The test results, presented Thursday (Jan. 11) at a closed industry meeting of broadcasters sponsored by the Association for Maximum Service Television (MSTV) and National Association of Broadcasters (NAB), also showed that neither vestigial sideband (VSB) nor coded orthogonal frequency division multiplex (Cofdm) performed well for indoor DTV reception or for so-called "pedestrian reception" portable DTV applications.

In fact, the test results "really replicated what FCC's Office of Engineering and Technology already predicted a year ago in their theoretical analysis," said an attendee of the meeting, who spoke on the condition of anonymity. "The purported advantage of Cofdm simply didn't appear on the field tests," he said.

Backers of Cofdm modulation heatedly disputed that conclusion, arguing that the testing was flawed because it used a European-made Cofdm receiver that was not set up to handle U.S. power levels or adjacent channel interference, which is considered worse in the United States than in Europe.

A spokesman for Sinclair Broadcasting, a leading proponent of Cofdm, attributed the deficiencies to "gross negligence." The report said European equipment had to be configured for the MSTV test because "there are no Cofdm systems currently employed in the U.S."

Broadcasters, looking for clear-cut answers to their DTV modulation questions, are reportedly disappointed with these test results.

Combing through the voluminous test results, one attendee said, "They've done a very good job in terms of comparing apple to apple." He added, "This is good science."

During the field tests, the same data rate and the same average signal power were applied to VSB and Cofdm. The field tests were carried out in two locations: Washington/Baltimore and Cleveland. Cleveland testing was conducted on a low VHF channel that had a National Television System Committee (NTSC) channel with a co-located transmitter, and the NTSC channel was adjacent to the DTV channel.

Throughout the field tests, the MSTV/NAB team took measurements on three areas: outdoor reception using a 30-foot mast mounted on a truck; pedestrian reception using a 6-foot outdoor antenna to determine how each modulation scheme performed for portable DTV applications; and indoor reception using a bow-tie and silver sensor antenna in the Washington/Baltimore area. The Cleveland field tests used a mega-wave antenna.

Better reception

For outdoor reception, VSB showed 75 percent reception in Washington/Baltimore and 73 percent reception in Cleveland. Cofdm, on the other hand, scored 48 percent and 60 percent reception, respectively.

Cofdm backers attributed the result to an overload due to the absence of a front-end filter in the receiver used for the tests.

For indoor reception, engineers familiar with the field test results agreed that many improvements are still needed for both systems. "Neither system exhibited the level of reliability that would be required of a practical broadcast service based solely on service to indoor antennas," a report on the field test concluded.

The MSTV/NAB report confirmed "What we've known all along — VSB has problems with multipath, while Cofdm is less efficient in its use of signal power," according to an industry source.

Broadcasters are expected to vote on recommendations based on the field trial results next week (Jan. 15). Another round of testing is also planned, although it remains unclear which companies will participate.