Parking
Lab squeezes HDTV into standard TV channel
eetimes.com
By R. Colin Johnson
EE Times
(03/05/01, 6:51 p.m. EST)
LOS ALAMOS, N.M. — The Department of Energy's
Los Alamos National Laboratory has announced an
encoding algorithm that squeezes a high-definition
television signal into the existing 6-MHz bands
already allocated to TV broadcasters. Existing
analog TVs can receive the broadcast as usual, but
HDTVs will be able to decode the embedded digital
information for rendering on progressively scanned,
1,280- x 720-pixel displays.
"When you flip back and forth between the original
and our encoded HDTV signal, you can barely tell
the difference, but the best thing is that you can
broadcast it using the same equipment already in
use today," said Los Alamos' George Nickel,
developer of the algorithm.
Congress has mandated that HDTV should be the
broadcast television format of choice by 2006, but
its road map for getting there involves allocating
new frequencies, making consumers buy a new TV to view the new
picture standard, then abandoning the old channels along with every
existing analog TV set.
Instead of requiring HDTV signals to use wider channels than the ones
already serving normal TV broadcasters, the Los Alamos encoding
algorithm works by squeezing the new information into the nooks and
crannies of the old channel — much as color TV was once squeezed
into the black-and-white TV bands.
"It doesn't retain all the information in a native HDTV signal, but it is
compatible with existing broadcasting equipment and receivers," said
Nickel. "It's like the way black-and-white TVs still worked when
broadcasters switched to color. Color could have been broadcast with
more resolution if it hadn't had to stay compatible with the older
black-and-white sets. Similarly, there are 120 million analog TVs that
would still work when receiving a HDTV broadcast encoded using our
method."
Current HDTV broadcast methods are incompatible with existing
television sets, because the picture is converted into digital data and
compressed into a non-image space like a computer file. When viewed
on an analog receiver, the MPEG-encoded digital images look like
"snow."
The down side
A drawback of the new method is that it requires additional processing
time that is not necessary in native HDTV transmissions. For example,
the Los Alamos encoder must still use the same inefficient
synchronization signals required by analog TVs.
Those signals contain "dead time," which the encoder must leave
untouched. That's because the electron gun in an analog TV scans to
the end of a line, then takes a few microseconds to reset itself to the
beginning of the next line. Since analog TVs have no memory, the
signal itself must momentarily pause — and take up dead time — to
allow the electron gun time to reset.
By contrast, digital TVs buffer up the incoming signal in internal
memory before sending it to the screen, so that native HDTV
transmitters can use the dead time to send more information rather
than pausing.
Nevertheless, the researchers still claim that a normal HDTV signal
using 0.5 bit/pixel will appear to be only a slightly lower-definition 0.4
bit/pixel; 80 percent of the high-definition signal is retained.
The lab's encoding method uses 5 MHz of the allocated 6-MHz band,
unlike broadcasts today that use only 4 MHz, Nickel said. The signal
includes all the usual synchronization signals for National Television
System Committee (NTSC) color broadcasts, including the blanking
signal — the black stripes between TV frames — and sends it
unaltered as usual.
The extra information for the higher-resolution display is encoded in
the existing image space, but the lab is tight-lipped about the details.
All it will say is that about half of the fine details are encoded in the
letterbox bands and the other half in the vestigial video sidebands.
"There is about a half a MHz of unused bandwidth in the letterbox,
and about a half a MHz in the vestigial sidebands, and we make use of
them both to encode the high-definition information," said Nickel.
To analog-TV viewers, the extra information will appear to be encoded
within the black bands at the top and bottom of the screen — the
so-called letterbox. Viewers will be able to tell when there is
information in the letterbox because it will be gray instead of black.
"We are telling people that the information content in the picture
using our algorithm will contain about 80 percent of that from an
unencoded HDTV transmission, as measured in bits per pixel, but you
will also have a gray letterbox instead of black," said Nickel.
Existing HDTV broadcasters have had to simulcast two distinctly
different signals on two separate channels, so that regular analog TVs
get an entirely different data stream from HDTVs. By compressing the
HDTV signal and disguising it to appear as a gray letterbox, the lab's
approach will let broadcasters send the signal over old broadcast
channels.
With added software, new digital televisions will be able to decode the
additional high-definition information to enhance their display. Existing
HDTVs will also only require a small software change to correctly
receive the Los Alamos signal.
To utilize the extra bandwidth in the vestigial sidebands, however,
broadcasters will have to make some slight hardware alterations to
their transmitters, since they do not currently put information into the
vestigial part of their channels. But the alterations will not require
them to buy new transmitters or build antennas. Existing NTSC
receivers will not even see the data in the vestigial sidebands, since it
is sent in quadrature with the carrier wave for the video signal,
thereby making it invisible to analog TVs.
Roots in nuclear test
According to the lab, the technology used in the compression
algorithm was initially invented there for processing images from
nuclear tests. A patent filed in November 2000 permits the lab to
license its encoder to television broadcasters and broadcasting
equipment manufacturers.
"Only 30 percent of the U.S. population is even familiar with HDTV,"
said Kathleen Herrera, a technology licensing specialist at Los Alamos.
"When the time comes to convert broadcast formats, our encoder will
ease the transition, enabling broadcasters to meet the Congressional
mandate without forcing everybody to go out and buy a new TV." |
