If
you've looked at television sets at any of the big electronics retailers in the
United States lately, you know that digital TV, or DTV,
is a big deal right now. Most stores have whole areas devoted to digital TV
sets. You're also hearing a lot about four other topics:
·
HDTV and
HDTV broadcasts
·
Digital satellite services
·
Digital cable
·
DVDs and DVD players
Unless
you are among the people in the United States who have purchased a DTV set,
what you have in your living room is a normal analog TV that
seems to be working just fine despite all the hype. Most people, faced with
this level of product proliferation, can only ask, "What the heck is going
on here?!"
On June 12, 2009, television stations in
the United States completed the transition from analog to digital broadcasting.
Consumers receiving local television signals over analog antennas now must use
converter boxes to receive programming on their TVs. This deadline was pushed
back several times in the last few years because of both broadcasters' and
consumers' inability to meet the FCC's criteria for a successful transition to
digital broadcasting.
The change was last scheduled to take place on Feb. 17,
2009, but was pushed back one final time to allow more people to purchase new
TVs or converter boxes to allow them to make the transition. Some stations,
however, switched to digital broadcasting in February 2009 anyhow because
they'd already contracted time to broadcast on digital transmitters and staying
analog would require a costly budgetary adjustment.
In this article, we will explore the world of digital television
so that you can understand exactly what is going on in this medium.
Understanding Analog
TV
To understand digital TV, it's helpful to understand analog
TV so that you can see the differences. (If you've read How Television Works, then you know how analog TV works).
The analog TV standard has been in use in the United States
for about 50 years. To review quickly, here are the basics of analog television
transmission:
·
A video camera takes
a picture of a scene. It does this at a frame rate of 30 frames per second.
·
The camera rasterizes the
scene. That is, the camera turns the picture into rows of individual dots
called pixels. Each pixel is assigned a color and intensity.
·
The rows of pixels
are combined with synchronization signals, called horizontal sync and vertical
sync signals, so that the electronics inside a TV set will know how to
display the rows of pixels.
This
final signal, containing the color and intensity of each pixel in a set of
rows, along with horizontal and vertical sync signals, is called a composite
video signal. Sound is completely separate. When you look on the back of
your VCR and
you see the yellow plug, that's the plug for composite video. Sound is either a
white plug (on VCRs that do not handle stereo sound) or a red plug and a white
plug (on VCRs that do handle stereo).
There
are lots of different things you can do with a composite video signal and a
sound signal. Here are just a few:
·
You can broadcast
them as radio waves.
When you attach an antenna to your TV set and pick up local stations for free,
you're receiving broadcast television from local TV stations.
·
You can record
them with a VCR.
·
You can transmit
them through a cable TV system along
with hundreds of other composite signals.
Many
different kinds of equipment understand composite video signals.
When
a composite video signal is broadcast over the airwaves by a TV station, it
happens on a specific frequency. In the United States, we know
these frequencies as VHF channels 2 through 13 and UHF channels 14 through 83.
The
composite video signal is transmitted as an AM signal and the sound as an FM
signal on these channels. See How TV Works for
details on transmission, and How Radio Works for
details on AM and FM. The FCC allocated three bands of frequencies in the
radio spectrum, chopped into
6-MHz slices, to accommodate these TV channels:
·
54 to 88 MHz for
Channels 2 to 6
·
174 to 216 MHz for
Channels 7 through 13
·
470 to 890 MHz for
UHF Channels 14 through 83
When
your VCR wants
to display its signal on a normal analog TV, it takes the composite video
signal and the sound signal off the tape and then modulates those signals onto
a 60-MHz (channel 3) or 66-MHz (channel 4) carrier, just like a TV station
would. Instead of broadcasting it, however, the VCR sends it straight to the
TV. A cable box or satellite box does the same thing.
Right
now you hear a lot about "digital satellite systems" and
"digital cable systems." The set-top box receives a digital signal
from the satellite or cable; the box then converts that signal to an analog
signal and sends it to your analog TV. That's why if you're a digital cable or
satellite TV subscriber, your provider probably told you that the June 2009 DTV
transition wouldn't require you to buy new equipment.
True
digital TV, on the other hand, is completely digital and involves:
·
Digital cameras working
at a much higher resolution than analog cameras
·
Digital
transmission
·
Digital display at
a much higher resolution
You
can see the difference in resolution in the next section.
What's Wrong with
Analog TV?
If you currently have an analog TV, and it works fine
with broadcast TV, cable TV, VCRs, satellite TV, camcordersand so on, an obvious question
would be, "What's wrong with analog TV?"
The main problem is resolution.
·
The resolution of
the TV controls the crispness and detail in the picture you see.
·
The resolution is
determined by the number of pixels on the screen.
·
An analog TV set
can display 525 horizontal lines of resolution every 30th of a second. In reality,
however, an analog TV displays half of those lines in a 60th of a second, and
then displays the other half in the next 60th, so the whole frame is updated
every 30th of a second. This process is called interlacing.
That's
been the way TV works for years. But now we've used to looking at computer monitors and
expect much better resolution. The lowest-resolution computer monitor displays
640 x 480 pixels. Because of the interlacing, the effective resolution of a TV
screen is perhaps 512 x 400 pixels.
So
the worst computer monitors you can buy have more resolution than the best
analog TV set; and the best computer monitors are able to display up to 10
times more pixels than that TV set. There is simply no comparison between a
computer monitor and an analog TV in terms of detail, crispness, image
stability and color. If you look at a computer monitor all day at work, and
then go home and look at a TV set, the TV set can look very fuzzy.
The
drive toward digital TV is fueled by the desire to give TV the same crispness
and detail as a computer screen. If you have ever looked at a true digital TV
signal displayed on a good digital TV set, you can certainly understand why --
the digital version of TV looks fantastic! There's no comparison. With 10 times
more pixels on the screen, all displayed with digital precision, the picture is
incredibly detailed and stable.
It's
hard to convey the difference between a DTV signal and an analog signal without
an actual demonstration, but here's a static comparison that can help you
understand the idea. Below is a picture of an odometer:
This
is a nice, crisp picture. Let's assume that this picture is being displayed on
a good digital TV so that this is what you actually see. The following photo
shows you what you would see on an analog TV:
You can see that the
analog TV picture is much fuzzier than the digital TV image. Look, for example,
at the teeth on the gears. There's a
significant difference in picture quality that's even more obvious when the
image is moving. It is that quantitative difference that drives the interest in
digital TV. And as if the incredible picture weren't enough, digital TV also
offers much better sound.
TV Goes Digital
The term "digital TV" is used in many different
ways right now, depending on whom you're talking to. There's also the term
"HDTV," which is the most advanced form
of digital TV in use in the United States. The reason it gets confusing is
because digital TV in the United States combines three different ideas.
The first idea that is new to digital TV is the digital
signal.
Analog
TV started as a broadcast medium. TV stations set up antennas and
broadcast radio signals to
individual communities. You can attach an antenna to your TV and pick up
channels 2 through 83 for free. What you receive, as described earlier, is a
single, analog composite video signal and a separate sound signal.
Digital
TV started as a free broadcast medium as well. For example, in San Jose, Calif.,
you can tune in to about a dozen different 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 until the
digital transition is complete, 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.
Each
broadcaster has one digital TV channel, but one channel can carry
multiple sub-channels if the broadcaster chooses that option.
Here's how it works:
On
its digital channel, each broadcaster sends a 19.39-megabit-per-second (Mbps)
stream of digital data. Broadcasters have the ability to use this stream in several
different ways. For example:
·
A broadcaster can
send a single program at 19.39 Mbps.
·
A broadcaster can
divide the channel into several different streams (perhaps four streams of 4.85
Mbps each). These streams are called sub-channels, and this type of
broadcasting is called multicasting. For example, if the digital TV
channel is channel 53, then 53.1, 53.2 and 53.3 could be three sub-channels on
that channel. Each sub-channel can carry a different program.
The
reason that broadcasters can create sub-channels is because digital TV
standards allow several different formats. Broadcasters can choose
between three formats:
·
480i -
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 second.
(The
"p" and "i" designations stand for "progressive"
and "interlaced." In a progressive format, the full picture updates
every 60th of a second. In an interlaced format, half of the picture updates
every 60th of a second.)
The
480p and 480i formats are called the SD (standard definition)
formats, and 480i is roughly equivalent to a normal analog TV picture. When
analog TV shows are upconverted and broadcast on digital TV stations, they're
broadcast in 480p or 480i.
The
720p, 1080i and 1080p formats are HD (high definition)
formats. When you hear about "HDTV," this is what is being discussed
-- a digital signal in the 720p, 1080i or 1080p format.
Finally,
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, as shown below:
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.
Digital
Compression
PREV NEXT
The
idea of sending multiple programs within the 19.39-Mbps stream is unique to
digital TV and is made possible by the digital compression system being used.
To compress the image for transmission, broadcasters use MPEG-2 compression,
and MPEG-2 allows you to pick both the screen size and bit rate when encoding
the show. A broadcaster can choose a variety of bit rates within
any of the three resolutions.
You
see MPEG-2 all the time on the Web on Web sites that offer streaming video. For
example, if you go to iFilm.com,
you will find that you can view streaming video at 56 kilobits per second
(Kbps), 200 Kbps or 500 Kbps. MPEG-2 allows a technician to pick any bit rate
and resolution when encoding a file.
There
are many variables that determine how the picture will look at a given bit
rate. For example:
·
If a station wants
to broadcast a sporting event (where there is lots of movement in the scene) at
1080i, the entire 19.39 megabits per second is needed to get a high-quality
image.
·
On the other hand,
a newscast showing a newscaster's head can use a much lower bit rate. A
broadcaster might transmit the newscast at 480p resolution and a 3-Mbps bit
rate, leaving 16.39 Mbps of space for other sub-channels.
It's
very likely that broadcasters will send three or four sub-channels during the
day and then switch to a single high-quality show that consumes the entire
19.39 Mbps at night. Some broadcasters are also experimenting with 1- or 2-Mbps
data channels that send information and Web pages along with a show to provide
additional information.
Buying
a Digital TV Set
PREV UP
NEXT
If
you go to an electronics store today to buy a new TV set, there are four types
of sets that you will see on the shelf:
·
Analog TV sets
·
Digital-ready sets -
They should be identified as standard definition (SDTV) sets. These TVs are
normally 480p displays with a digital tuner built in. The problem with these
sets is that their maximum resolution is the low 480p SD resolution, so if you
want to watch high-definition TV, you won't be able to use these sets.
·
HDTV-ready sets -
These sets are essentially monitors able
to display 1080i/p resolution in the 16:9 aspect ratio. They may or may not
have tuners built in.
·
Integrated HDTV
sets - These sets have a digital tuner for broadcast
DTV signals integrated into an HDTV display. With the standards changing so
much, you may end up paying for an integrated tuner that becomes obsolete.
The
preferred way to handle HDTV is to purchase the components separately:
·
A 16:9 HDTV
display capable of 720p and 1080i/p resolution
·
A digital receiver
·
An antenna
Since
the HDTV display will be the most expensive piece and will likely last 10 years
or more, buying the components in this way allows you to change the receiver if
you need to. There are currently three types of receivers:
1.
You can purchase a
set-top box and a Yagi antenna to receive broadcast HDTV signals.
2.
You can purchase a
set-top box and a small satellite dish to receive HDTV signals from a
satellite.
3.
You can purchase a
board for your computer that lets you use your hard disk as an HDTV storage
device, along with a Yagi antenna, and use it to receive signals on both your
computer monitor and your HDTV display.
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Source: electronics.howstuffworks
