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.
|Mark Aitken, director of technology for Sinclair Broadcast, helped coordinate the station group’s mobile DTV demonstration at the NAB Show. |
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!
|Harris’ Maxiva UHF transmitter uses high-power LDMOS amplifiers developed in cooperation with Freescale. |
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 firstname.lastname@example.org