Doug Lung /
01.01.2001
8-VSB vs. COFDM: The Debate Continues
Over the last few months, me and many others have written about the relative merits of COFDM and 8-VSB for DTV. It is the most pressing issue facing broadcasters today. A wrong decision could delay or even destroy the plans for a complete transition to digital TV broadcasting.

Each system has its merits and it is an interesting technical exercise to consider which is best for a particular application. However, until Sinclair Broadcast Group filed its Petition for Expedited Rulemaking urging the FCC to modify its DTV rules to allow broadcasters the flexibility to use COFDM technology, it remained a debate among engineers.

Now, broadcasters and manufacturers are taking sides in the debate. The FCC will have to decide whether to accept the Sinclair petition or dismiss it without allowing it to be put out for public comment as requested by the Consumer Electronics Manufacturers Association.

This month I'll consider some of the new technical questions raised in an FCC Office of Engineering and Technology (OET) Report comparing 8-VSB and COFDM, and Sinclair's Petition for including COFDM in the U.S. DTV standard.

The FCC's DTV Report on COFDM and 8-VSB Performance is available on the FCC Web site. It provides a background on the COFDM and 8-VSB DTV transmission systems and summarizes the Sinclair testing in Baltimore, which I covered in my column last month (TV Technology, Oct. 6, p. 34). The results of 8-VSB field tests, outlined in Gary Sgrignoli's paper Preliminary DTV Field Test Results and Their Effects on VSB Receiver Design are also summarized. Comments from press reports, broadcast engineers, the NAB, MSTV, Harris Corp. and other manufacturers are discussed. I won't repeat most them here because they have been extensively reported on in this and other broadcasting publications.

One comment I had not seen before was from Ed Kott of Philips, who indicated there have been significant problems with interference to COFDM service in Great Britain from impulse noise. This noise comes from electric motors in vacuum cleaners and other household appliances as well as from light dimmers and power lines, among other things.

The FCC report stated Bruce Allen of Harris said "COFDM is 8 dB more susceptible to impulse noise commonly found in consumer homes."

A footnote in the report regarding the Sinclair tests said that "neither the COFDM nor the 8-VSB signals used for the demonstration complied with the out-of-band requirements contained in the FCC rules." It also said, "While COFDM systems can have higher occupied bandwidths than the 5.38 MHz of 8-VSB, we believe that the 5.7 MHz occupied bandwidth of the equipment used by Sinclair may be overly ambitious." A COFDM occupied bandwidth of 5.625 MHz was suggested, which would give the ATSC 8-VSB system "about a 5 percent data rate advantage over the DVB-T 6 MHz COFDM used by Sinclair."

FRINGE ADVANTAGE

One interesting portion of the report compared the service availability of COFDM with 8-VSB in the top 10 TV markets. The comparison gave COFDM an "urban advantage" based on the assumption that 80 percent of the urban center population would have a COFDM signal above threshold, and 50 percent of the locations would have enough multipath that 8-VSB would not work but COFDM would.

8-VSB was given a "fringe advantage" based on the assumption that there is a 4 dB transmitter power advantage for 8-VSB that would extend coverage beyond that provided by COFDM. In this extended coverage area, it was assumed 50 percent of the population would receive service with 8-VSB but not with COFDM.

This comparison showed the best case for COFDM was in the Dallas/Fort Worth, Texas, market, where 3.69 percent of the population were assumed to be able to receive COFDM but not 8-VSB. The best case for 8-VSB was in Washington, D.C, where 1.13 percent of the population were shown to be able to receive the 8-VSB signal and not the COFDM signal.

For the 10 markets added together, COFDM would be able to reach an additional 67,008 people. The total advantage for COFDM in the urban center areas was 1,536,652 people. The advantage for 8-VSB in the fringe areas totaled 1,469,644 people. This fringe advantage would decrease if the COFDM transmitters used greater power. This should be possible, according to the report.

The FCC OET investigated the impact on interference to existing NTSC stations of increasing the power of DTV stations by 4 dB. The report said. "This analysis indicates that if the 1 MW cap is retained, the overall increase in interference to NTSC service would be generally small."

The FCC report found both COFDM and 8-VSB are capable of providing viable DTV service. However, it recommended the ATSC 8-VSB standard be retained because "we believe that 8-VSB has some advantages with regard to data rate, spectrum efficiency and transmitter power requirements."

It also said "we find the relative benefits of changing the DTV transmission to COFDM are unclear and would not outweigh the costs of making such a revision." The benefits COFDM provides for single-frequency network operation and mobile service "may not be important or meaningful, given the current structure of broadcasting in the United States."

The report's recommendation did not ignore the reception problems uncovered in the Sinclair tests but concluded, "Based on our discussions with CE manufacturers and recent announcements by semiconductor manufacturers, we believe that reasonable solutions to the multipath issue and indoor reception problems raised by Sinclair are being developed and should be available in the near future."

SINCLAIR PETITION

By this time, most readers should be familiar with Sinclair's position on 8-VSB and COFDM. Sinclair's Petition for Expedited Rulemaking emphasizes the problems with 8-VSB reception in urban areas subject to multipath and the problems caused by the critical antenna placement required for 8-VSB.

For example, if an antenna has to be rotated when switching channels, channel surfing will become difficult. In addition, if different antenna positions are required for different channels, it will not be possible to record one TV channel while watching another.

The petition also questions the fringe area advantage of 8-VSB, noting that Sinclair's tests showed COFDM had a power disadvantage of only 2 dB and, under real-world conditions, even this disadvantage disappears. It also commented that if additional signal levels are required in fringe areas, reception could be improved using a preamplifier.

However, it states that "there is no reasonable technological solution for the urban viewer whose location suffers from multipath distortion. Short of deploying an expensive rooftop antenna or subscribing to cable, urban households relying on simple antennas will be powerless to overcome 8-VSB multipath effects."

There has been some confusion about what Sinclair is requesting. It is important to point out that Sinclair is not requesting the U.S. DTV transmission standard be switched from 8-VSB to COFDM. Sinclair requested the FCC appoint an industry task force to conduct "a study and issue recommendations regarding the integration of COFDM digital modulation technology into the ATSC DTV standard" and conduct "a rigorous scientific analysis to determine the interference ratios for COFDM transmissions into existing NTSC and 8-VSB DTV signals."

The task force should complete these studies and "issue its recommendations and findings within 120 days of its appointment." Once this is completed, Sinclair said "the commission should review its recommendations and adopt an alternative, COFDM-based ATSC DTV standard, and establish simple procedures whereby broadcasters could demonstrate, using interference ratios provided by the COFDM Task Force, that they will not cause interference to any operating NTSC or 8-VSB DTV broadcasters."

MORE FLEXIBILITY

Sinclair's petition did not focus solely on 8-VSB's deficiencies. It noted that COFDM gives broadcasters much more flexibility to provide a variety of fixed, mobile, and portable DTV video services. Using COFDM, broadcasters could vary data rates from 4 to 24 Mbps "to achieve a wide range of operational modes and meet a variety of service goals."

Sinclair explained, "While CEMA recently asked the commission to reallocate returned broadcast spectrum specifically to mobile data services (with COFDM as the digital modulation technology), such action would be unnecessary if the commission allowed DTV broadcasters to commence COFDM operations and provide these advanced services over already allocated broadcast frequencies."

In my next column I'll cover Sinclair's response to the FCC DTV Report on COFDM and 8-VSB Performance and the Consumer Electronics Manufacturers Association's Opposition to Petition for Expedited Rulemaking and Motion for Its Immediate Dismissal. These two documents get to the core of the 8-VSB and COFDM debate on both the technical and consumer/marketplace issues. I'll also have some comments on the debate.

E-mail your comments to me at dlung@transmitter.com or dlung@xmtr.com. Look for updates on my main Web site at www.transmitter.com. You are also welcome to try my alternative site for RF Current at www.xmtr.com if the main site is slow.

Thanks for your comments and encouragement this year. I wish you a joyous holiday season and a Happy New Year.


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