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Modulation Standard Debate Intensifies - TvTechnology

Modulation Standard Debate Intensifies

We might as well start with the bad news. After spending time at NAB2000 with proponents of both COFDM and 8-VSB, it looks like reliable consumer reception of DTV with indoor antennas is at least a year away and perhaps as much as three years away.
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We might as well start with the bad news. After spending time at NAB2000 with proponents of both COFDM and 8-VSB, it looks like reliable consumer reception of DTV with indoor antennas is at least a year away and perhaps as much as three years away. Sinclair demonstrated a COFDM solution for indoor and portable DTV reception at the show, but before it can be rolled out in the U.S. a lot of work is required – some technical, some non-technical – political and legal.

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I didn't see or hear of any demonstrations of indoor or portable 8-VSB reception at the Las Vegas Convention Center. However, manufacturers were showing improved 8-VSB demodulators and an increased understanding of some of the problems encountered with indoor reception.

Unfortunately, according to presentations by NxtWave and Zenith, it could take up to a year or more for current designs to make it into consumer products. ATSC chairman Robert Graves said it could take two or three years before 8-VSB would able to deal with 1 dB echoes. (I assume he meant complex echoes at any phase.)

BLACK MOUNTAIN

Unless you spent NAB hiding in your hotel room or buried in computer graphics at the Sands, you have already heard about Sinclair's demonstration of indoor reception of COFDM at the Acrodyne booth. Sinclair was transmitting COFDM at 300 kW ERP from its KVWB-TV, Channel 29, on Black Mountain, about 12 miles from the Convention Center. What made the demonstration unique was that KVWB was using DVB-T Hierarchical COFDM modulation.

The main signal was transmitted as a DVB-T 8k 64 QAM COFDM signal. Using hierarchical modulation, two data streams were encoded on the same signal. One was a high-priority stream at 4.9 Mbps, which appeared as a QPSK signal, and the other was a low-priority stream at 13 Mbps, which appeared as a 16 QAM signal. I won't explain how this is done this month, but you can read about it in the paper "DVB-T Hierarchical Modulation: A Brief Introduction," available on my Website at www.xmtr.com. [See the figure "DVB-T Hierarchical 64 QAM constellation with an imbedded QPSK stream," the DVB-T paper for a visual representation of this.]

ROBUST RECEPTION

In the Acrodyne booth, around the Convention Center floor, and at various spots around Las Vegas, Sinclair, DVB, and their partners were using three Nokia Mediascreen portable DTV receivers/terminals to demonstrate robust indoor reception of DTV. People playing around with the Mediascreen in the Acrodyne booth found it wasn't easy to find a location or position where the Mediascreen didn't work.

The Mediascreen was receiving the high-priority datastream at 4.9 Mbps, so HDTV reception wasn't possible. The SDTV picture looked perfect on the notebook-sized screen and there were enough bits left over to include some Web pages.

Regardless of the attractiveness of the Mediascreen application, most U.S. broadcasters also demand HDTV capability. Sinclair was demonstrating this using the low-priority 13 Mbps datastream. HDTV reception required a more complex antenna setup. Sinclair used one of the Radio Shack dual bow-tie and reflector indoor antennas. It was propped on top of the flatscreen HDTV monitor.

If the antenna was removed from the top of the monitor and moved around, the picture took errors, indicating antenna positioning was more critical for the low-priority 16 QAM datastream. Still, reception did not appear to be affected by people moving around in front of the monitor.

AN IMPRESSIVE DISPLAY

The Sinclair demonstration was not a scientific test. It was, however, an impressive display of how robust the DVB-T platform can be. Due to the QPSK nature of the high-priority datastream and a high level of forward error correction (1/2), the required signal-to-noise threshold is between 10 and 11 dB, significantly better than 8-VSB.

This 4 to 5 dB advantage over the 8-VSB standard is more than enough to compensate for any average power reduction that may be required to handle the higher peak power requirements of COFDM. It also allows an additional margin if, as some laboratory tests have shown, COFDM is more susceptible to co-channel interference. While not capable of providing HDTV service, this high-priority datastream should be capable of providing standard-definition DTV service in locations where even marginal analog reception is difficult.

If the DVB-T standard is capable of providing a "channel" with such robust reception characteristics as well as a more delicate HDTV "channel," all in the same 6 MHz of spectrum, it would appear the DTV modulation battle has been won by the DVB-T standard. In the United States, however, things are rarely so clean-cut.

LENGTHY PROCESS

First, after going through a lengthy process to select a DTV modulation method and choosing 8-VSB, it is not easy to admit that during the delay in rolling out DTV, the U.S. standard has been superseded by a relatively new international standard.

Second, although the FCC made the 8-VSB standard the law for U.S. DTV broadcasting more than two years ago, receivers are just now becoming widely available. Many companies have invested time and money in developing consumer products to the 8-VSB standard. Consumer purchases of DTV receivers are starting to grow. Any change in standards would disenfranchise these "early adopters." Of course, if there is going to be a change, it is better to do it before even more 8-VSB products are sold.

Third, although manufacturers such as Pace say they can quickly bring 6 MHz DVB-T set-top boxes to market in the U.S., I've heard some companies wishing to test 6 MHz COFDM are having difficulty finding receivers. Considering it took more than two years to bring somewhat reasonably priced 8-VSB set-top boxes to market, it is likely to take at least a year to get inexpensive 6 MHz COFDM set-top boxes and receivers on the U.S. market.

In addition to changing the IF bandwidth, it will also be necessary to add VHF capability to the UHF-only tuners sold in European markets.

SETTING A STANDARD

Finally, many engineers, with some notable exceptions, believe it is important the FCC set a DTV standard. Defining a COFDM-based standard for the U.S. will take some time. The Brazil tests of the Japanese ISDB-T standard have shown how longer interleaving can greatly improve the performance of COFDM-based systems in the presence of impulse noise. I found there is interest in incorporating longer interleaving into a U.S. COFDM standard, should one be adopted.

There are, of course, other trade-offs. Hierarchical modulation provides a way to reach TV sets in very difficult environments. However, it comes at the cost of reducing the low-priority data rate below what many believe is needed for difficult HDTV programs, such as sports. Hierarchical modulation theoretically limits reception of HDTV to a smaller service area (based on C/N only) than 8-VSB for a given average radiated power.

PROS AND CONS

After reading this, you are probably thinking I'm recommending the U.S. stick with the existing standard. That is certainly the easiest position to take. However, I find problems with that position as well. One of the major problems is that the 8-VSB story at NAB this year was very similar to the 8-VSB story last year.

"We've identified the problem and will have a solution for it soon" is the best way I can summarize it. As far as I know, there were no demonstrations of 8-VSB reception using indoor antennas in the Las Vegas Convention Center. I tried to talk the Zenith engineers into attempting indoor reception with their demonstration setup in Room N207, but wasn't successful. Granted, it was a difficult location, certainly as bad or worse than the Sinclair demo location. It was unlikely it would have worked, even with a "miracle" chip. I was hoping, however, to see some indication 8-VSB is getting close to reliable indoor reception.

Zenith's demonstration in Room N207 did show substantial improvements in 8-VSB DTV receiver performance. Zenith was comparing the performance of four generations of 8-VSB receiver designs under various reception conditions, including impulse noise and dynamic multipath. Three HP echo simulators were used to generate the impairments for the multipath tests. Throughout the tests, the third- and fourth-generation receivers maintained their 4 dB threshold advantage over COFDM receivers operating at a comparable data rate.

"BREADBOARD" STAGE

The demonstrations showed the third-generation receivers were capable of handling dynamic multipath changing phase at a 7 Hz rate. The prototype fourth-generation receiver, still in the "breadboard" stage, was able to maintain reception at a 9 Hz rate.

The impulse noise test, using noise from a hair dryer motor coupled from the power line into the test setup, provided similar results to the earlier demonstrations. 8-VSB receivers outperformed the DVB-T COFDM receivers used in the demonstrations by a wide margin. After criticisms about earlier demonstrations, Zenith took care to avoid overloading the input to the COFDM receivers.

Recent measurements have shown that 8-VSB receiver equalizers need to correct for pre-echoes outside the 3 to 4 microsecond range or less common on today's 8-VSB demodulator chips. It was suggested that equalizer ranges be adjusted to allow for pre-echoes of 10 microseconds or more. In addition, the AGC (automatic gain control) response of the RF system in the tuner needs to be faster.

Zenith indicated these are two relatively easy modifications to improve reception of 8-VSB when using indoor antennas.

PHILLY TEST

NxtWave and CBS described tests in Philadelphia using one of the most advanced 8-VSB receivers. Thirty-two of the 42 sites tests for indoor reception were able to receive the KYW DTV transmission. Ninety-four percent of the sites with better than CCIR grade-1 analog reception on Channel 29 were able to receive the DTV transmission from the KYW, operating on Channel 26 with 770 kW ERP.

The Antiference indoor log periodic style antenna worked best followed by the ubiquitous Radio Shack dual bow-tie and reflector antenna.

However, NxtWave's Chris Strolle said that in two years, multipath wouldn't be seen as a problem for indoor reception. He based his estimate on the typical 12- to 18-month design-to-market time frame needed for consumer electronics. He did note that PC card designs are able to make it to market in as little as 6 months.

MORE IS BETTER

In his presentation "Feasibility of Reliable 8-VSB Reception" at NAB, he said the height of the DTV transmitting antenna was important, as well as the DTV power. In both cases, more is better.

For DTV reception, Strolle said we need better antennas, better tuners, better equalizers and some type of error concealment. Immunity from RF compression in tuners also needs to be improved. He explained that newer 8-VSB demodulator designs use both the in-band (I) and quadrature (Q) components of the 8-VSB signal. With some improvements, Strolle stated that, contrary to popular belief, 8-VSB tuners could cancel 0 db multipath, and blind equalization will work.

At the SET breakfast meeting to report on the Brazil DTV testing, ATSC's Robert Graves wasn't quite as optimistic. I've posted a full report on the breakfast in my April 17, 2000 RF Current newsletter. To summarize, in discussions on multipath performance, Graves said that over 2 to 3 years it should be possible for 8-VSB to handle 1 dB ghosts.

During the final statements from each of the proponents attending the breakfast, the topic turned to data bandwidth available from each of the systems. Touting 8-VSB's data capacity even with low C/N ratios, Robert Graves stated, "The important thing is transmitting bits." Peter Macavok from DVB immediately replied, "No, the important thing is receiving bits."

For many in the audience, that summarized the situation we face today.

Contact Doug Lung via e-mail atdlung@xmtr.com.