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The New Aspect Ratio
Current televisions are based upon the aspect ratio of 4:3. This means a
TV with an aspect ratio of 4:3 will be 3 feet high and 4 feet wide. With
HDTV the aspect ratio is 16:9, making the image much wider than a normal
TV. The new screen size is 33% wider to be exact.
The increased width is a huge improvement because it
allows you to view TV and movies as if you were watching them in a
theater. When watching TV on a 16:9 screen you use your peripheral
vision, which truly makes you feel like you are a part of the program.
Resolution
The usual resolution a normal NTSC TV can display is 525 scan
lines with 480 actually being visible. The usual TV can display a
resolution of about 210,000 pixels per image. High Definition TV's can
go all the way up to 1920 (horizontal) x 1080 (vertical) pixels, with a
total of 2,073,600 pixels per image. That is ten times
the resolution and picture quality. Ten times!
In fact, when you squeeze all these pixels down to the
same sized TV you get impeccable quality. Instead of stretching these
pixels out, they are crammed together so tight that resolutions get as
high and as sharp as they do.
Sound Quality
High Definition television also has 5.1 channels of
CD-quality surround sound (left, right, center, left rear, right rear,
and low frequency effects - AKA bass). These multiple channels bring you
true surround sound at an amazing quality.
With the older NTSC standard you only get two channels
of audio, which is just enough to make it stereo. With HDTV you are
listening to Dolby Digital/AC-3 which is the same that is used in most
movie theaters, DVD players and home entertainment systems!
No More Ghost Images
Many times signal quality is not perfect when watching
regular NTSC television, which gives you what are called 'ghost images'.
A ghost image is when you see two of the same person next to each other,
as if there is a ghost of that person next to himself.
With a digital signal you will always get a perfect
image as long as the TV is receiving a quality signal. This means no
more fuzz, no more snow and no more ghosts. Television the way it was
meant to be watched!
Progressive Scanning
Regular NTSC signals are shown as interlaced signals. This
means every other horizontal line of the monitor is displayed in one
frame, and the other half of the horizontal lines are drawn in the next
frame of the picture. This works because it happens so fast that the eye
is deceived into believing that only one picture is being shown. Below
this is shown in slower motion and on an image that is not moving for
explanation purposes.
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Direct TV HDTV Specials
Dish Network HDTV Specials
When the first
high-definition television (HDTV) sets hit the market in 1998,
movie buffs, sports fans and tech aficionados got pretty excited,
and for good reason. Ads for the sets hinted at a television
paradise with superior resolution and digital surround sound. With
HDTV, you could also play movies in their original widescreen
format without the letterbox "black bars" that some people find
annoying.
But for a lot of people, HDTV hasn't delivered a ready-made source
for transcendent experiences in front of the tube. Instead, people
have gone shopping for a TV and found themselves surrounded by
confusing abbreviations and too many choices. Some have even
hooked up their new HDTV sets only to discover that the picture
doesn't look good.
Fortunately, a few
basic facts easily dispel all of this confusion. In this article,
we'll explain the acronyms and resolution levels and give you the
facts on the United States transition to all-digital television. We'll
also tell you exactly what you need to know if you're thinking about
upgrading to HDTV.
Analog, Digital and HD
For years, watching TV has involved analog signals and cathode ray
tube (CRT) sets. The signal is made of continually varying radio waves
that the TV translates into a picture and sound. An analog signal can
reach a person's TV over the air, through a cable or via satellite.
Digital signals, like the ones from DVD players, are converted to
analog when played on traditional TVs. (You can read about how the TV
interprets the signal in How Television Works.)
This system has worked pretty well for a long time, but it has some
limitations:
Conventional CRT sets display around 480 visible lines of pixels.
Broadcasters have been sending signals that work well with this
resolution for years, and they can't fit enough resolution to fill a
huge television into the analog signal.
Analog pictures are interlaced - a CRT's electron gun paints only half
the lines for each pass down the screen. On some TVs, interlacing
makes the picture flicker.
Converting video to analog format lowers its quality.
Analog TVs like this one can't use a digital signal without a set-top
converter.
United States broadcasting is currently changing to digital television
(DTV). A digital signal transmits the information for video and sound
as ones and zeros instead of as a wave. For over-the-air broadcasting,
DTV will generally use the UHF portion of the radio spectrum with a 6
MHz bandwidth, just like analog TV signals do.
DTV has several
advantages:
The picture, even when displayed on a small TV, is better quality.
A digital signal can support a higher resolution, so the picture will
still look good when shown on a larger TV screen.
The video can be progressive rather than interlaced - the screen shows
the entire picture for every frame instead of every other line of
pixels.
TV stations can
broadcast several signals using the same bandwidth. This is called
multicasting.
If broadcasters choose to, they can include interactive content or
additional information with the DTV signal.
It can support high-definition (HDTV) broadcasts.
DTV also has one really big disadvantage: Analog TVs can't decode and
display digital signals. When analog broadcasting ends, you'll only be
able to watch TV on your trusty old set if you have cable or satellite
service transmitting analog signals or if you have a set-top digital
converter.
This brings us to the first big misconception about HDTV. Some people
believe that the United States is switching to HDTV, that all they'll
need for HDTV is a new TV and that they'll automatically have HDTV
when analog service ends. Unfortunately, none of this is true.
HDTV is just one part of the DTV transition. We'll look at HDTV in
more detail, including what makes it different from DTV, in the next
section.
DTV vs. HDTV
The Advanced Television Standards Committee (ATSC) has set voluntary
standards for digital television. These standards include how sound
and video are encoded and transmitted. They also provide guidelines
for different levels of quality. All of the digital standards are
better in quality than analog signals. HDTV standards are the top tier
of all the digital signals.
Standard vs.
high-definition aspect ratio
The ATSC has created 18 commonly used digital broadcast formats for
video. The lowest quality digital format is about the same as the
highest quality an analog TV can display. The 18 formats cover
differences in:
Aspect ratio - Standard television has a 4:3 aspect ratio - it is four
units wide by three units high. HDTV has a 16:9 aspect ratio, more
like a movie screen.
Resolution - The lowest standard resolution (SDTV) will be about the
same as analog TV and will go up to 704 x 480 pixels. The highest HDTV
resolution is 1920 x 1080 pixels. HDTV can display about ten times as
many pixels as an analog TV set.
Frame rate - A set's frame rate describes how many times it creates a
complete picture on the screen every second. DTV frame rates usually
end in "i" or "p" to denote whether they are interlaced or
progressive. DTV frame rates range from 24p (24 frames per second,
progressive) to 60p (60 frames per second, progressive).
Many of these standards have exactly the same aspect ratio and
resolution - their frame rates differentiate them from one another.
When you hear someone mention a "1080i" HDTV set, they're talking
about one that has a native resolution of 1920 x 1080 pixels and can
display 60 frames per second, interlaced.
The 18 Primary DTV
Standards
Broadcasters get to decide which of these formats they will use and
whether they will broadcast in high definition - many are already
using digital and high-definition signals. Electronics manufacturers
get to decide which aspect ratios and resolutions their TVs will use.
Consumers get to decide which resolutions are most important to them
and buy their new equipment based on that.
Until the analog shutoff date, broadcasters will have two available
channels to send their signal - a channel for analog, and a "virtual"
channel for digital. Right now, people can watch an over-the-air
digital signal only if they are tuned in to the broadcaster's virtual
digital channel. After analog broadcasting ends, the only signals
people will receive over the air will be digital.
However, even though a digital signal is better quality than an analog
signal, it isn't necessarily high definition. HDTV is simply the
highest of all the DTV standards. But whether you see a
high-definition picture and hear the accompanying Dolby Digital® sound
depends on two things. First, the station has to be broadcasting a
high-definition signal. Second, you have to have the right equipment
to receive and view it. We'll look at how to get an HDTV set and
signal next.
Progressive scanning, on the other
hand, displays the entire picture in one frame. This provides a sharper
picture to the eye and looks much better than an interlaced image of the
same resolution. With DTV there are two progressive formats for
broadcasters to choose from: 480p and 720p. A 480p image is without
question much sharper than a regular, NTSC broadcast, although it is
only considered a DTV signal and not a true High Definition signal. 720p
has 720 horizontal scan lines and is considered to be true HDTV. 720p
can many times rival a 1080i picture, which is 1080 horizontal scan
lines displayed as an interlaced image. This is because of the sharper,
and more accurate picture 720p displays, even though 1080i has many more
scan lines and a much higher resolution.
Multicasting
Multicasting is when a broadcaster
sends multiple signals out over the same channel. Usually when
multicasting, the quality of the broadcast is less than that of HDTV,
but it is almost always better than regular NTSC television.
The benefit of multicasting is
that a television station can air four or more signals at the same time
and give you the choice of what you want to watch. There have been
reports of stations multicasting a HDTV signal and an EDTV signal
without much loss in the High Definition Signal, however this is usually
not the case.
Norm Samat, the directory of ABC's
Monday Night Football HDTV feed put it perfectly:
You almost get into a trance
looking at HDTV pictures because you really see more. The wider 16:9
aspect ratio makes a big difference in covering football. With the old
4:3 aspect ratio screens it was often tricky to cover the defense. In
HDTV they are already in your picture, so you can see more of the play
develop.
http://electronics.howstuffworks.com/hdtv.htm
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