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This is an unofficial announcement of Commission action. Release of the full text of a Commission order constitutes official action. See MCI v. FCC. 515 F 2d 385 (D.C. Circ 1974).
FCC OFFICE OF ENGINEERING AND TECHNOLOGY ISSUES REPORT ON
The FCC Office of Engineering and Technology (OET) today released a report following its independent assessment of two standards for the provision of digital television (DTV) services in the United States: the 8-Level Vestigial Side-Band (8-VSB) system which had been adopted as the U.S. Standard in a 1996 FCC order, and Coded Orthogonal Frequency Division Multiplex (COFDM). The OET study was initiated in response to issues raised by COFDM indoor reception demonstrations by Sinclair Broadcasting Group (Sinclair). OET recommended that the ATSC 8-VSB standard be retained.
The OET study concluded that both systems have certain advantages and disadvantages and are both capable of providing viable DTV service. OET further concluded that, based on discussions with consumer equipment manufacturers and recent announcements by semiconductor manufacturers Motorola and NxtWave, reasonable solutions to the multipath issue and indoor reception problems raised by Sinclair are being developed and should be available in the near future.
The OET study reiterated some of COFDM's benefits, such as its advantages for single frequency network operation and mobile service. OET also said that 8-VSB has some advantages with regard to data rate, spectrum efficiency and transmitter power requirements. On balance, however, OET concluded that the relative benefits of changing the DTV transmission to COFDM are unclear and would not outweigh the costs of making such a revision, and therefore it recommended that the ATSC 8-VSB standard be retained.
The Sinclair demonstrations consisted of an A/B comparison of 8-VSB versus COFDM reception made at two to three locations in downtown Baltimore. Reception was attempted using a "bow-tie" type UHF antenna with two 8-VSB receivers and two COFDM receivers. The antenna was rotated 360 degrees and the range over which reception was achieved was recorded. In general, reception of COFDM was better than 8-VSB at the two sites visited. Sinclair engineers indicated that at both locations analog TV reception was either very poor or not acceptable. Independent measurements indicated that the reception difficulties at the Sinclair sites were due to the presence of strong static signal reflections, or "multipath."
OET review of field tests conducted in a number of cities showed good performance for 8-VSB for outdoor reception. These studies showed outdoor service availability of 80-95 percent for cities with a small to moderate percentage of obstructed sites and 63-79 percent for cities with a large percentage of obstructions.
The field test data also indicated that indoor reception of DTV signals is more challenging. Indoor service availability ranged from 75-100 percent in cities with a small to moderate percentage of obstructed sites and from 31-40 percent in markets with a large percentage of obstructed sites. The Report said that the above test results indicated that DTV service availability approaches that of NTSC service in most instances and with expected receiver improvements will exceed it in the near future.
OET held discussions with a number of industry representatives. Sinclair stated that its tests have raised concerns as to the ability of the 8-VSB standard to provide service using simple indoor antennas. Sinclair also indicated that it has completed additional observations in the Baltimore area, including sites located near the edge of the demonstration's predicted DTV service area. Sinclair indicated that there were no indoor sites where antenna pointing was not a significant factor in obtaining satisfactory reception of 8-VSB DTV service. It also observed that the necessity to re-orient the antenna to receive stations at different locations would be a significant impediment to DTV "channel surfing." It also stated that at their edge of service sites, there were no locations where 8-VSB was significantly easier to receive than COFDM.
The report found that in general, with the exception of Sinclair, other parties continued to support the 8-VSB system as the DTV transmission standard. They generally stated that all of the factors that have been identified regarding COFDM performance in the Sinclair demonstrations were well understood and considered at the time the DTV transmission system decision was made. They stated that the demonstration locations had very strong ghosts that were outside of the correction range of the 8-VSB receivers used by Sinclair. Most of the industry representatives stated that, in theory, 8-VSB and COFDM should be able to perform nearly the same where there is static multipath. Most also stated that COFDM can generally be expected to perform better in situations where there is dynamic multipath, e.g., in mobile operations.
A number of parties also stated that 8-VSB offers a number of advantages over COFDM for broadcast DTV service, including superior overall coverage, lower costs of construction and operation, and immunity to impulse noise from household appliances. Industry representatives also asserted that 8-VSB receivers that perform better than the units used in the demonstration are, in fact, available now. The consumer electronics (CE) manufacturers all viewed multipath performance as an issue that will be worked out in the normal process of improving new products.
The study found that each system has its unique advantages and disadvantages. The 8-VSB system, in general, has better threshold or carrier-to-noise (C/N) performance, has a higher data rate capability, requires less transmitter power for equivalent coverage, and is more robust to impulse and phase noise. The COFDM system, on the other hand, has better performance in dynamic and high level static multipath situations, and offers advantages for single frequency networks and mobile reception.
The study examined and estimated the difference in service availability between COFDM and 8-VSB operation in the top-10 TV markets. In this analysis, COFDM was assumed to have an advantage in urban areas close to a station's transmitter and 8-VSB was assumed to have an advantage in fringe area coverage. These estimates appear to indicate that the relative advantages/disadvantages of either system with regard to overall coverage are generally small and vary by market.
The study also investigated the impact on interference to existing NTSC stations of increasing the power of DTV stations by 4 dB in order to make up for the disadvantage of COFDM for fringe reception noted above. This analysis indicates the overall increase in interference to NTSC service from higher power COFDM operations would be generally small. OET indicated that further study is needed to examine whether COFDM could support satisfactory service on VHF and lower UHF channels due to impulse noise concerns.
The study also found that the adaptive equalizer performance of 8-VSB receivers is very important for reception in multipath conditions. It has been suggested that a value of 22 s seems to be a reasonable minimum equalizer range, but that longer ghost canceling ranges may be beneficial. While quantitative measurements on the 8-VSB DTV receivers used in the Sinclair test were not available, it has been implied that the adaptive equalizer performance for these receivers was in the range of about 10 s or less. This appears to be a reasonable explanation for the relatively poor performance of the 8-VSB receivers in the Sinclair test, especially with regard to indoor reception.
The study found that the Sinclair demonstration has provided useful insight into certain indoor reception conditions, particularly with regard to strong multipath conditions, and possible deficiencies of some early DTV receiver designs. However, the study concluded that the multipath reception problems identified by Sinclair are solvable with improved adaptive equalizer performance and that a well-designed 8-VSB receiver should be able to provide satisfactory reception at the Sinclair locations. It further noted that signal strength and immunity to interference from impulse noise are also important factors in successful indoor reception and that 8-VSB may have some advantage over COFDM with regard to these factors.
The study also concluded that, as with most products, performance improvements in DTV receivers will continue to be made over time. From recent announcements and claims regarding the availability of improved equalizer chips, it appears reasonable to conclude that manufacturers are working to improve 8-VSB receivers, including the receivers' indoor reception and signal acquisition capabilities. In this regard, CE manufacturers indicated that improved receivers will be available this fall and that further improvements will be introduced next year.
The study further found that 8-VSB has about a five percent data rate advantage over COFDM. While a 5 percent data rate difference is relatively small, it could have some impact on the ability to provide certain high definition television programming.
- FCC -
OET Contact: Bruce Franca, Alan Stilwell (202) 418-2470