Image Processing Basics - Digital Signal Processing: Fundamentals and Applications - page 747

FIGURE 14.44B
The RGB color video sequence.
14.9.1 Analog Video
In computer systems, progressive scanning traces a whole picture, called frame via row-wise . A higher-
resolution computer uses 72 frames per second (fps). The video is usually played at a frame rate
varying from 15 frames to 30 frames.
In TV reception and some monitors, interlaced scanning is used in a cathode-ray tube display, or
raster. The odd-numbered lines are traced first, and the even-numbered lines are traced next. We then
get the odd-field and even-field scans per frame. The interlaced scheme is illustrated in Figure 14.45 ,
where the odd lines are traced, such as A to B, then C to D, and so on, ending in the middle at E. The
even field begins at F in the middle of the first line of the even field and ends at G. The purpose of using
interlaced scanning is to transmit a full frame quickly to reduce flicker. Trace jumping from B to C is
called horizontal retrace, while trace jumping from E to F or G to A is called vertical retrace.
The video signal is amplitude modulated. The modulation levels for NTSC video are shown in
Figure 14.46 . In the United States, negative modulation is used, meaning that the bright and dark
intensities are inverted before modulation. In the negative modulated video signal, less amplitude
comes from a brighter scene while more amplitude comes from a darker one. Since most pictures
contain more white than black levels, the video signal level is reduced. Hence, with negative modu-
lation, possible power efficiency can be achieved for transmission. The reverse process will apply for
display at the receiver.