Page 33 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

10 Chapter 2. Video Coding: Fundamentals

signal of time that is obtained by sampling the video intensity pattern in
the vertical and temporal coordinates and converting intensity to electrical
representation. This sampling process is known as scanning.
Raster scanning begins at the top-left corner and progresses horizontally,
with a slight slope vertically, across the image. When it reaches the right-
hand edge it snaps back to the left-hand edge (horizontal retrace) to start
a new scan line. On reaching the bottom-right corner, a complete frame has
been scanned and scanning snaps back to the top-left corner (vertical retrace)
to begin a new frame. During retrace, blanking (black) and synchronization
pulses are inserted.
The most commonly used raster scanning methods are progressive and inter-
laced, as illustrated in Figure 2.1. In progressive (also known as noninterlaced
or 1:1) scanning, a frame is formed by a single scanning pass. In interlaced
(or 2:1) scanning, however, a frame is formed by two successive scanning
passes. In the rst pass, the odd lines are scanned to form the rst eld, then
the even lines are scanned to form the second eld. When interleaved, the
lines of the two elds form a single frame.
The aspect ratio, vertical resolution, frame rate, and refresh rate are impor-
tant parameters of the video signal. The aspect ratio is the ratio of the width
to the height of a frame. The vertical resolution is related to the number of
scan lines per frame (including the blanking intervals). The frame rate is the
number of frames scanned per second. The e6ect of smooth motion can be
achieved using a frame rate of about 25–30 frames=s. However, at these frame
rates the human eye picks up the /icker produced by refreshing the display
between frames. To avoid this, the display refresh rate must be above 50 Hz.

vertical retrace horizontal retrace field 1 field 2

Progressive Interlaced
Figure 2.1: Raster scanning methods