11. Real-time Audio and Video
11.1 Introduction
Real-time audio and video applications
are two important multimedia applications on today's
computer networks. These applications enable efficient communications
through computer networks. For example, videoconferencing enables a
face to face meeting between groups of people at two or more different
locations through both speech and sight. Videoconferencing can also be used as
distance learning and collaborate work with remote teams.
World Wide Web was designed to focus on static
documents retrieval. These documents may include text files, images,
audio and video files. Through there are some real-time applications
(i.e., telephony and videoconferencing) products were made
that allow users to hear and see real-time audio and video
over World Wide Web, the current World Wide Web technology is unsuitable
for real-time applications. The main reason is that current World Wide
Web use HTTP as transfer protocol, while HTTP is based on TCP which is
unsuitable for real-time applications
(see section 4 for details).
With rapidly growing of real-time audio and video applications,
more and more researchers involve developing new World Wide Web technologies
that support real-time audio and video applications.
Real Time Video and Audio in the World Wide Web discusses a
protocol called VDP that specialize for handling real-time video over
the WWW.
This chapter provides inside looks of the networked audio and video.
Audio and video signals are analog signals by nature.
To be transmitted, these signals must be first converted to
digital form. In section 11.2 and 11.3, the concept of digitization
is introduced. To reduce the transmission bandwidth, audio
and video files are often compressed before transmission.
The compression formats are also introduced in these two sections.
Real-time audio and video applications require large network
bandwidth and low data latency. If an application involves more than two
parties, then multipoint communications need be considered.
Section 11.4 describes the characteristics of all major types
of networks and their possible impacts on these applications.
Section 11.5 describes the telephony and videoconferencing systems.
Hyperlinks are provided for readers who have deeper interests
in these areas.
The information provided each section is relatively independent from
the other sections.
However, readers are encouraged to finish reading the sections in sequence
to understand the entire concept of real-time audio and video applications
for computer networks.
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Last modified: Sun Dec 8 1996