Additions to standard promise robust HDTV, better coverage
WASHINGTON
Two additions to the ATSC suite of standards-Distributed Transmission and Enhanced 8-VSB-could give DTV broadcasters greater flexibility and options to achieve better market coverage and more reliable over-the-air reception.
Although the two additions to the standard aren't expected to be formally approved by the ATSC until mid-summer (followed by FCC review), transmitter vendors are already preparing to offer their customers the necessary equipment modifications.
Distributed transmission enables broadcasters to construct Single Frequency Networks (SFNs) using multiple synchronized transmitters to better replicate their existing analog service area with a digital signal, according to Mark S. Richer, president of the Washington-based Advanced Television Systems Committee. The "candidate" standard provides the methodology to synchronize transmitters in a distributed-transmission SFN.
Enhanced 8-VSB (also referred to as "E-VSB") gives broadcasters the flexibility to reduce the bit-rate and provide two separate data streams, with a more robust signal relative to noise threshold. "The robust mode could be used to provide new services while simultaneously providing services such as HDTV to existing receivers," says Richer.
ATSC is considering the use of new video and audio coding technologies that may be used with the robust mode. These technologies promise solutions to DTV transmission problems, but broadcasters already over-burdened with capital investments in transmitter equipment may find themselves having to spend yet more money to modify or expand their existing installations to support these new options.
ALTOONA OR BUST
One broadcaster, public station WPSX-DT, in Clearfield, Pa., has already obtained an experimental license to use a distributed-transmission network to solve its DTV transmission problems. (See Doug Lung's RF Technology column, "Single Frequency Networks," Feb. 5.) Although WPSX didn't have a problem serving its entire coverage area from its Channel 3 VHF analog station, when the FCC allotted their digital operation Channel 15 UHF, that made it difficult for the main transmitter to reach target markets blocked by mountainous terrain, including Johnstown, Altoona and State College, Pa.
"Since UHF is a higher frequency, it doesn't propagate as well as VHF, making it difficult to get the signal up over the mountains and trees," says Rich Schwartz, director of marketing and product management for transmitter maker Axcera, in Lawrence, Pa. So by installing a distributed-transmission network, which synchronizes the signals from several DTV transmitters operating on the same channel, a station is able to boost the signal over challenging terrain to reach the desired population centers within its market.
Axcera offers a distributed-transmission solution with its DXA2B Adaptor. In the WPSX configuration, the DXA2B (in the main transmitter in Clearfield) provides SMPTE 310 streams to two "slaved" DT2B modulators, which then distribute a signal that can be received by both legacy and advanced receivers.
Axcera also developed an RF watermark system-based on another part of the candidate standard being considered by the ATSC-that inserts a low amplitude symbol sequence into the 8-VSB signal to identify which digital transmitter is broadcasting it. This allows field test equipment to identify which slave transmitters are being received and to resolve the separate impulse responses of each slave transmitter at a particular receiving location. The RF watermark may also be used to identify transmitters that are well below normal reception threshold or a source of co-channel interference.
"Distributed transmission allows you to lock transmitters together so they're all broadcasting exactly the same data," says Schwartz. "This precise synchronization significantly increases the chances that multiple signals will fall within the equalizer window of receivers located between multiple transmitters. This allows the receivers to reject the weaker signals and lock onto the strongest."
With distributed transmission, broadcasters that would prefer to use low-power DTV transmitters to save energy could set up an SFN using several low-power transmitters. These can be positioned close to densely populated sections to focus their transmission for the biggest audience.
"Normally, for broadcasters to take advantage of distributed transmission or E-VSB, they'd have to change their transmitter's exciter," adds Schwartz. "But since our solution is software-based, we're able to easily modify our exciters to support these proposed improvements to the ATSC standard."
NOT A SLAM DUNK
"Neither of the [ATSC] proposed changes is a 'slam-dunk' fix for all transmission problems. But each can improve over-the-air reception, especially when used in tandem," says Brett Jenkins, vice president of Engineering for Thales Broadcast and Multimedia in Southwick, Mass. "Should broadcasters wish to broadcast E-VSB on a distributed-transmission network, E-VSB must be used at each of the transmitters for a uniform, synchronous signal."
Thales offers its Adapt exciter, which could be modified to support E-VSB for a small cost. Also, the Thales Affinity Series DTV Solid State UHF transmitter, a low-power DTV transmitter of 50 W to 1 kW, is ideal for distributed transmission. "Customers considering distributed transmission must factor in the cost of extending their STL to all the transmitters on the network," says Joe Turbolski, Thales' director of marketing.
Chatsworth, Calif.-based KTech is also closely following E-VSB and distributed transmission. "Our hardware is ready to accept and provide the new 8-VSB technology," says President Steve Kuh, of E-VSB. "KTech will be ready to upgrade in the field when and if such enhanced services are approved by ATSC."
GREATER RELIABILITY
"The intent of E-VSB is to give broadcasters the option to devote a portion of their 19 Mbps channel to a lower data-rate stream that's been encoded in a more robust fashion for lower signal-to-noise thresholds and more reliable reception," says Joe Seccia, product manager for digital transmission products at Harris Broadcast in Mason, Ohio, which has been active in the T3/S9 committee currently evaluating E-VSB. "In a practical sense, if environmental noise interferes with the over-the-air reception of an HDTV signal, the receiver could then defer to a more robust SDTV version of the same programming that's been multiplexed into that transport stream," he says.
The implementation of E-VSB is relatively simple for broadcasters, mainly involving software changes to the company's third-generation Apex ATSC exciter, which also serves as the platform for distributed transmission, according to Seccia and David Glidden, Harris' director of TV transmission products. "Customers can also employ our Flexicoder MPEG encoder to encode the signal according to E-VSB, then send that output to their STL chain," says Glidden.
Seccia and Glidden add that if the broadcaster wanted to devote 4 Mbps of the 19 Mbps channel to a more robust, redundant SD signal, that would not leave 15 Mbps for a HD signal, because some capacity is needed for management overhead. But the Flexicoder, with its encoding and algorithm enhancements, can produce a good HD picture at only 14 Mbps, they say.
NOT ROCKET SCIENCE
For Acrodyne Industries Inc. (Ai), the technology needed to support distributed transmission is very similar to the translator networks that broadcasters have used for decades to re-transmit their signals to small, remote communities that otherwise get poor reception from the main transmitter.
"We can make this simple for broadcasters," says Mark Polokov, national sales manager, for Ai, in Baltimore. "The key is to set up the DTV transmitters and antennas properly, and to lock to an external 10 MHz signal with a GPS reference from a satellite system-which is accurate to within 1 Hz over a month's period of time." (Polokov says this concept was first presented by independent consultant/engineer Merrill Weiss to an IEEE conference last year.)
Polokov explains: "If you have two digital channels next to each other in the same market, for example Channels 23 and 24, by using an external 10 MHz GPS reference, the two could avoid interfering with each other."
Although the expense can be kept down by using low-power DTV transmitters, Polokov also added that it's important to use high-quality systems, such as the Rohde & Schwartz SV7000, a low-cost, compact "tabletop design" UHF Solid State digital transmitter, with average UHF DTV power of 75 W to 300 W.
"There used to be so many holes in the UHF spectrum that you never had to worry about drifting into someone else's channel," he says. "But now, with double the amount of channels, there are a lot of adjacent channel problems, so you have to ensure that your system never drifts."
