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

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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