Page 41 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

18 Chapter 2. Video Coding: Fundamentals

Table 2.2: Raw data rates of typical video formats
Format Raw data rate

HDTV 1:09 Gbits=s
CCIR-601 165:89 Mbits=s
CIF @ 15f.p.s. 18:24 Mbits=s
QCIF @ 10 f.p.s. 3:04 Mbits=s

Table 2.3: Typical video applications
Application Bandwidth
HDTV (6-MHz channel) 20 Mbits=s
Desktop video (CD-ROM) 1:5Mbits=s
Videoconferencing (ISDN) 384 kbits=s
Videophone (PSTN) 56 kbits=s
Videophone (GSM) 10 kbits=s

2.5.2 Elements of a Video Coding System

The aim of video coding is to reduce, or compress, the number of bits used to
represent video. Video signals contain three types of redundancy: statistical,
psychovisual, and coding redundancy. Statistical redundancy is present be-
cause certain data patterns are more likely than others. This is mainly due to
the high spatial (intraframe) and temporal (interframe) correlations between
neighboring pels. Psychovisual redundancy is due to the fact that the HVS is
less sensitive to certain visual information than to other visual information. If
video is coded in a way that uses more and=or longer code symbols than ab-
solutely necessary, it is said to contain coding redundancy. Video compression
is achieved by reducing or eliminating these redundancies.
Figure 2.3 shows the main elements of a video encoder. Each element is
designed to reduce one of the three basic redundancies.
The mapper (or transformer) transforms the input raw data into a represen-
tation that is designed to reduce statistical redundancy and make the data more
amenable to compression in later stages. The transformation is a one-to-one
mapping and is, therefore, reversible.

Symbol
Mapper Quantizer
encoder
Figure 2.3: Elements of a video encoder