG4 [20-35].
These compression standards have since formed an essential component in the development
of Video CD, Digital Versatile Disc (DVD), HDTV broadcasting, and so on. In the following,
we briefly review the basic concepts in audio and video compression.

2.1.1 Digital Audio
Audio data are typically encoded in PCM at a certain data width and sampling frequency.
For example, Figure 2.1 illustrates the audio data sequence for CD audio, which employs a
sampling frequency of 44.1 kHz, with each sample represented by a 16-bit word. Thus, a
two-channel audio recording will generate data at a rate of 1.4 Mbps, not an insignificant
number even in today's broadband access networks. Thus, researchers have since developed
many audio compression algorithms to reduce the audio bit-rate, with the MP3 [36, 37] being
the best known and most widely adopted.
In addition to the CD audio format, the industry has recently introduced (two actually)
new formats for next-generation audio distribution. The first one, called DVD audio [38], is