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DTV Explained By Zenith
Analog
Television
The analog television broadcast system used in the United States for
the past 50 years transmits signals as an electronic wave. In the
wave, images and sounds are represented by continuously changing
frequencies and voltage levels. Transmitters broadcast this signal
over the air, and the waves are translated back into images and sounds
by TV sets. Unfortunately, the shape of this wave is susceptible to
degradation as it travels to your home. This degradation can introduce
unwanted elements, called artifacts, into the picture. Many things can
interfere with an analog signal, such as planes passing overhead and
household appliances.
Digital
Television
When a digital television signal is transmitted, images and sounds are
divided into tiny components of computer information, the ones and
zeros of computer language. The computer data provides a more
consistent signal that is highly resistant to interference. Unlike the
analog wave, there is no chance of misinterpreting what the computer
data means. Digital television sets can then receive this data and
decode it back into images and sounds. The result is a perfect,
studio-quality picture free from the interference, snow and ghosts
that appear on analog TV broadcasts.
Advantages of Digital Television
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Picture
Pictures add realism to the television experience and crisp digital
pictures are one of the biggest advantages of digital television.
Unlike analog TV, digital TV is free from snow, ghosts and
interference. What's more, with HDTV or high-definition television,
the highest form of digital television, you get 4 to 5 times more
picture information than conventional TV pictures. Combine that with
the new widescreen format, and you have the ultimate home theater
viewing experience.
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Sound
Sound adds emotion to the television experience and digital
television is enhanced with CD-quality theater-like audio. The
digital television signal includes audio in the 5.1 channel Dolby
Digital format — separate left and right front channels, left and
right rear channels, center and subwoofer channels.
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Multicasting and Datacasting
Digital broadcasts can carry huge amounts of digital information —
more than 19 megabits per second. Because of this, a broadcaster may
choose to split their channel into two or more streams of
programming to offer more choice and flexibility to the viewer.
Multicasting allows a broadcaster to transmit several
standard-definition digital programs at the same time. For example,
a broadcaster may show a high-definition movie, a popular sitcom and
a news program all at the same time.
Datacasting links information from various sources, such as the
Internet, to the television broadcast and allows the information to
be displayed on-screen at the same time as a television program or
delivered to your PC. You will then be able to get the latest stock
quotes, access statistics on your favorite player during a football
game, or order the product you just saw advertised right from your
TV.
Digital
Video Formats
The Advanced Television Systems Committee (ATSC) digital TV standard
adopted by the FCC in 1996 contains 18 scanning formats. They include
interlaced and progressive scan HDTV and SDTV formats with varying
frame rates, and two aspect ratios.
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Active Lines
Scanning formats are measured in active lines of picture elements or
pixels transmitted, both horizontal and vertical. The number of
total pixels is calculated by multiplying the amount of vertical
lines by the density of horizontal picture elements. For example,
the 1920x1080 HDTV format has more than 2 million pixels. (1920 X
1080 = 2,073,600). Today's NTSC video only has a resolution of
211,200 pixels (480 vertical lines and 440 horizontal pixels).
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Aspect Ratio
The aspect ratio refers to the shape of the video image. Today's
television has a 4:3 aspect ratio, which is almost square in
appearance. Digital television will have either a 4:3 or 16:9 aspect
ratio. A 16:9 aspect ratio is widescreen, and appears rectangular:
this looks more like a movie screen, filling more of the viewer's
field of vision for a more lifelike vieweing experience.
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Scanning Method
Frames of video are made up of many lines of video that are scanned
onto a television screen so closely they appear to be a solid
picture. The scanning method can be either interlaced or
progressive. Interlaced scanning (also used in today's analog TV),
indicated by an "i" in the video format, fills in the odd number
lines (1,3,5,7...) and then fills in the even number lines
(2,4,6,8...) until the frame is complete. Progressive scanning, "p",
fills in each line consecutively until the frame is complete, like a
computer display.
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Frame Rate
Frame rate, also known as picture rate, is the speed at which the
lines are scanned in order to create a video frame, 60-, 30-, or
24-frames per second.
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