US DTV doesn't work. Sinclair pushing for the DVB standard.......................
tvbroadcast.com
As reported in the June issue of Television Broadcast, "The Rabbit Test," now that the ATSC standard is being deployed, The Sinclair Broadcast Group has raised a red flag, questioning the ability of the 8-VSB modulation scheme to deliver reliable DTV service in the city grade contours of a broadcaster's coverage area.
Using the transmission facilities of one of its stations in Baltimore, during the months of June and July Sinclair conducted comparative reception tests of the 8-VSB modulation system developed by Zenith for the ATSC standard, and the COFDM modulation system now being deployed as the terrestrial component of the European Digital Video Broadcasting (DVB) standard.
Sinclair has been conducting the tests on Channel 40 in Baltimore, using a side mount antenna located 1,200 feet above average terrain. According to Mark Aitken, a vice president in Sinclair's New Technology Group, the facility provides for an A/B comparison of 8-VSB versus COFDM, with 8-VSB and COFDM being alternately transmitted, and reception capability/parameters being documented at multiple fixed locations.
The 8-VSB system is being used to transmit 19.39 Mbps of usable data. COFDM is being used to provide for a usable data rate of 18.67 Mbps. The COFDM mode selected is operating configured for 6 MHz bandwidth, two thousand carriers, 64 QAM modulation, 3/4 FEC, and 1/8 Guard Interval. The COFDM system can be configured for other data rates, with varied operating characteristic.
Although the results are still preliminary, first-generation DTV receivers developed for the U.S. standard have failed a number of critical indoor reception tests--due to their inability to deal with dynamic multipath. Where 8-VSB reception has been possible using the indoor test configuration, it typically requires very precise antenna alignment--not always with the strongest signal level. The 8-VSB reception was also impaired at times by the movement of people around the room.
According to Aitken, no locations have been found where it is possible to receive the 8-VSB signal, where COFDM did not work. Sinclair plans to make the test results available on their website. Most likely the results will be posted at: [http://www.sbgi.net/dtv/]. At the time this was written, the page contained results of earlier 8-VSB reception tests conducted by Sinclair in Philadelphia.
Broadcasters and other engineers who have traveled to Baltimore to witness the tests are expressing grave concerns that problems with reception of the 8-VSB broadcasts could cause consumers to reject the new DTV standard, even as they scramble to meet the FCC's transition schedule.
"We are in the process of a government-mandated build out of a system that appears to have serious problems under very common reception conditions," said LeBon G. Abercrombie, senior executive vice president for Planning & Development at Pappas Telecasting Companies. After traveling to Baltimore to witness Sinclair's demonstration with Dale Kelly, senior vice president and director of engineering for Pappas, Abercrombie said: "I believe that broadcast DTV will fail unless a solution to the reception problems can be found. It must be portable, and it must work out-of-the-box on indoor antennas in high-signal areas. It is time for the ATSC, the FCC and the others responsible for the current standard to demonstrate a practical, working system."
Inside Pandora's Set-Top Box
The ATSC standard has been under assault on many fronts. Sales of DTV-compatible receivers have gotten off to a slow start. Many manufacturers have chosen to decouple the tuner and digital decoder functions of the system from the expensive HDTV display components. Only a small percentage of the displays sold to date have included an 8-VSB tuner.
Most of the "HDTV compatible" systems sold to date are being used as the display component of upscale home theater systems with new digital services such as DBS and DVD-Video players. DirecTV has announced that it will begin selling HDTV capable set-top decoders in September when it will begin to deliver HBO's new HDTV service; soon after they will begin to offer pay-per-view movies in HD.
No one is willing to say how many receivers have been built, or how many of these have been sold to consumers. Anecdotal reports from consumers who have purchased first-generation ATSC set-top decoders appear to confirm the problems that Sinclair is documenting with its Baltimore tests.
The ATSC continues to claim that the problems with the modulation standard are overstated; that the system works just fine with the outdoor antennas they assumed people would use to receive these broadcasts. They also claim that second-generation receivers with improved equalizers will solve the reception problems.
Television Broadcast has also spoken to several broadcasters who feel that 8-VSB should be given a fair chance to get worked out. For example, Diane Krach, vice president of engineering at WTXF-TV, in Philadelphia, remarked that for her station the COFDM controversy is a non-issue.
"We settled on ATSC and it's going to stay that way," she stated. "It will work. I put a Panasonic decoder on my set at home and I'm receiving our signal. Panasonic's ATV Lab in Burlington, New Jersey is receiving our signal. I've gone to high-end video stores in Philadelphia and they're getting the signal. Sure, in the city, there are problems. People have to understand that indoor reception is not what we'd like it to be. The technology is not optimal on the receiver end. It's an evolving technology but I think it's a good one. I believe the committee made the right decision. There's no going back now. It's happening. It works."
"We can't wait ten minutes."
David Smith, President of Sinclair Broadcast Group has issued a call to action for broadcasters, in a July interview published on the website of the HDTV Newsletter [http://web-star.com/hdtv/smithintr.html]. Smith relates:
"We must finish testing for the public purposes. We are then going to expand for the express purposes of creating statistical relevance. We are then going to issue a report that is going to say "here is what we have concluded using the best receivers of today.
It will be contrasted against the current iteration of COFDM receivers. I think what happens then is that we will start to draft up a petition that will invite the FCC to rethink the issue. If the FCC says, 'We are not interested,' the next step will be the House and Senate commerce committees. In our view, they are ready to hear us if for no other reason than 'seeing is believing.' We are beyond the faith phase. You can see that it doesn't work."
While Sinclair will not reveal which stations have agreed to sign the petition, public statements by Smith place the number above 500. And at least six members of Congress have witnessed the demonstrations in Baltimore.
Given the preliminary results of the Baltimore tests, a number of broadcasters are calling for additional testing by an impartial organization, with well defined testing parameters. Several broadcasters are offering facilities that are operational, or will be on the air soon, to conduct these tests. David Felland, Director of Engineering and Operations for WMVS and WMVT--Milwaukee Public and Educational Television-is offering a range of facilities for potential tests. These stations are part of an eight-station Wisconsin State Educational Network, for which Felland is Director of Engineering/Delivery Services.
"We need to understand what the alternatives are at a point in time when we can still make appropriate choices," Felland said. "We need to do it now...we can't wait ten minutes."
Felland believes it is time for receiver manufacturers to step up to the opportunity or step aside. He is also concerned that the FCC has not made important decisions about options for the replacement of thousand of NTSC translators; he believes that COFDM can deal with this problem through the use of on channel repeaters.
Other industries with a stake in the DTV outcome are expressing concern as well. After making a presentation to members of SMPTE technology committees meeting at Microsoft's Television Studios, Craig Mundie, senior vice president of Consumer Strategy, said he's concerned about two things: "We are perilously close to creating a situation that will alienate consumers against free-to-air TV. [And] we remain concerned that the system as chosen is potentially flawed for the use of DTV as a direct replacement for the way TVs are used today."
Clearly this story is not going away. FCC commissioners are beginning to express concern and solicit input regarding appropriate steps to resolve the issues that threaten the future viability of free-to-air television broadcasts. There will be much to report in the coming days. It should be noted at this point, that a wide range of issues surround the selection of an appropriate transmission infrastructure for the conditions that exist in each market.
In this, it appears that COFDM offers far greater flexibility and spectral efficiency, through the use of Single Frequency Networks. Portable and even mobile reception have been demonstrated using COFDM modulation and low complexity omni-directional antennas. |
