start issuing licenses for the new
broadcasts. Despite the protest of
some who felt that the new licenses
should be auctioned off (with the
proceeds going to reduce the
national debt), the FCC decided to
give all existing TV stations a second
channel for Digital TV (DTV) broadcasting if they so desired. This
was deemed necessary considering
the million-dollar-plus cost that
each station would incur to make the
transition to digital.
Money was not the only
stumbling block facing a nationwide
implementation of DTV. A great deal
of effort went into the assignment of
new channels (mostly in the UHF
spectrum) to avoid interference
between the new stations, as well as
to reduce interference with — and
from — the existing NTSC stations
which would remain on the air for
many years during the transition.
The exact date for the NTSC
stations to go dark has been changed
several times, mostly due to the huge
task of installing new antennae and
transmitters for over 1,700 existing
commercial TV stations. But the latest
legislation (DEFICIT REDUCTION
ACT OF 2005, Title III — Digital
Television Transition and Public
Safety Act) firmly calls for the end
of most NTSC broadcasting on
February 17, 2009.
Why call it the “Digital
Television Transition and Public
Safety Act?” One of the reasons
for the transition to DTV is to free
up a portion of the UHF band,
which will be given to Homeland
Security for first responders.
When analog broadcasts cease
in the 700 MHz spectrum
(current Channels 52-69), those
frequencies will be used to
improve communications between
police, firefighters, etc. Any
revenue generated from the sale
of this spectrum will be used to
offset federal deficit spending.
off-air recording devices will require a
digital (a.k.a., ATSC) tuner to receive
them free, over the air terrestrial
broadcasts. All televisions sold since
March of this year have been
required to have such tuners built in
(technically, the law only requires
this of sets shipped in interstate
commerce or imported into the
United States). This, of course, is in
addition to the standard NTSC tuner
that is still needed to receive analog
signals until 2009.
Older sets will require a converter
box between the antenna and TV to
translate the digital signals into an
analog signal that the TV can receive.
Starting in 2008, a federal program
will commence to provide $40 vouchers towards the purchase of up to two
converters per household. More on
this later.
So does this mean that all TVs
with a digital tuner or converter box
will display HDTV? Unfortunately, the
answer is NO! Every digital broadcast
can carry one or more programs, each
of which can be in one of 18 different
formats (see Table 1).
Each station broadcasts a 19. 39
Mbit/s (million bits per second)
data stream which can be divided
into any number of different signals
or services. Most video streams
will be either Standard Definition TV
(SDTV) or HDTV; many Extended
Definition TV (EDTV) formats are
defined but none are currently in
common use.
DTV pictures are clearer due to
several factors: no NTSC artifacts
(moiré and chroma crawl), less
interference (static and impulse
noise), and no multipath distortions
(ghosts and airplane flutter). Audio
quality is also vastly superior, with 5.1
surround sound channels widely used.
HDTV
Probably the most exciting part of
the DTV standard is the inclusion of
two basic HDTV formats that allow
much higher resolution images. Both
720p60 and 1080i30 formats are
in widespread use by the major
networks. The other major difference
with HDTV is the wider aspect ratio of
the screen — 16: 9 vs. the old 4: 3. This
widescreen aspect gives a more
“movie theater like” experience and
allows most films to be broadcast
Vertical Lines
Horizontal
Pixels
Aspect Ratio
Frame Rate
1080
1920
16: 9 sq
60i, 30p, 24p
HDTV
720
1280
16: 9 sq
60p, 30p, 24p
480
704
16: 9 ns
60p, 60i, 30p, 24p
EDTV
480
704
4: 3 ns
60p, 60i, 30p, 24p
480
640
4: 3 sq
60p, 30p, 24p
SDTV
480
640
4: 3 sq
60i
NTSC
486
720
4: 3 ns
60i*
Vertical lines = number of active lines in the picture
Horizontal pixels = number of active pixels (picture elements) per line
Aspect ratio = ratio of picture width to picture height/pixel aspect ratio
sq = square pixels; ns = non-square pixels
Frame rate = number of frames (p) or fields (i) per second
p = progressive scanning; i = interlaced scanning
Rates may be based on either 60 Hz or 59. 94 Hz
* = based on ITU-R BT.601 specification
WHAT DOES IT ALL
MEAN?
With the demise of analog
transmissions, TVs and other
1080-line video is actually encoded with 1920×1088 pixel frames, but the last eight
lines are discarded prior to display. This is due to a restriction of the MPEG- 2 video
format, which requires the number of pixels to be evenly divisible by 16.
TABLE 1
October 2007 73
