COFDM appears to have lost a significant battle according to a report presented in early January to a closed industry meeting sponsored by NAB and MSTV.
DTV reception tests were conducted in the Washington D.C./Baltimore area and Cleveland in late summer. The Washington/Baltimore tests relied on transmission from four different UHF DTV stations at some 200 outdoor and 45 indoor locations. The Cleveland tests involved transmissions from a low-channel VHF DTV station to about 100 outdoor sites and 20 indoor sites.
The outdoor receive antennas were positioned at elevations of either 30 feet or 6 feet. Indoor antennas were also used at each test site. According to an early report draft, the Washington/Baltimore location presented DTV signal challenges from severe multipath, interference and low signal areas. The Cleveland location suffered from impulse noise and other RF propagation characteristics associated with low VHF operation. Because the Cleveland DTV facility (channel 2) was collocated with an NTSC facility (channel 3), the tests also compared DTV and NTSC performance.
Did 8VSB get a KO? The results reportedly show that in outdoor tests, VSB enjoyed a 75 percent success rate in the Washington/Baltimore area and 73 percent in Cleveland. COFDM signals were less receivable, scoring only 48 percent in the Washington/Baltimore area and 60 percent in Cleveland. Neither modulation scheme performed well for indoor DTV reception or in mobile or portable DTV applications. Successful indoor reception at the Washington/Baltimore sites was achieved at only 30 percent of the sites for either modulation. Reception success with either modulation method on the six-foot antenna was not much better, only 50 percent.
The apparent besting of COFDM by VSB prompted Nat Ostroff, vice president, New Technology, Sinclair Broadcast Group to fire off a strongly worded response to the report. He claimed that the COFDM receivers' front-end design was defective. The receivers, according to Ostroff, were configured with an unprotected high-gain preamplifier connected directly to the antenna input. Without an input bandwidth filter, Sinclair suggested that the receiver's preamp was being overloaded by extraneous UHF, VHF and FM signals.
To support its contention, Sinclair then conducted its own set of tests. These tests used two BT COFDM DTVM 200(T) receivers obtained from the Canadian Research Center. At seven locations, using a COFDM signal from WBAL-DT, Ostroff reported that the COFDM receivers were easily overloaded and desensitized by 3dB to 7dB at all seven test sites. The same seven sites were then checked by switching into the antenna feed a bandpass filter that provided a broad (approx 40MHz 3dB bandwidth) pre-selection of the off-air signal prior to being fed to the receivers. The results were dramatically different and are summarized in Table 1.
The Sinclair tests showed that without the bandpass filter, COFDM reception was intermittent or not possible at all seven locations. Reception from VSB signals was possible at six of the seven locations. However, when the bandpass filter was switched into the antenna feed, COFDM reception was possible at all seven locations. The reason for VSB's success over COFDM in the earlier MSTV/NAB tests was, according to Ostroff, that the VSB receivers included input filtering. Ostroff called the MSTV/NAB's use of COFDM receivers without similar protective channel filters as a "gross engineering error."
Overall, the preliminary report doesn't offer much hope for OTA broadcasters' DTV future, stating "...although some viewers would be able to enjoy indoor reception with either system, neither system exhibited the level of reliability that would be required of a practical broadcast service based solely on service to indoor antennas." This emphasizes the crucial issue for broadcasters, the requirement for DTV must-carry by cable.
Despite the wrangling, sources report the Committee adopted a non-binding resolution by a 26-3 vote. The resolution states in part: "We conclude that there is insufficient evidence to add COFDM and we therefore reaffirm our endorsement of the VSB standard. .... We therefore will take all necessary steps to promote the rapid improvement of VSB technologies and other enhancements to digital television and direct the staffs to develop a plan and promptly submit it to the Boards."
The latest report conflicts with the DTV tests conducted by other countries. In similar shootouts, five countries have recently announced the adoption of DTV (COFDM) technology: Australia, Brazil, Hong Kong, India and Singapore. The United States group has planned a Phase Two round of testing. However, given the first round's results and the Committee's apparent support for VSB, it remains unclear which companies might participate.