UHF
Broadcasters
Are Going Digital
by Ken
Freed.
.
The
pulsing tube atop a high tower is being replaced by
powerful digital technologies.
Chief
engineers at UHF stations in Denver have the same
concerns as engineers at any other ultra-high frequency
TV stations.
"We have old
klystron tubes at both transmitters, and they're big
consumers of electrical energy," says Jim Hollinger,
chief engineer at KDVR, Channel 31 (Fox). "I'd like to
see us convert both to IOT operations, which would cut
our electric bills quite a bit.
Hollinger says
conversion to an inductive output tube (IOT) has not yet
begun at TV31. "We're very much on hold with Advanced TV,
which has a lot of problems to sort out before I'll be
ready to invest in digital equipment, and I see no value
right now in simulcasting the same programming in analog
and digital since there will be so few viewers for quite
a while."
Transmitters with
induced output tubes also interest Mike Dant, chief
engineer at KTVD, Channel 20 (UPN). "The IOT is a more
linear device that allows you to operate with higher
power at less cost." TV20 presently has three klystrons,
two visual and one aural, which consume 10.5 amps at 24
kV. "When we lose an amplifier, we have to shut off water
and other functions just to get up a new one. If we lose
a bunch of amplifiers in a series, the signal goes
out."
Dant reports he had
to power down the system because it used coaxial
switching from amplifier to combiner, a "mechanical
problem" solved with a unitized RF (URF) system. "URF
coax feeds to a series of waveguide pads lets us switch
hot. VHF has similar device, but it's not as massive
because of the power involved on UHF transmitters
operating at 30 kW to 220 kW before you account for
antenna gain."
Transmitter
replacement at TV20 is not out of the question, Dant
adds. "Some of the new solid state transmitters I saw at
NAB look very promising." He mentions the latest Acrodyne
transmitter and the Westinghouse demonstration of ATV
transmitted with silicon carbide drivers on Channel 52 in
Las Vegas.
"I like the idea of
a solid state transmitter," he says, "because silicon
carbide semiconductors are tolerant of high voltage and
high heat. Of course, solid state with 5 megawatts ERP
[estimated isotropic radiated power] has been
around for years, but we haven't had high power until
lately. That's a real advance.
"The good news
about the IOT is that it has a third less current
consumption than previous transmitters," Dant says,
"which cuts overall operating costs as well as the
overall cost of ownership. The bad news is that solid
state transmitters need linear amplifying, and klystrons
don't lend themselves to that very well. The good news,
again, is that there are improvements all the time."
.
Transmitter
Improvements
What specific
improvements may alter UHF operations? Who better to ask
than UHF transmitter manufacturers?
The inquiry can
start in the neighborhood. Larcan TTC is based in
Louisville, Colorado, the town once known for Italian
restaurants that now boasts a growing population of
high-tech companies. Offering their perspectives are
engineering director Alvaro Medina and RF design engineer
Derek Jasnoch.
"The main
development in the future is going to be the solid state
transmitter," says Medina. "We need less power for HDTV
than for NTSC, between 10 kW and 30 kW. Right now it's
feasible to have 10 kW. We'll see 20 kW in six years, I
think, and 30 kW will take a bit longer. The companies
working on silicon carbide systems may be a little
optimistic in their predictions, but what they're doing
is very impressive, and it is going to happen.
"Another important
development," he says, "is the diacrode. Developed in
France by Thomson and now being used here in America by
Acrodyne, the diacrode is a small tube, essentially a
tetrode with new architecture, 17 centimeters in height
and 20 centimeters in diameter, capable of producing 60
kW of combined video and aural power. "This tube, "Medina
says, "will be ideal for HDTV. You can have all the power
you need, so its a very good development. I think the
diacrode is the way to the future &emdash; if you decide
to use tubes."
"I think there's
still a lot of life left in the IOT," argues Derek
Jasnoch. "IOT companies see the diacrode as a threat, and
so they will develop the IOT a lot more. Competition will
improve both technologies, which will only be better for
us manufactures."
"Take digital
modulation, for example." Jasnoch says. "Low-power TV
transmitters can rebroadcast major or small networks
using four to six channels in the 6 MHz of bandwidth on
one NTSC carrier, which gives broadcasters a means for
multichannel competition with cable or MMDS. If your
station has several analog IOT transmitters, with digital
modulation, you could start offering 24 to 60
channels."
What still needs to
happen for UHF stations to go digital?
"We need to lobby
the FCC to approve low-power TV," suggests Jasnoch. "We
already have the technology for digital transmission, but
the set-top boxes still need some work. The boxes being
developed for DBS could be used for UHF, too, instead of
only being set for satellite frequencies. We just need
the FCC not to be so stiff on the
regulations."
Medina sees another
priority &emdash; broadcasters having the capacity for
digital compression. "There's a company developing
compression technology that's using Larcan equipment to
study the feasibility of doing digital compression on
UHF. We sent them our latest 100 kW transmitter for their
tests. The problem is that the exact methodology for
digital compression is still up in the air.
"Come to think of
it," Jasnoch jumps in," what's really going to change in
the transmitter is the modulator now being developed by
companies like Zenith. The idea is to remove the NTSC
modulator and install a digital modulator. Mechanically,
it will be an easy switch. Financially, the DM may be
significantly more expensive, but prices will drop with
economies of scale."
Medina identifies
another improvement. "The new low-power TV transmitters
are introducing digital controls, which will migrate to
high-power TV transmitters. Microprocessors easily can
handle all the information for transmitter parameters
like voltage and temperature. Microprocessor also can
control the gain by monitoring transmitter conditions, so
if anything needs attention, the controls can lower
transmitter power automatically until the condition
disappears.
"And when you
include remote monitoring over phone lines," says
Jasnoch, "using either a Windows or Macintosh interface
with password or key encryption security controls, taking
very good care of a UHF transmitter soon will be much
easier than it is today."
.
An
Informal Survey
What exactly still
needs to happen for UHF to go digital? The question was
posed to six other equipment manufacturers. Their replies
reveal the future.
* Acrodyne
sales and marketing manager, Joe Wozniak, believes the
shift to digital television technology is gaining
momentum. "We've been using tetrodes for high power
transmission of a separate duplex visual signal since
1980, and we've had a 30 kW tetrode UHF transmitter since
1988. We introduced the diacrode transmitter to the
American market in 1995, and this double-ended tetrode
tube doubles the output of a 30 kW transmitter to 60 kW.
It's the world's largest UHF transmitter tube for a
single amplifier, and an excellent solution for HDTV. Now
that the hardware is available, broadcasters are showing
genuine interest in delivery. "
Wozniak explains
that Acrodyne in 1994 introduced an 800 watt solid state
amplifier (500 watts to 10 kW) that's used as driver for
the tetrode. The system has four modules with two to 25
drivers to create a 60 kW transmitter. As an example, he
reports selling a 5 kW, 8-module system to a station in
Medford, Oregon, home town of Scala. The 5 kW system has
an average radius of 20-30 mile with landscape
variables.
Wozniak foresees a
contest between two modulation techniques &emdash; the
Grand Alliance HDTV modulation scheme at 8 VSB or else
digital compression at 64 QAM. "Even though there's no
standard yet approved for terrestrial broadcasting, and
we still need FCC approval to digitize our transmitters,
I can say now that the Acrodyne tetrode already supports
either modulation."
* Advanced
Broadcast Systems (ABS) president Don Adams sees the
IOT and solid state transistors as crucial developments,
but he looks for surprising innovations to come from the
microwave TV industry. "Broadcasters want a technology
proven before they'll deploy it," he says, "but microwave
people seem willing to take more risks."
Adams offers an
example. "Microwave people are marrying higher end
technology to the transmitter by using process logic
controllers and computers to monitor their transmitters,
providing them with better remote control. A side benefit
is data acquisition for long-term trend analysis, which
helps the engineer more than anyone."
"Most of the
excitement in the next few years," he says, "will be
around the exciter. The exciter takes in the digital
transmission and makes a baseband signal to go out on the
air. The exciter converts and modulates the signal,
brings it up to the operating channel. After that, it's
all a matter of brute force amplifiers to bump a 10 watt
signal up to 60,000 watts."
"Work still needs
to be done on the upper-end filter system," Adams says,
"and we're going to have to make the antenna be able to
control group delay since it turns out that digital
transmissions can be degraded by wire or antenna
problems, plus I have concerns about co-channel
interference while both NTSC and digital signals are
simulcast for the next 15 years or so. And the biggest
problem of all is that the transition to digital is going
to cost stations and the consumer lots of money, and how
can any station justify the expense of buying a new HDTV
transmitter without many receivers out there? It's going
to be interesting to see how all this is going to come
down the pike."
* Comark
director of marketing Mark Aitken, who's held various
engineering positions in the company over the past two
decades, expresses excitement about Comark's
participation in the HDTV Joint Venture Team, funded
through the National Institute of Standards and
Technology in the Commerce Department. Other players
include Philips, Thomson Multimedia, Sun Microsystems,
IBM's T.J. Watson Research Center, MCI
Telecommunications, Advanced Modular Solutions, and NBC.
The team is managed by the David Sarnoff Research Center.
"On a cost-sharing
basis in a three year program," Aitken says, "we're
working on technologies to demonstrate all of the
interrelated requirements for a complete digital HDTV
studio and transmission facility. Comark's participation
is centered around the IOT and aimed at digital
correction of nonlinear amplifier systems. We're
confident that our many years of experience with IOTs and
our many years of operating the forefront of HDTV arena
gives us a good deal of inside knowledge of the
technology requirements as we're putting all the
requisite pieces together."
What does Comark
see as the core issues for UHF? "There are no core
technology issues in the way," Aitken replies, "but a lot
of political issues still need to be resolved, like a
channel allotment table that everyone will stand by, and
proposals for more spectrum auctions with the value for
spectrum based on the PCS auction, the cash cow milked to
feed the budget deficit. If not settled sensibly, such
issues could make it exceptionally difficult to migrate
from NTSC to HDTV."
* Harris is
focusing energies on solid state technology, according to
Bob Weirather, director of the television product line in
the Harris Broadcast Division. "The growth and
improvements of solid state transmission is what's
happening out there. Solid state transmitters are
available from 1 kW to 10 kW, which is suitable for
everyone in broadcasting." He announces that Harris has
just sold a 10 kW solid state Ultra UHF transmitter to
KRMA, Channel 6 in Denver. (KRMA's chief engineer, Joe
McGee, says the transmitter will be used for
re-broadcasting the station in Grand Junction, the
boomtown at the western edge of Colorado on the Grand
Mesa.)
"The demarcation
for choosing between solid state and tube transmitters is
around 20 kW," he says, "and acceptance of the
liquid-cooled or air-cooled IOT as the best tube solution
is now pretty much a given. After running lots of tests,
however, we are not convinced that either technology is
head and shoulders above the other, so we're comfortable
with both tubes and transistors."
What really needs
attention, Weirather asserts, is antennas. "What's being
looked at by the industry is a wideband antenna for
multiplex signals, both stacking antennas and panel
antennas with a dipole sticking out. Folks are still
looking for reliable operations with good quality of
sound and pictures. That's why Harris is working with a
group of TV stations in Denver to reconfigure the antenna
array on Lookout Mountain west of the
metroplex."
As for digital
broadcasting, Weirather says, "We're receiving lots of
serious inquiries about digital hardware, not just blue
sky phone calls, but genuine requests for quotes and
delivery dates from stations figuring out next year's
capital budget. So, I think we're just now seeing the
very top of the tip of the iceberg, which is still below
the horizon; we can't quite see the waterline yet, but
sooner or later well be up next to the digital TV iceberg
and realize how huge it is."
* ITS
broadcast product manager Dale Dalesio confirms that
Information Transmission Systems Inc. (recently purchased
by ADC Telecommunications) is pursuing a dual track of
both IOT and solid state transmitters. "If you could
squeeze more power out of a transistor, such as with
silicon carbide, then solid state transmitters would be a
lot more competitively priced, and they could dominate
the market."
Dalesio also sees
politics as the chief stumbling block to digital UHF
broadcasting. "The FCC needs to approve Advanced TV,
Grand
Alliance HDTV, or
some other format, as soon as possible, and then stations
will start investing in digital facilities. Until then,
there's a lot of uncertainty over whether digital
facilities will be forced upon the industry before there
are enough digital receivers available for consumers to
view the digital signal."
In the meantime, he
proposes, "What we may need is a truly affordable
consumer electronics device or box that receives digital
broadcast signals and converts them analog on existing TV
sets."
* Westinghouse
has publicly hitched its wagon to the digital star
and the silicon carbide transistor, according to Paul
DeGonia, director of HDTV/ATV development for
Westinghouse Wireless Solutions, a division of the
Communications and Information Systems division.
Westinghouse wowed the crowds at NAB '96 with a highly
theatrical HDTV demonstration of their silicon carbide
technology with low power UHF broadcasts from a
dielectric antenna mounted on the roof of KLAS (CBS),
located a half mile away from the Las Vegas Convention
Center.
"Everything depends
on what happens with the FCC over the next year or so,"
DeGonia says, and it isn't clear what they're going to
do. With the selection and approval of the Grand Alliance
standardby NAB, we thought the issue was behind us, but
there's still noise in the background saying HDTV may not
be the right thing to do because of concerns about having
flexibility for being able to do other things in the
future, that locking in HDTV now may prohibit
developments in the future. But there are always other
things being developed, so we can't let such thinking
stop us. Nothing will move forward until that fundamental
decision is made on the digital standard. Manufacturers
like Westinghouse can't really do anything until that
issue is settled."
Where is there
hope? "What's encouraging," he answers," is that
broadcasters are indicating a willingness to commit some
money to testing our silicon carbide transistors to see
how they work in the real world, which is an ad hoc way
of getting started immediately. The idea now is to build
solid state transmitters, and we're getting ourselves to
set for full production by mid 1997."
As for UHF itself,
DeGonia reveals the Westinghouse game plan. "We're
thinking that most or all of the VHF stations will
convert to a UHF allocation for digital channels, so
Channel 4 becomes Channel 28. God only knows what will
happen to the VHF bandwidth, which the FCC likely would
auction off when returned for re-allocation. The VHF
spectrum likely will still used for some form of
communications, but it's unlikely to be television. If it
goes that way, there's a clear path ahead to make the
conversion to digital in the U.S., but I have not yet
talked to people overseas to see how they feel about UHF
terrestrial broadcasting. Guess we'll have to wait and
see." 
.
