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Sec. 8–9 Television 649
encryption techniques. Subscribers to the premium services are provided with appropriate
decoders that, in some systems, are addressable and remotely programmable by the CATV
company.
Digital TV (DTV)
DTV is now a reality in the United States and in many other parts of the world. In July
2009, analog on-the-air TV transmission ceased in the United States (except for some very
low-power stations), and DTV stations were on the air. This was the result of much work in
the United States by the FCC and the TV broadcast industry. Since 1987, the FCC has been
encouraging the development of a high-definition television (HDTV) system to replace the
NTSC system. The HDTV system has high resolution that approaches the quality of 35-mm
film and a widescreen aspect ratio (width to height) of 16:9 instead of the narrowscreen 4:3
aspect ratio of NTSC. More than 20 HDTV methods were proposed [Jurgen, 1988; Jurgen,
1991]. By 1993, the number of competing proposals were reduced to four digital systems
[Challapali et al., 1995; Harris, 1993; Zou, 1991]. In May of 1993, the proponents of these
four competing systems joined forces to develop a single digital television (DTV) system that
used the best ideas of the four competing systems [Challapali et al., 1995; Hopkins, 1994; and
Petajan, 1995]. This joint group is called the Grand Alliance (GA), and its members are:
AT&T, General Instrument Corporation, Massachusetts Institute of Technology (MIT),
Philips Electronics North America Corporation, David Sarnoff Research Center, Thomson
Consumer Electronics, and Zenith Electronics Corporation. This resulted in the 1996 FCC
DTV standard, known as the ATSC (advanced television systems committee) standard, and
ATSC stations are now on the air in U.S. cities.
Table 8–14 summarizes the U.S. ATSC standard. The standard is actually several standards
combined, allowing for different resolutions and aspect ratios. There are five major
formats. Two are high definition (HD), and three are standard definition (SD). Widescreen pictures
(16:9 aspect ratio) are specified in both the HD formats and in one SD format. The other
two SD formats use conventional-width pictures (4:3). In addition, the frame rates may be 24,
30, or 60 framess with interlace scan or progressive scan. The 24-framess mode is included
to more easily accommodate motion picture film (which is 24 framess). These variations give
a total of 18 possible picture formats. All ATSC receivers are designed to decode and display
pictures for all 18 formats (although the resolution may not be preserved). The broadcaster
may switch formats from program to program (or from program to commercial). However, it is
not cost effective for broadcasters to be able to produce all formats. For HDTV, the NBC and
CBS television networks use 1080i (i.e., 1080 × 1980 pixels with interlace screening). ABC
uses 720p for HDTV, since they believe that progressively scanning 720 lines is visually superior
to 1080i. For SDTV, NBC and ABC use 480p, and CBS uses 480i. Note that the vertical
resolution for SDTV, which is 480 lines, is equivalent to the vertical resolution for U.S. analog
TV, which is 483 lines, as given by Eq. (8–59b). The horizontal resolution for 4:3 SDTV, 704
or 640 pixels, is larger than that of 445 pixels, as given by Eq. (8–60b) for analog TV.
The bit rate for uncompressed video data is tremendous. For example, referring to
Table 8–14, for 1080 active lines, 1920 samples (pixels) per line, 8 bits per sample, 30
frames (pictures) per second, and 3 primary colors (RGB), the bit rate is 1,080 × 1,920 × 8 ×
30 × 3 = 1,500 Mbs. However, a TV channel with a 6–MHz bandwidth can only support a
data rate of about 20 Mbs if 8-level (3 bitssymbol) multilevel signaling is used. (From