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LATER THIS month CBS, the American TV network, will broadcast the first
major drama that it has recorded in a high-definition TV format. But few
people will be any the wiser when The Littlest Victims, a dramatised documentary
about babies with AIDS, is transmitted on 23 April. Before the broadcast,
CBS will have to convert the programme from an HDTV format to the national
standard so that people can view it in their homes on conventional TV sets.
This means transforming high quality pictures made up of 1125 lines interlacing
60 times a second, similar to the Hi-Vision system developed in Japan, to
the traditional format of NTSC, which uses pictures of 525 lines interlacing
59.94 times a second (see feature, ‘Confused signals blur the future of
television’, this issue). And the way things are going, it may be years
before HDTV signals are transmitted directly into American homes. The causes
of the delay are as much political as technological.

American electronics companies are worried that foreign competitors
might capture a potentially large market for HDTV. They earlier lost their
dominance in making radio and TV receivers, and missed altogether the two
newest home electronics products, video recorders and compact discs. The
industry believes that success in HDTV is critical for its future.

Congress and three industry groups are investigating these fears. The
House Telecommunications and Finance Subcommittee, chaired by Edward Markey,
a Representative for Massachusetts, has held a series of hearings since
October 1987. The goal, explains one of Markey’s aides, is to ‘drag some
sort of consensus from the industry’ on how to develop HDTV in the US and
how to ensure that as many American firms as possible are involved in the
programme.

Some companies want the government to support the development of HDTV
and nurture the domestic industry. ‘Active government involvement is necessary,
indeed critical, to assuring that the US is able to participate fully in
HDTV and related technologies,’ says Jerry K. Pearlman, chairman of Zenith
Electronics, the only remaining American producer of TV sets. Pearlman also
wants stronger laws to prevent overseas companies from selling their products
at artificially low prices in the US, and he urges tougher enforcement of
existing legislation.

Zenith is a member of a group of 27 companies that was formed in January
this year by the American Electronics Association to develop, fund and manage
research on HDTV. Other participants include Apple Computer, Digital Equipment,
Hewlett-Packard, IBM, ITT, Motorola and Texas Instruments. The partnership
is drawing up a business plan, and hopes to get some funds from the government.
The association, which represents 3500 firms with products ranging from
semiconductors to mainframe computers, warns in a submission to Markey’s
subcommittee that ‘failure to participate in (the advanced television market)
will make US national security dependent upon the availability of technology
capability controlled by other nations’.

The Department of Commerce and some members of Congress seem sympathetic,
and the Pentagon appears to think that HDTV might have a military role.
Its main research agency, the Defense Advanced Research Projects Agency,
announced last December that it plans to invest $30 million in HDTV technology
over the next two years. Half will go to display technology, including cathode-ray
tubes, projection displays, back-projection systems and solid-state flat
panel displays. The other half will go on processors to receive HDTV signals,
manipulate them, and convert them into a form for display. Noting that ‘computer
technology and video/audio technology are converging’, DARPA wants to explore
both analogue and digital processors. The agency has received 87 proposals
from researchers keen to spend the money, including two from Sony of Japan.

Some people in the industry view standards as one way to preserve the
domestic market for American firms. This may be the effect of last September’s
judgment by the Federal Communications Commission. The commission announced
that HDTV broadcasts should fit within the present 6-megahertz bandwidth
of transmission and that they must not make existing TV sets obsolete. These
decisions appear to rule out both the 1125-line format of the Japanese Hi-Vision
system, which requires a bandwidth of 8 megahertz, and the European MAC
system, which is designed for the 12-megahertz bandwidth of satellite broadcasting.

In through the back door

But two important groups, the American National Standards Institute
and the Society of Motion Picture and Television Engineers, have already
accepted a system for studio production of HDTV programmes that is similar
to the Japanese format: the system uses pictures of 1125 lines interlacing
60 times a second. But the move does not indicate support for the Japanese
format, says Alex Shapiro, a New York consultant and spokesman for the two
groups. ‘The production standard does not need to be and probably should
not be the same as the transmission standard.’ Both American and Japanese
companies make 1125/60 production equipment. Other production standards
have been suggested, including the European 1250/50 format and a system
based on pictures of 1050 lines (twice the NTSC standard) interlacing 59.94
times a second. This approach is supported by North American Philips, a
manufacturer, and the National Broadcasting Corporation, a major American
TV network. While NBC and Philips claim the 1050/59.94 format is the most
compatible with the current NTSC system, Shapiro says that 1125/60 is easily
convertible to 1050/59.94 for distribution.

Others disagree. Lynn Claudy, HDTV staff engineer at the National Association
of Broadcasters in Washington DC, says conversion is possible, ‘but it takes
a lot of technical tricks’. Converting the number of horizontal scanning
lines is easier than making a small change in the rate of projection, according
to Ben Crutchfield of the Advanced Television Test Center in Alexandria,
Virginia, which is sponsored by the broadcasters. Japanese companies have
developed equipment for converting signals from one format to the other,
but early versions cost hundreds of thousands of dollars apiece. Some observers
believe those prices will come down; others are not convinced.

The Electronics Industries Association, also based in Washington and
whose members include foreign-owned companies, lists 23 proposals for broadcasting
advanced television systems. Many proposed HDTV formats generate pictures
made up of 1050 lines that interlace 59.94 or 60 times a second. Those formats
are different enough from the Hi-Vision and MAC systems to require conversion.
The differences are not to make life difficult for Japanese and European
manufacturers, but are because the new formats must be compatible with the
NTSC system, and NTSC is incompatible with Hi-Vision and MAC. One option
is to try to improve the transmission of the NTSC signals to make better
use of their 525 horizontal lines; in practice, most NTSC signals, and the
625-line ones outside the US and Japan, do not exploit the potential of
the system. ‘Improved definition’ TV sets, which are just appearing in shops
in the US, Japan and Europe, store picture information. This allows them
to double the rate at which picture lines interlace on screen to create
the illusion of images in greater detail and clarity. Other options include
broadcasting a supplementary signal that could be combined with current
or upgraded NTSC signals for HDTV, or simultaneous transmission of separate
NTSC and HDTV signals. Each system has its attractions and proponents.

The David Sarnoff Research Center at Princeton, New Jersey, has proposed
a two-step approach that would first enhance NTSC, then add a supplementary
signal to generate a true HDTV signal. The centre, which was formerly known
as the RCA Laboratories, is a subsidiary of SRI International, a consulting
firm based in Menlo Park, California.

Using a $3 million grant from the NBC TV network and 208 affiliated
stations, the centre has come up with an Advanced Compatible TV (ACTV) system
that doubles the number of picture lines of the NTSC standard from 525 lines
to 1050 lines, and increases the ratio of the width to the depth of the
picture on screen, the aspect ratio, from 4:3 to 5:3 or 16:9. The centre
demonstrated a prototype of its ACTV system in October 1987, which it is
still developing.

For the first phase, known as ACTV-1, a studio camera records high-definition,
wide-screen pictures in digital form, which an electronic circuit strips
of fine detail. This leaves a picture signal that is compatible with existing
NTSC equipment. Some extra information is also transmitted on an additional
signal. Conventional NTSC TV receivers ignore this additional signal while
ACTV receivers use it to stretch the picture and enhance detail. The second
stage of development, ACTV-II, is a system of two channels. One channel
carries the ACTV-I signal and the other carries more picture information.
ACTV-II receivers will tune into both channels simultaneously and combine
the two sets of information to display HDTV on a wide screen with 1050 lines.

Several schemes rely on broadcasting HDTV or supplementary signals on
unused, or ‘taboo’, channels in the present television spectrum. NTSC signals
are noisy: they spill beyond the boundaries of their channels and cause
interference. To minimise this effect, engineers separate active stations
in any area with ‘taboo’ channels, which they do not use in that area but
which they can use in cities beyond about 150 kilometres away. HDTV developers
say they can transmit primary or supplementary signals in these ‘taboo’
channels without causing interference. One reason is that the signal formats
differ so much that interference is unlikely. Another is that HDTV signals
require much less power. The NTSC format was developed in the days of vacuum
tubes, when frequencies could easily drift from their nominal oscillations.
The accurate display of a video signal requires synchronisation, between
the receiver and the transmitter, of the rates at which images are scanned.
Pioneers of NTSC used synchronisation signals to keep vacuum-tube sets working
properly, and these signals accounted for much of the power needed for NTSC
broadcasts; semiconductor circuits, on the other hand, are much more stable
and do not require as much synchronisation. (People who remember vacuum-tube
sets will also remember the need to adjust them precisely to keep the picture
from fluttering; semiconductor circuits are so stable that many modern TV
sets do not even have the knobs to make the adjustments.)

Making the most of ‘taboo’ channels

Philips Laboratories in Briarcliff Manor, New York, run by North American
Philips, a subsidiary of Philips of the Netherlands, has an approach that
‘does not touch or damage’ the 6-megahertz NTSC signal, explains engineer
Michael Tsinberg. In its High Definition System for North America, HDS-NA,
Philips plans to transmit a separate 3-megahertz signal, at much lower power
than that required for the complementary NTSC signal. The supplementary
signal has such low power that TV stations could broadcast it in the ‘taboo’
channels not used for NTSC transmission. Philips demonstrated the system
for broadcast and cable television in April 1987; by December last year
it had developed a complementary satellite system. Conventional TV sets
would receive only the NTSC signal of the Philips HDS-NA system. HDTV sets,
on the other hand, would contain a second tuner to receive the 3-megahertz
supplementary signal. Circuits in an HDTV set would combine the NTSC signal
with the supplementary signal to generate the added lines for a high-definition
display of a NTSC channel. The decoded HDTV format would show 1050 lines
in a 16:9 aspect ratio, and deliver up to four channels of digital sound
of the same quality as that from compact discs.

The short bandwidth offers a number of advantages. Two of the 3-megahertz
signals, for two HDTV programmes, could fit into one ‘taboo’ channel, which
would ease transmissions in urban areas. And with the airwaves already crowded,
the Federal Communications Commission seems more likely to endorse an HDTV
system that needs only a supplementary signal of half the bandwidth of standard
TV. Like NTSC, picture lines of the Philips HDS-NA system would interlace
images on screen 59.94 times a second. The same format could be used for
the transmission of cable television. For satellite transmission, where
compatibility with NTSC is unnecessary, Philips would use a signal with
a 9.5-megahertz bandwidth.

Zenith has proposed its own Spectrum Compatible HDTV System, which would
transmit a 6-megahertz HDTV signal in a ‘taboo’ channel that a special receiver
would pick up: broadcasters would continue to transmit NTSC signals separately.
One unusual aspect of Zenith’s approach is its shift from the interlaced
display, which is standard in present TV sets and most proposed HDTV formats.
NTSC signals normally scan every other line of the screen in sequence in
1/60th of a second, then they scan the alternating (interlaced) lines in
the next 1/60th of a second, painting the entire 525-line screen in two
successive passes. The eye does not notice the interlacing; it sees a steady
picture made up from 525 lines, which changes 30 times a second; the set,
however, is displaying 262.5 lines every 1/60th of a second. Signals in
the Zenith format scan a 787.5 line screen, one line after the other, at
59.94 frames a second; there is no interlacing. The company says the result
is as good as an interlaced display of more than 1000 lines.

For transmission, the Zenith system separates picture information into
two parts; it distinguishes between frequencies above and below 200 kilohertz
within a total bandwidth of 6 megahertz. Higher frequencies (above 200 kilohertz),
which account for less than 1 per cent of a signal, will be compressed and
transmitted in analogue format, which requires less bandwidth. The more
powerful, lower frequencies (below 200 kilohertz), will be digitally encoded;
although this requires more bandwidth, it reduces interference and demands
less power for transmission. This way, says Zenith, an HDTV station would
need only 0.2 per cent of the power of an NTSC station to cover the same
area. Because the Zenith format for HDTV differs from the NTSC format and
its power demand is so low, the company says it could transmit the signal
in a ‘taboo’ channel without interfering with adjacent NTSC stations.

Broadcasters and the FCC are now preparing to evaluate the proposed
systems for HDTV. It will not be easy, says Ben Crutchfield, programme officer
at the Advanced Television Test Center, which plans to open its test laboratory
in October. He says comparison will be difficult because some systems require
inputs of different video formats, for instance 1050 lines or 1125 lines,
and some need conversion equipment that does not exist. Viewing pictures
from different systems on different receivers, which may be necessary because
of the range of formats, will make it harder for researchers to decide if
the system or the receiver is being tested. So far, the cable television
industry has been prepared to wait for broadcasters to decide upon a standard.
However, unlike terrestrial broadcasters, cable companies do not have to
wait to be allocated space in the airwaves. And they are not likely to wait
indefinitely if the Federal Communications Commission and broadcasters cannot
reach agreement on a standard.