Understanding Digital Television
Most people in the United States are not aware of it yet,
but the U.S. television system is in the midst of a massive change.
The type of signal, format and aspect ratio have all changed in the process of
converting from analog TV to digital TV in the United States.
The standard-analog or NTSC-TV
systems that served so well in the 20th century are
quickly being replaced by a digital TV system that offers stunning picture quality and multi-channel surround sound.
Both systems will coexist until DTV (often referred to as
HDTV) reaches an 85 percent market penetration. When that happens, analog
transmitters will be turned off, and broadcasters will return their
old licenses to the Federal Communications Commission.
One of the most confusing aspects of DTV is the
proliferation of standards. The FCC didn't mandate a single
standard, but chose to "let the market decide," so now there are 18 possible variations on the digital theme. Fortunately, only three seem to have enough
support to survive:
Standard Definition: 480P, the
DVD format. 480I is shorthand for analog video; the name refers to
480 visible horizontal scanning lines interlaced in alternating
fields. 480P is a progressive-scan mode that basically doubles the
resolution of the old video, producing a better image.
High-Definition: 720P and
1080I are both High-Definition formats. Some video experts contend
that the difference in picture quality between 720P and 1080I is
could be thought of as a line-tripling scheme, and is favored by ABC
alternates fields of 540 lines each to create combined frames of
1080 lines. 1080I is used by CBS, NBC, and PBS.
Some programming, especially shows
produced by Showtime and HBO, is delivered in Dolby® Digital 5.1TM
surround sound. Stereo receivers "fold down" surround signals to two
channels. A DVD or broadcast movie with a surround soundtrack is
still enjoyable through a two-channel system, but lacks
front-to-rear localization cues.
DTV FAQ, Terms, Definitions
transmitting in HDTV, broadcasters will be able to transmit
four or more channels of standard definition television (SDTV)
programming simultaneously. This is called multicasting. The
option to multicast was made possible by digital technology to
allow each digital broadcast station to split its bit stream
into 2, 3, 4 or more individual channels of programming and/or
data services. For example, on channel 55, you could watch
55.1, 55.2, 55.3 or 55.4, or if the station were
broadcasting in HDTV, you could watch a single High-Definition
(The "p" and "i"
designations stand for "progressive" and
"interlaced." In a progressive format, the full
picture updates every sixtieth of a second. In an interlaced
format, half of the picture updates every sixtieth of a
- The picture is 704x480 pixels, sent at 60 interlaced
frames per second (30 complete frames per second).
- 480p - The picture
is 704x480 pixels, sent at 60 complete frames per second.
- 720p - The picture
is 1280x720 pixels, sent at 60 complete frames per second.
- 1080i - The picture
is 1920x1080 pixels, sent at 60 interlaced frames per
second (30 complete frames per second).
- 1080p - The picture
is 1920x1080 pixels, sent at 60 complete frames per
The 480p and 480i formats are
called the SD (standard definition) formats, and 480i
is roughly equivalent to a normal (old) analog TV picture.
The 720p, 1080i and 1080p
formats are HD (high definition) formats. When you
hear "HDTV," this is what is being discussed
-- a digital signal in the 720p, 1080i or 1080p format.
The HD formats of digital TV have a different aspect ratio
than analog TVs. An analog TV has a 4:3 aspect ratio, meaning
that the screen is 4 units wide and 3 units high. For example,
a "25-inch diagonal" analog TV is 15 inches high and
20 inches wide. The HD format for digital TV has a 16:9 aspect
ratio. The screen is 16 units wide and 9 units high.
Analog TV broadcasts you
receive are a single, analog composite video signal and a
separate sound signal. You can get, over the air (OTA),
channels 2 thru 83 in most large cities if you live within 50
miles of the station's antenna.
Digital TV as well can be
received from local commercial digital TV stations if
you have a digital TV receiver and an antenna. The FCC gave
television broadcasters a new frequency to use for their
digital broadcasts, so right now each broadcaster has an
analog TV channel and a digital TV channel. The digital
channel carries a 19.39-megabit-per-second stream of digital
data that your digital TV receives and decodes.