DIGITAL TELEVISION: THE
FACTS AND NOTHING BUT THE FACTS Peter H. Putman, CTS, ISF
Digital television
is here, and it's going to change the way you watch and interact
with your television. Datacasting, HDTV, Dolby surround soundtracks,
and multiple sub-channels of programs are all part of the
DTV revolution.
There's no end
of confusing information being circulated about DTV. Perhaps
you've heard about the "18 different HDTV standards" or the
fact that your old analog TV "will be obsolete as of 2006."
You're probably wondering just how much money you'll need
to spend to enjoy DTV. Maybe you aren't sure if the end result
will be worth all the time, money, and aggravation.
To help you get
started, this guide will answer the most common questions
about digital television and HDTV, and help you cut through
all the "hype" and "clutter".
What exactly
is digital television?
DTV is a new all-digital
system for transmitting, receiving, and viewing higher-quality
television images and stereo (surround) audio.
What are the
goals of digital television?
To offer better picture and
sound quality through the use of digital signal processing,
and to allow the introduction of new digital services such
as multicasting and datacasting. Another goal is to transmit
multiple streams of video on a single channel. The FCC has
mandated that DTV signals be freely available over the air
in every locality nationwide that has analog television.
How do current TV broadcasts
and DTV compare to each other?
There are some similarities.
Both use VHF and UHF broadcast frequencies, although some
stations may wind up switching their channels after the full
implementation of DTV.
While analog and digital television
broadcasts have a modulated carrier wave, the way that signal
is modulated is quite different. While analog TV uses an amplitude/modulated
(AM) signal for pictures and frequency modulation (FM) for
audio, DTV signals use digital "packets", or bursts of data,
to transmit pictures and audio. Four modulation systems are
currently being used for DTV:
* The United States
uses Eight Level Vestigial Sideband (8VSB)
*Europe, parts
of South America, and much of Asia and Australia/New Zealand
use Coded Orthogonal Frequency Division Multiplexing (COFDM).
Japan has its own variation of COFDM.
· Digital cable
television uses Quadrature Amplitude Modulation (QAM).
· Satellite DSS
systems use Quadrature Phase Shift Keying (QPSK).
I've heard
that all television broadcasts must be digital by 2006. True?
The FCC's plan for DTV implementation commenced in November
1998 with 26 "pioneer" DTV stations. DTV stations representing
the top four networks in the top 10 markets were all supposed
to be on-air by May 1999, with an additional 119 by November
1999. At this writing (4/02) there are about 270 DTV stations
broadcasting.
All commercial
DTV stations are supposed to be on-air by May 1, 2002, but
over 850 have applied to the FCC for an extension of the deadline.
On January
1, 2007, all current analog commercial TV broadcasts are
scheduled to cease, provided 85% of homes are capable of receiving
DTV. Given the slow pace of the digital TV transition, that
deadline is likely to be pushed back a few years.
What will
happen to my old analog TV set after the transition?
Good news! You'll still be
able to use it, if it has AV inputs. All current DTV set-
top boxes provide at least one downconverted video signal.
This is usually composite video (sometimes S-video too), which
plugs into the AV inputs on your TV set. It is expected that
newer and cheaper set-top converter boxes will continue to
come to market and support older analog TV sets.
I'm confused
by all the new DTV standards. Can you explain them?
It's not as confusing
as it looks. Actually, there are four basic DTV picture resolution
standards. The additional options have to do with how fast
the pictures are refreshed and scanned on your TV screen.
Our current NTSC
analog TV system makes pictures with 525 total scan lines
(about 480 visible) from the top to bottom of the TV screen
that are refreshed 30 times per second. These are actually
interlaced (alternating) scan lines -half the picture is traced
in 1/60 of a second, and the other half in 1/60th of a second.
The DTV transmission
standards do include a digital version of this 480-line, 30
Hz signal known as 480i. These scan lines can also be traced
progressively, like a computer monitor. This 480p system results
in a picture with fewer motion artifacts and no visible flicker;
as if you were using a line-doubler.
Both 480i and
480p digital signals are considered to be Standard Definition
Television (SDTV), for the images they present are not transmitted
with any more vertical lines of picture information than the
current NTSC system, and will often have the same aspect ratio
(4:3). However, 480p images will appear to have improved vertical
picture detail over 480i images, thanks to the use of progressive
scanning
Only two DTV
picture scan rates -- 1080 interlaced (1080i) and 720
progressive (720p) -- are considered High Definition
Television (HDTV).
In the 1080i
system, 1080 picture scan lines are traced from top to bottom
as interlaced fields (540 lines in the first field, and 540
lines in the second). There are 1920 pixels (picture-forming
elements) on each line. Therefore, the total image resolution
is 1920x1080, or just over 2 million pixels.
The 720p system
scans 720 picture lines from top to bottom in 1/60th of a
second, to eliminate flicker. There are 1280 pixels on each
line, resulting in a total image resolution of 1280x720 or
921,600 pixels.
In the same time
interval that one-half of a 1080i image is shown -- or about
1 million pixels -- all of the 720p image will be scanned,
or just under 1 million pixels. For this reason, proponents
of the 720p system claim that it has the same perceived image
resolution as the 1080i system. Indeed; tests have been done
to verify this equality in perceived image detail.
OK, but where
does the 'digital' part come in?
All of the
program information - video, audio, time, and program guides
- is sent out as compressed data, using the MPEG-2 compression
system. These packets of data are received and decoded by
your digital set-top receiver or integrated DTV set. It's
just like listening to a CD or watching a DVD, only the data
rate is much higher.
Which networks
use which system?
CBS television
and CBS affiliates broadcast 1080I, as do NBC and its affiliates.
PBS affiliates have also adopted the 1080i standard for prime-time
shows, using multicasting of SDTV channels in the daytime.
Subscription
services HBO and Showtime use the 1080i format
to broadcast movies and special programs. A new DirecTV service,
HDNet, features sports and other programming in 1080i.
ABC Television
and its affiliates are broadcasting 720p. WB Network
has also announced it will broadcast prime-time shows in the
720p format.
Fox stations
are currently broadcasting 480p digital television,
with no immediate plans for any carriage of high-definition
TV signals.
You mentioned
"multicasting". What does that mean?
Multicasting
is a process whereby more than one program (called a minor
channel) is transmitted by a broadcaster on the same DTV channel.
The maximum data rate for digital TV is 19.39 megabits per
second (Mb/s). How each broadcaster chooses to use the space
in that digital "shoebox" is up to them.
CBS, ABC, and
NBC O&O stations use the maximum data rate for their HDTV
broadcasts. Some stations put two minor channels into one
data stream. PBS transmits four SDTV minor channels each day,
allocating about 4 Mb/s to each channel. At night, they can
switch to a pair of channels - one SD, and one HD.
Do I need a
special TV set to watch DTV?
Depending on the
TV set you have now, you may be able to watch DTV simply by
adding an external DTV receiver, or set-top box (STB) New
models with a simple interface are coming to market for under
$400.
* If your TV
is a "multimedia" type with progressive-scan inputs for connection
to a computer display card or DVD player, you'll be able to
see some DTV signals by adding an set-top box receiver (STB).
Depending on the set's scan rate, you'll have compatibility
with 480p at minimum, and possibly 1080i and 720p.
* If you have
a newer set with component video inputs (marked "RGB" or "YPbPr"),
you'll be able to watch all DTV formats. However, you won't
be able to watch true widescreen TV unless your TV set has
a widescreen picture tube or projection screen. Widescreen
DTV signals will appear as letterboxed images on 4:3 DTV sets.
Is the difference
in image quality that much better in a digital broadcast?
Absolutely! Just
moving from an analog TV system that is subject to pulse noise,
ghosts, color errors, and "snow" to an all?digital system
makes a tremendous difference. And it gets even better as
image resolution is increased AND the picture aspect ratio
becomes wider.
Many people have
commented that watching HDTV is like "looking through binoculars,"
"looking at a photograph", or "actually being there". That's
how good the picture quality is.
What will
it cost me to get DTV and HDTV?
Right now, a DTV set-top box
will cost from $400 to $800, depending on the model. DTV-compatible
direct-view TVs range from $1,700 to $4,500, while projection
TVs with DTV inputs start at $2,000 and go up from there.
You can also
watch DTV on a front projector. LCD and DLP models range from
$3,000 to $12,000, while CRT projectors range from $9,000
to $35,000. Plasma TVs will also show DTV signals, but these
are costly. The cheapest start at just under $6,000 and run
as high as $20,000 to $30,000. (All prices given as examples
are suggested retail.)
At least three
companies manufacture DTV tuners that plug into a desktop
PC, and use its monitor (or an external monitor or projector)
to show DTV and HDTV signals. These tuners retail for about
$400.
If I buy an
HDTV/ready set today, will it be obsolete tomorrow?
That depends on
whether you buy an "all-in-one" integrated DTV set, where
the tuner is part of the set. More and more integrated HDTVs
are coming to market each year. Otherwise, you could buy a
separate TV and set-top DTV tuner. This way, the most you'd
have to do in the future is to switch to a new set-top box
if you wanted other services, and not replace your entire
DTV set.
Will I be
able to receive DTV in my area?
The FCC has mandated
that DTV broadcasts cover existing analog TV service areas.
In many cases, a small antenna will be all you need to receive
the signals.
If you are on
cable, you may have to wait a while until your cable system
carries the DTV signals. "When" and "How" is up to the cable
system operator, and the cable DTV modulation system (QAM)
won't work with most over-the-air DTV set-top receivers.
If you have satellite
TV (such as DirecTV and Dish), you'll be able to watch selected
DTV broadcasts by selecting a satellite?ready STB and subscribing
to the appropriate service package.
Are DVD and
HDTV products compatible with each other?
As stated earlier,
DTV sets with 480p inputs for DVD players will also be able
to show 480p broadcasts, and may also be able to show 720p
and 1080i. The component video formats are almost the same.
DVD interlaced component video connections are usually labeled
"YCbCr" or "YPbPr".
Do VCRs, PVRs,
and other video equipment work with DTV sets?
All DTV sets have
at least one composite input (sometimes two or three), and
at least one S-video input (possibly more). You can connect
your existing VCRs, PVRs, and other accessories to these jacks
as you would with a regular TV. Also, DTVs usually have an
analog TV tuner, so your cable set?top box can also be connected
to your DTV.
What's the
difference between the DTV component video standards? There
are two different ways to connect a component video signal.
The first, identified as "YPbPr", is a three-wire connection.
It consists of one luminance signal marked "Y" and two color
difference signals marked "Pb" and "Pr". This is a pretty
common connection on all Asian-made DTV-compatible TV sets.
The second standard is identified as "RGB". This can be a
three-wire, four-wire, or five-wire connection, and is similar
to that used by your computer monitor. In the four-wire RGBS
and five-wire RGBHV formats, there are three color signals
- Red, Green, and Blue - and either one or two sync signals.
This format has been used by RCA.
Are DTV signals
broadcast on special frequencies?
No. DTV broadcasts
use the same channels as regular analog television. While
many DTV stations are now occupying UHF broadcast channels,
the plan is to allow many broadcasters to move back to their
original VHF or UHF TV channel once the transition to DTV
is complete. During and at the end of the transition, channels
51 through 69 would be auctioned off for other uses.
Why is it
so hard to receive DTV signals in some locations?
When it comes
to digital television, it's an "all or nothing at all" proposition.
Once the signal is acquired, a steady stream of data assures
you'll get a perfect picture and great audio. If that bitstream
is interrupted, however, there will be nothing - just a blank
screen.
In areas with
lots of buildings and multipath, frequent drop-out causes
this "cliff effect" to kick in. The fix is to use a higher-gain
antenna and perhaps even a preamp - assuming the multipath
can be tamed. Testing continues to determine the optimal designs
for antennas, as well as improving error correction in set-top
receivers.
New 8VSB receiver
designs - such as the 'Casper', recently tested by Link Electronics
in Canada - have shown tremendous improvement in handling
ghosts and multipath, making the goal of indoor reception
in urban areas practical.
The key to widespread
rollout of digital TV will be carriage on cable TV systems.
Consider that better than 70% of all US households are getting
television via cable or satellite, and you can see how important
it is for broadcasters and cable/DSS providers to sign carriage
agreements.
Will there
be much to watch on DTV?
CBS carried all
18 of its prime-time filmed shows in 1080i HDTV this past
season (2001), as well as NCAA college football, college basketball,
The US Open golf and tennis championships, and movies.
PBS continues
to run numerous widescreen SDTV shows and a few HDTV programs
each month. NBC offered "Crossing Jordan", "The Tonight Show
with Jay Leno", and the 2002 Winter Olympics in 1080i, and
it's expected that letterboxed NBC shows such as "ER" and
"The West Wing" will follow next season.
ABC presented
all of its fall 2001 scripted prime-time shows in 720p, along
with Dolby 5.1 surround soundtracks.
HBO and Showtime
continue to present a selection of movies and live concerts/specials
in 1080i. Mark Cuban's HDNet carries selected Major League
Baseball, National Hockey League, and National Lacrosse League
games in 1080i, and provided shared HDTV coverage of the 2002
Winter Olympic Games with NBC.
Fox broadcasts
several prime-time filmed programs in widescreen 480p, and
also originated the 2002 Super Bowl in a widescreen 480I component
format, converted to 480p at each affiliate station.
Where can
I go to get more information about a specific DTV topic?
There are many places to look. Here are a few suggested web
sites:
PBS
Digital web site
HDTV
magazine
Roam
Consulting Inc.
Get
The Picture
Copyright ©2002
Peter H. Putman / ROAM Consulting Inc. Used by permission.
All other rights reserved.
Peter Putman is a senior contributing editor
for Primedia Business publications. Over the past two decades,
he has authored hundreds of technical articles, reviews, and
columns for magazines including Video Systems, Sound & Video
Contractor, Millimeter, Entertainment Design, The Perfect
Vision, and EMedia magazines; on topics such as large-screen
projection and interfacing technology, electronic cinema,
HDTV, DVD, and screening/presentation room automation and
design
