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.
. The OET report, FCC/OET 99-2, "DTV Report on CODFD and 8-VSB Performance," is
available on the FCC web site at www.fcc.gov
- FCC -
OET Contact: Bruce Franca, Alan Stilwell (202) 418-2470
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