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Unformatted text preview: es every second (called refresh rate) to provide continuity of
motion and to avoid flicker.
A complete line travelled by the electron gun from left to right is called a scanline.
In one pass of raster scanning, if the electron beams travel all the scanlines
sequentially from top to bottom, it is called & progressive scan (see Figure
19.5(a)). On the other hand, if the electron beams travel only the odd scanlines in
one pass and the even scanlines in the next pass, it is called an interlaced scan (see
Figure 19.5(b)). For the interlaced scan, each pass is called & field, and two
complete fields comprise a full display screen.
When the electron beam has travelled the bottom-most scanline, it retraces back to
the top. Similarly, when the electron beam reaches the end of a scanline, it retraces
back to the beginning of the next scanline to be traveled. To prevent the retrace
lines from being visible on the screen, blanking pulses are applied to the picture
tube. The time taken to move the electron gun from the end of the bottom-most
scanline to the beginning of the top-most scanline is called the vertical blanking
interval, while the time taken to move it from the right end of one scanline to the
left end of the next is called the horizontal blanking interval. The vertical blanking
intervals are often used to embed other signals, such as synchronization
information, in broadcast television (TV) systems.
The two commonly used types of monitors for video display are TV monitors and
computer monitors. TV monitors use interlaced scanning technique with a refresh
rate of 60 fields per second or 30 frames per second (the actual refresh rate
depends on the broadcasting standard used by the TV system - NTSC, PAL or
SECAM described later in this chapter). On the other hand, computer monitors use
progressive scanning technique with a refresh rate of 60 frames per second. The
aspect ratio, which is the ratio of the horizontal to the vertical dimensions, is 4:3
for both TV a...
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- Spring '14