What I Want at NAB
This month I'll outline some of the things I'll be looking for at NAB2002. Rather than talk about specific new products, I'll focus on the areas where I'm expecting to see interesting developments. I'll also begin a discussion of spread spectrum technology and show how it can be used by broadcasters.
At NAB2001, RF-related exhibits tended to focus on products and technology promised at past NAB conventions rather than big, exciting new technology innovations. Considering that many stations will be building DTV plants from the ground up this year, the market for DTV transmission and monitoring equipment is huge. I expect to see companies that haven't catered to broadcasters joining existing manufacturers in offering new products for DTV transmission and monitoring.
Now that the FCC is allowing use of less than the authorized facilities to meet the rapidly approaching May 1 DTV deadline, I expect to see more standardized packages for quick implementation of DTV, similar to the K-Tech XMT-100 low-power DTV transmitter with PSIP generator, ATSC encoder and ATSC modulator - but packaged with a simple antenna system. I'm also looking for more reasonably priced, easy-to-use ATSC monitoring systems.
ALL DIGITAL, ALL THE TIME
In the studio transmitter link area, the dual carrier technology used in Microwave Radio's "Twin-Stream" systems has been the most common way of getting DTV signals to transmitter sites. As the cost of digital encoders and multiplexers drop, I expect to see more "all digital" microwave systems offered with the multiplexer and perhaps even an SDTV encoder as part of the package.
"All digital" transmission is now commonly used for satellite newsgathering. While terrestrial ENG has been slow to transition to digital, I expect the pace to accelerate as older ENG vans are replaced and the reduction in 2 GHz channel bandwidth obsoletes older analog microwaves. COFDM has been the modulation of choice for terrestrial digital ENG, but as smaller stations look for lower-cost options, don't be surprised if some 8VSB systems such as those offered by Nucomm start showing up in ENG vans.
Even though it seemed like all possible, practical TV-transmitting antenna designs had been built, over the past few years I've been surprised by new products like the TCI Model 888 wideband slot antenna and the Dielectric combination VHF/UHF slot antenna. It will be interesting to see what's new in antennas this year. In my June column, I'll tell you about products that attracted my interest at NAB2002.
The NAB Broadcast Engineering conference has a number of topics of interest to broadcast RF engineers this year, most of them in Monday sessions.
Looking through the list of papers on the NAB Web site, one stood out. Richard Citta, chief scientist at Linx Electronics, is presenting a paper on a Near Optimal Multipath Combining Receiver at the DTV reception engineering session Monday morning.
The receiver is based on an oversampled offset QAM demodulator and is designed to get around the problems present in existing ATSC receivers using adaptive equalizers with FIR filters. The paper's abstract states, "The new receiver does not cancel ghosts but combines the multipath signals in a near optimal manner." Multipath improves receiver performance instead of degrading it. Other papers in this session include a DTV Progress Report from ATSC and a report on DTV Data Coverage with Translator Enhancements.
On Monday afternoon, Victor Tawil from MSTV and a panel of engineers report on the status of the 8VSB enhancement test program. William Meintel will present A Guide to Selecting a Post Transition DTV Channel. Other papers to be presented cover adaptive equalization in transmitters (Harris Corp. and Axcera), on channel repeaters (R.W. Zborowski), the Dielectric VHF/UHF slot antenna and Andrew Passive Power Products' new multichannel UHF-combining network.
Papers at Tuesday afternoon's "Television Design for Efficiency" session that may be of interest to my readers include Integrating COFDM Microwave into the Electronic News Gathering Environment at WCVB, by WCVB's Director of Engineering Michael Keller, and Ian Trow's Portable Digital Links for Mobile Video Applications. Trow is from Tandberg.
According to the abstract, Trow will discuss "how digital video links for mobile applications can be achieved by combining MPEG compression, terrestrial modulation, RF upconversion and low-power radio amps in a small, DC-powered unit." Visit the NAB Web site at http://www.nab.org/conventions/nab2002/bec.asp for a complete list of sessions.
SPREAD SPECTRUM LINKS
In many markets, it is becoming difficult to find frequencies for new microwave links. Recently I noticed some broadcasters, especially LPTV broadcasters, are taking a new approach to microwave links. Rather than using the traditional, licensed, analog microwave on 7 or 13 GHz, they are using unlicensed spread spectrum radios operating in the 2.4 or 5.8 GHz bands for transmitting digital video.
By now I'm sure most of my readers are familiar with IEEE 802.11b wireless networking. Wireless access points cost a bit over $100 and wireless PC cards and PCI cards are now available for under $100. Many new laptops include the technology. Experimenters have added external antennas to extend the range of IEEE 802.11b although, as far as I know, the standard is not being used by broadcasters for spread spectrum video links.
Spread spectrum links are usually sold as data links, so before they can be used for broadcast video applications, a digital encoder/decoder will be needed. While one company offers spread spectrum equipment that can operate at OC-3 data rates, the data rates will be limited to DS-3 (approximately 45 Mbps) or less if the link has to span more than a few miles.
At longer distances, the data rate on unlicensed spread spectrum systems will be limited to 12 Mbps or less. Therefore, some form of video compression equipment will be required. Most links designed for data applications do not offer SMPTE-310 or DVB-ASI connections, so additional equipment will be required to interface encoding equipment designed for DTV encoding.
You are probably wondering about the practical range of unlicensed spread spectrum equipment. Data rates suitable for high-quality, standard-definition video are available with links operating in the 2.4 and 5.8 GHz bands. Western Multiplex has a program on its Web site (http://www.wmux.com/products/micsrolink/) that will calculate the system performance of its 2.4 GHz microwaves.
Based on data sheets for various spread spectrum systems, it appears that at 5.8 GHz, OC-3 links are usable up to 7 miles; DS-3 links are usable up to 15 miles; and T1 to 8 x T1 links are usable up to 40 and 50 miles, depending on the data rate. Links operating at 2.4 GHz have a slight advantage in range, with ranges up to 60 miles listed for single T1 links.
The 2.4 GHz spectrum is crowded and there are often high-power interference sources just outside the unlicensed band, so interference may offset any gains in margin due to the lower frequency.
Unlicensed spread spectrum devices operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands are regulated under FCC rules Part 15, Subpart C, Intentional Radiators. The exact bands are 902 to 928 MHz, 2.400 to 2.4835 GHz, and 5.725 to 5.850 GHz. As data rates for 902 to 928 MHz systems I've seen are limited to about 2 Mbps - a bit low for high-quality standard-definition TV - I won't cover them here. However, they may be useful in monitoring applications.
FCC section 15.247 (b) describes the power levels allowed in these bands. Peak transmitter power outputs up to 1 W are allowed in the 2.4 and 5.8 GHz bands for frequency-hopping systems and for all direct-sequence spread spectrum systems. I'll discuss the difference between frequency-hopping and direct-sequence spread spectrum next month.
The FCC allows the use of antennas with gains up to 6 dBi in multipoint applications without a reduction in transmitter output power. However, for antennas with gains exceeding 6 dBi, the transmitter output power must be reduced; by the amount (in dB), the antenna gain exceeds 6 dBi. Higher effective radiated power is allowed on point-to-point links.
In the 2.4 GHz band, the output power must be reduced by 1 dB for every 3 dB that the antenna gain exceeds 6 dBi. At 5.8 GHz, antennas with more than 6 dBi gain can be used without any reduction in transmitter peak output power. Therefore, when maximum link distance and reliability is needed, you could use a large 8-foot diameter microwave dish fed with waveguide to achieve a very high effective radiated power at 5.8 GHz.
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Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.