410 Electronic Communication and Data Systems
overlaid, producing a picture rich in color. Since the full
array of each CCD is used for each color, the three-CCD
camera is capable of higher HDTV resolutions.
HDTV Picture Quality
The most impressive attribute of HDTV is the pic-
ture's visual quality. To compare the HDTV image to the
analog television image, we must first convert the typical
analog image to an equal digital resolution.
The National Television Standards Committee
(NTSC) formulated the standards for analog television
and video in the United States. The NTSC standard calls
for 525 scan lines at a 60 Hz refresh-rate based on the
interlace technique. NTSC is not compatible with most
computer video systems and must be converted before it
can be displayed. The Advanced Television Systems
Committee (ATSC) was established in 1983. The com-
mittee spent years developing standards that were eventu-
ally adopted by the FCC for digital television broadcast-
ing and receivers. These standards have been designed to
eventually replace the NTSC standards.
There are 18 scanning formats described in the
ATSC standards. Variations in the standards are derived
from concerns about interlace scanning and progressive
scanning, frame rates, and aspect ratio. Earlier, in the sec-
tion about the analog television system, you learned
about interlacing. Interlace scanning is a two-step process
of transmitting the odd lines of the scan and then going
back over the image, filling in the even lines to make a
complete image, Figure 23-37. Progressive scanning is
the capture and transmission of the entire image at one
time. Each line is placed on the screen progressively in
one sweep.
Frame rate is how often the image is updated on the
screen. Currently, three frame rates (in frames per sec-
ond) exist: 60, 30, and 24. Aspect ratio is the relationship
of the horizontal to vertical screen presentation measure-
ments. Aspect ratio standards can be either 4:3 or 16:9.
The 16:9 is a wide-screen aspect ratio similar to common
movie theaters. The 4:3 aspect ratio is a standard televi-
sion rectangle. The various factors of aspect ratio, frame
rates, and scanning method combine to form the 18 dif-
ferent screen standards. Figure 23-38 lists the 18 ATSC
digital TV compression formats.
HDTV has a vertical scanning rate equal to 720p
(progressive) and 1080i (interlaced) vertical lines. The
actual display may have a higher vertical scan rate than
the 1080i standard. This is especially important as the
size of the display area increases.
To compare the quality of analog television to
HDTV, you must convert scan lines to maximum number
of pixels. A pixel is the smallest unit of an image on a
graphic display. It can be thought of as a single dot in the
entire image.
Analog television approximates a screen composed
of 480 × 440 pixels, producing 211,200 total pixels.
HDTV approximates a screen composed of 1920 × 1080
pixels, producing 2,073,600 total pixels and by far a
greater detailed image than the analog system.
The Moving Picture Experts Group developed the
MPEG2 image compression standard to increase the
amount of video data transmitted in an HDTV system. By
compressing the broadcast video data, more information
could be broadcast in the same amount of bandwidth. The
MPEG2 compression technique can reduce the image
information by as much as 97 percent, but an average of
50 percent is typical. The compression technique is based
on the fact that the majority of the video images on a tel-
evision screen do not change from frame to frame. For
example, a news broadcast has a persistent image, such as
a background, with very little movement requiring new
data. Parts of the image that are persistent do not need to
Odd Rows
240 lines 30 × /sec
All lines scanned at once
480 lines 60 × /sec
Even Rows
240 lines 30 × /sec
A
B
Figure 23-37. Interlaced and progressive scanning.
A—Interlaced scanning captures and transmits the odd
lines first, and then the even lines. B—Progressive scan-
ning captures and transmits the whole image at once.