[opendtv] RF at the 2009 NAB Show - Part 1

From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Date: Sat, 4 Jul 2009 19:40:33 -0400

http://www.tvtechnology.com/article/83442
RF at the 2009 NAB Show - Part 1
by Doug Lung, 07.02.2009

Much of my time at this year's NAB Show was devoted to mobile DTV and ways to
improve DTV coverage. The success of mobile DTV will depend on reliable
coverage.

ATSC mobile DTV was on the air on multiple stations during the convention.
Receivers were on display in several booths and suites, although attendees
looking for bus tours demonstrating system performance and coverage were
disappointed.

I worked with Rohde & Schwarz, Echostar Technologies LLC, Triveni Digital, LG
Electronics, Microwave Radio Communications, NBC's technology group and KBLR,
the local Telemundo O&O in Las Vegas, to set up a distributed transmission
system using KBLR's main transmitter and two low-power transmitters.

In addition to KBLR's HDTV programming, the system transmitted two mobile DTV
channels. One of the channels was a simulcast of KBLR Telemundo programming and
the other was a special feed from CNBC received at the KBLR studios using a
video-over-IP link on the corporate network using equipment from T-VIPS.

LESSONS LEARNED

Perhaps the most important lesson learned from this demonstration was that
synchronized low-power transmitters using vertical polarization can
significantly improve mobile DTV reception in obstructed or congested areas
even when the primary station is predicted to have a strong signal in
surrounding areas.

I'll have more on the demonstration in next month's RF Technology column. As
far as I know, this was the first over-the-air public demonstration of a
distributed transmission system carrying the ATSC candidate standard mobile
stream.

Sinclair Broadcasting transmitted mobile DTV on its Las Vegas stations. These
and other mobile DTV demonstrations included real-time and non-real-time
content and interactive programming developed with MobiTV. Harris
demonstrations included audience measurement opportunities with partners
Nielsen and Rentrak. LG Electronics played a key role in all the
demonstrations, from providing the essential technical assistance in setting up
the demonstrations to providing most of the receivers needed to view the mobile
DTV transmissions.

Sessions and exhibitors described how mobile DTV works and the equipment needed
to implement it. Many of the papers presented in the Broadcast Engineering
Conference focused on mobile DTV and Saturday's IEEE-BTS Technology Tutorial
explained the ATSC Mobile DTV candidate standard in detail.

I was happy to see some exhibitors offering tutorials and technical
presentations in their booths, which offered exhibit-only attendees a chance to
learn about the latest technology.

Axcera presented tutorials in its booth on distributed transmission for DTV and
DTV translators. Richland Towers arranged sessions with antenna manufacturers
Dielectric, Electronic Research Inc., Micro Communications Inc. and Radio
Frequency Systems in its booth to discuss broadband antennas for mobile DTV.
During the annual ERI breakfast its customers got an opportunity to hear
excerpts from the papers ERI's Myron Fanton presented in the Broadcast
Engineering Conference.

Mobile DTV can be confusing. Fortunately, major transmitter manufacturers were
showing complete turnkey solutions. Existing transmitter amplifiers and
antennas will work for mobile DTV, but more transmitter power and a different
antenna design might be needed to optimize coverage.

You can keep your existing ATSC MPEG-2 encoders and multiplexer, but a special
multiplexer is required to combine the ATSC MPEG-2 stream with the MPEG-4
mobile DTV stream. The bandwidth devoted to conventional ATSC has to be reduced
to make room for mobile DTV data.

Mobile DTV cannot be transmitted with conventional ATSC exciters. Several
manufacturers offered new exciters compatible with ATSC mobile DTV and others
offered the possibility that it may be available in the future through firmware
upgrades.

At this time, it is probably safest to purchase the mobile DTV multiplexer and
exciter from the same manufacturer. A separate encoder is needed to create an
ATSC compliant AVC (H.264) compressed program stream.

Finally, ATSC mobile DTV receivers need some basic information to decode the
programs. This signaling data performs a function similar to the basic ATSC
PSIP data. Triveni Digital and DTV Innovations were demonstrating signaling
products for mobile DTV at the NAB Show. Adding interactivity, file transfer
and a program guide requires additional software.

All the antenna manufacturers I talked to felt circular polarization provided
the best signal for mobile DTV. If all other antenna parameters remain the
same, this means transmitter power will have to double to maintain the same
coverage area.

If that isn't possible and providing reliable mobile DTV service is the main
objective, it may be worth reducing effective radiated power in the horizontal
plane to achieve full circular polarization. If transmitter power is limited
and reducing coverage is not an option, mobile DTV broadcasters should aim for
as much vertically polarized power as their budget and transmitter power allow.

DISTRIBUTED TRANSMISSION

Another option for improving coverage is to build out a distributed
transmission system like the one KBLR demonstrated at the NAB Show. Coverage
can be improved in terrain shielded areas through the use of translators or
on-channel digital repeaters (OCDR). While there are several options for
translators (Larcan, KTech Telecommunications, Axcera, etc.), on-channel
repeaters are more challenging. To provide the most power, they need to include
circuitry to reject echoes, including the transmitted on-channel signal.

I was very impressed with Axcera's echo-canceling OCDR. The technology was
developed in-house at Axcera and I was told it is able to reduce echoes
(signals from the repeater's transmitter, including nearby reflections) by 20
dB on average and by up to 40 dB best case. A 50 watt repeater lists for
$17,000 and is available now. Imagine dropping one of these into a community
where you are having reception problems!

Acrodyne Industries (AI) showed OCDR based on the ETRI echo-canceling
technology that I've reported on before. In selecting an echo-canceling
on-channel repeater, specifications to focus on are the delay through the
repeater, its echo canceling capability and the isolation required between the
transmitter and receiver.

To take full advantage of this technology, FCC rules for licensing on-channel
repeaters need to be streamlined. When areas with reception problems are
discovered, broadcasters can't afford to wait months or even years for
applications to work their way through the FCC.

The FCC has already done much to make it easier for broadcasters to fill in
coverage areas lost in the move from a low VHF analog channel to a UHF channel
or to improve signal levels in areas where analog reception may have been
possible, but due to the digital cliff edge effect DTV reception doesn't work.

Streamlining processing for low-power on-channel boosters located inside a
station's DTV service area contour would provide a way for broadcasters to
respond immediately to resolve reception problems without requiring new
spectrum or creating new interference. I'll discuss this in more detail next
month.

Solid state transmitters are now challenging tubes at medium power levels.
Harris' new Maxiva UHF transmitter uses high-power LDMOS amplifiers developed
in cooperation with Freescale to generate 12.3 kW of ATSC DTV power with
equipment (excluding external pumps and heat exchangers required for
water-cooling) that fits in one standard height 19-inch rack!

I was very impressed with the modular nature of the Maxiva amplifiers. Unlike
other solid state amplifiers that require you to send or swap out the entire
amplifier tray or power supply for repair, the individual amplifier modules and
their associated power supplies in the Maxiva are easily changed. If an LDMOS
transistor fails, simply replace it rather than swap the entire amplifier tray.

Axcera managed to fit a 10 kW ATSC transmitter, with N+1 power supply
redundancy, in a standard 19-inch rack. Don't need that much power? Rohde and
Schwarz showed a 900 watt transmitter that will fit into seven 19-inch rack
units or a 400 watt unit that takes up only 4 rack units.

Larcan showed a new exciter for its DTV transmitters made by Pro-Television. It
doesn't support ATSC mobile DTV now, but the Pro-Television representatives
said it is being considered. Unlike the Zenith exciters Larcan previously used,
the Pro-Television exciters include adaptive pre-correction, eliminating the
need to manually transfer tap settings from a compatible VSB analyzer.
Upgrading an existing Larcan DTV transmitter with the new exciter should be
less expensive than purchasing a compatible VSB analyzer.

I'll have much more to report from the 2009 NAB Show in future RF Technology
columns!

E-mail Doug at dlung@xxxxxxxxxxxxxxxx

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[opendtv] RF at the 2009 NAB Show - Part 1

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