RF Ramblings – Anticipating NAB 2001

The annual NAB convention is the most visible forum in North America where engineers report on the achievements of the previous year and show what they hope to achieve in the following year.
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The annual NAB convention is the most visible forum in North America where engineers report on the achievements of the previous year and show what they hope to achieve in the following year. This year attendees will hear about improvements in the ATSC VSB standard, see new products for DTV transmission and monitoring and, I hope, see demonstrable improvements in reception of over-the-air digital VSB signals.

8-VSB PERFORMANCE

At NAB 2000, I was looking for demonstrations of easy, reliable, indoor DTV reception. Sinclair and Acrodyne showed reliable reception using the DVB-T COFDM standard, but ATSC 8-VSB demonstrations were limited to hotel suites and demodulators hooked to outdoor antennas on top of the convention center. This year, MSTV and NAB voted to continue supporting the VSB standard after their tests showed COFDM didn’t improve reception enough to warrant changing the U.S. standard.

Although there have been criticisms of the way the MSTV/NAB 8-VSB/COFDM tests were conducted and the lack of testing at lower data rates, I expect ATSC VSB improvements to be the main topic at NAB this year. At least two manufacturers indicated late last year that they were developing technology to allow hierarchical modulation and/or scalable data rates to allow for more-robust VSB reception.

If it begins to appear indoor reception of the 19.3 Mbps 8-VSB signal isn’t practical, then a more-robust VSB mode will be needed. Perhaps we will hear some more details about the new modes and how compatible they will be with existing receivers.

Perhaps one of the receiver or chip manufacturers will be brave enough to attempt to demonstrate over-the-air indoor 8-VSB reception at NAB this year. Even if it doesn’t perform perfectly, station engineers and executives will appreciate seeing if there has been progress in this area. While looking at demodulators on display, remember that receivers designed for test and measurement applications are not likely to be optimized for performance under difficult conditions.

Of course, the entire DTV market could change if new technology developed at the Los Alamos National Laboratory (LANL) is adopted. Scientist there have come up with a way to encode digital data on an existing analog TV signal, making the system compatible with existing TV sets and eliminating the need for a second channel.

The LANL press release is short on details, but it appears the system can operate at about 80 percent of the data rate of the current ATSC standard. It may be less, because another article indicated the system was based on a 720p video format. Space for the extra data comes from letterboxing the analog picture and putting the digital data in the formerly black, now gray, enlarged vertical blanking interval.

I’m not sure how this new technology is going to play out. While consumer electronics manufacturers haven’t exactly rushed to offer low-cost DTV sets, a fully compatible system eliminates the need for a low-cost DTV for standard-definition pictures.

The FCC and Congress may not be as happy, however, because any system that depends on existing analog TV reception technology will be subject to the same taboos and channel spacing requirements as current analog TV. The spectrum efficiencies of an all-digital system with robust interference performance (if not robust multipath performance) won’t be realized with the LANL technology as long as the analog signal is still there.

LOOKING AT TRANSMISSION GEAR

Over the last 6 months, the FCC has made monumental progress granting nonchecklist DTV applications. However, many stations have yet to receive authorization to begin DTV construction. With the FCC’s delay in processing some applications, I believe those stations that are still waiting on construction permits will have a good argument to request an extension of their build-out time. Many people are certainly expecting an extension of the May 2002 construction deadline for commercial DTV stations. We may find out at NAB2001 if the FCC is willing to oblige.

The FCC Consolidated Database (CDBS) shows 121 DTV licenses issued as of March 2, 2001. NAB reports 183 DTV stations on the air as of Feb. 28, 2001. Clearly, this indicates there will be large demand for DTV transmission equipment this year if the FCC deadline holds. More than 1,100 commercial TV stations will be rushing to complete their DTV facility in the next 13 months.

Transmitter manufacturers are aware this. There is also recognition that not every broadcaster can afford a high-power DTV transmission plant to reach a very small number of viewers. Look for lower-cost DTV transmitters at NAB this year, particularly low-power solid-state transmitters.

When considering low-power options, keep in mind that recent DTV field tests have shown signal strength is important, especially for indoor reception. When comparing manufacturers, look for an upgrade path. Some manufacturers make it very easy to increase power by adding additional cabinets. In other cases, if the control system and exciter system are not designed for expansion, it may take a lot of expensive, custom engineering to boost power.

Another item to consider when evaluating a newly introduced solid-state transmitter is availability of replacement parts. For some reason, tube types seem to be available almost forever. You can still buy an 807 or a 6146 power amplifier.

When looking at a new DTV transmitter, consider the total cost of a working package – not the transmitter by itself. Software in transmitters from ADC, Comark, Harris and some other manufacturers can take information from the demodulator in the exciter used for automatic precorrection and use it to drive a display capable of showing the VSB constellation diagram, signal-to-noise ratio, peak-to-average power ratio and more. A standalone high-quality VSB signal analyzer is likely to cost $40,000 or more.

Some early DTV exciters required an expensive HP vector signal analyzer to set up the correction. Consider cooling. Depending on the transmitter’s location, a liquid-cooled solid-state transmitter with an external heat exchanger may save transmitter room floor space and make installation simpler. On the other hand, if plenty of cool, clean air is available and space isn’t an issue, an air-cooled system may be simpler. The size and performance of the mask filter has to be considered as well.

I’m not expecting any major advances in transmitting antenna technology this year, but it would be nice to see a broadband antenna that offers elliptical polarization. Where possible, I’ve used around 20 percent vertical polarization on my DTV antenna designs. Intuitively, adding a vertically polarized component to the main horizontally polarized signal should give the receiver more signal level to work with.

Unfortunately, when it’s been necessary to install an antenna that has to work on non-adjacent, sometimes widely spaced channels, the only options I know of are full circular polarization, which doubles transmitter power requirements, and horizontal-only polarization.

Will anyone demonstrate a "smart" receive antenna system for VSB? Some companies have developed ways to use multipath and multiple antennas to improve reception of complex digital signals.

There are different ways to look at multipath. Take the example of a received signal consisting of the dominant signal and an almost equal amplitude reflection. Looking at frequency response, the multipath would appear to create a deep notch in the channel. Looking at the same signal in the time/space domain, using multiple antennas, we would see two signals, slightly separated in time, with a flat response. Once it is possible to separate the multiple paths, they can be used to improve channel performance.

DEVELOPMENTS IN DIGITAL MICROWAVE

Although digital microwave has been shown in various configurations at NAB for several years, most TV stations saw no reason to replace reliable analog microwave gear with something much more complex. The FCC’s reluctance to add digital modulation as permitted modulation under Part 74 and the lack of data on interference between analog and digital systems made it more difficult to coordinate and license digital broadcast microwave.

After demonstrations using COFDM-equipped vehicles running around Las Vegas at the last two NAB conventions and a growing number of reports showing the advantages of COFDM digital modulation for ENG, curiosity about digital ENG is changing to interest in purchasing digital ENG. Early implementations were large and somewhat difficult to operate.

This year Broadcast Microwave Systems (BMS) plans to show digital ENG systems with an MPEG-2 encoder, DVB-compliant COFDM modulator and microwave exciter in a 2 RU package. BMS will even have a configuration suitable for backpack use. The receiver, demodulator and MPEG-2 decoder takes up only 1 RU.

When comparing digital ENG (DENG) microwave equipment, be sure to look at how difficult the equipment is to use. As DENG becomes more common, less-skilled operators will be using it. Access to key functions should not require drilling down through three or four levels of tersely labeled LCD menus. Also, consider the encoder. A built-in MPEG-2 encoder will make the installation more compact. However, an external encoder can easily be replaced or upgraded to take advantage of new technology. Don’t underestimate the difficulty in combining various pieces of equipment from different suppliers. There are standards for interconnecting compressed digital video and audio. In fact, there are several of them, which can make it impossible for two pieces of equipment to talk to each other without extra format converters.

The situation with fixed terrestrial digital microwave for broadcast studio transmitter links (STL) is already complex. It is possible a digital fixed link could include a multiplexer from one manufacturer, a modulator/demodulator from another and the actual RF hardware from a third manufacturer. Given the variety of digital interfaces, RF levels and frequencies involved, it isn’t surprising most microwave manufacturers will be showing packaged solutions at NAB this year.

There is one option to consider if you will be replacing your analog microwave with an all-digital path – fallback to analog. Given the complexity of the digital path and the difficulty in tracking down interference, having a backup analog path could keep you on-the-air if something goes wrong. I suggest designing the new digital system with the older analog link as a backup or simply providing a switch to allow an analog modulator to be used in place of the digital modulator on the transmit side. On the receive side, a splitter or power divider can be used to provide an IF signal to an analog demodulator.

I’ll be reporting on RF trends and new discoveries at NAB2001 in my June column. As always, your comments are welcome. Write me at dlung@transmitter.com and visit my Web site at www.transmitter.com or www.xmtr.com.