FCC stays course on DTV transmission standard
eetimes.com
By Junko Yoshida and George Leopold
EE Times
(10/01/99, 5:53 p.m. EDT)
WASHINGTON — Federal regulators have rebuffed efforts by a small
but vocal minority of broadcasters to reopen the U.S. digital TV
transmission standard, concluding in an engineering report released late
Friday (10/1) that the vestigial sideband modulation scheme is technically
sound.
The Federal Communications Commission in a report compiled by its
Office of Engineering and Technology (OET) said that it studied a
modulation demonstration performed by critics of the 8-VSB spec but
found there was no reason to re-evaluate it. The FCC office "concluded
that the VSB standard should remain in place," an FCC official said Friday.
The OET study reiterated some of benefits of the alternative approach,
called COFDM (coded orthogonal frequency division multiplexing), such
as its advantages for single frequency network operation and mobile
service, the FCC said in releasing the report. The engineering office also
found that "8-VSB has some advantages with regard to data rate, spectrum
efficiency and transmitter power requirements. On balance, however, the
FCC engineering experts 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 demonstration was staged by Sinclair Broadcasting Group Inc.
(Cockeysville, Md.) Sinclair executives confirmed that they will petition the
FCC as early as next week to have the government reconsider the
Advanced Television Systems Committee's (ATSC) transmission standard
based on 8-VSB.
While Sinclair executives declined to discuss their petition, industry sources
said that their real motive behind the petition may be less on technical issues
than on a Nov. 1 FCC deadline for broadcasters to file their DTV
construction plans. Of the roughly 1,600 licensed DTV stations, as many as
one third still haven't filed their plans. Sinclair, which owns many stations in
mid-size markets, has been leading the debate over the technical merits of
VSB versus COFDM in hopes of extending the FCC deadline and drawing
out the debate, industry sources suggested.
Sinclair made one of the most significant public attacks on the VSB
reception problem after a July transmission test, in which Sinclair engineers
compared DTV reception via VSB versus COFDM. Results showed that
multipath reflections particularly interfered with VSB-based DTV
reception.
Sources said preparing a DTV construction plan is not a trivial task for any
broadcaster. It needs to prepare everything from getting construction
permits for DTV towers, balancing programming budget to dealing with the
DTV signal reception issues. "Everything is so tightly connected. You can't
just single out a VSB issue to change the entire picture," said one industry
expert.
Others, however, argued that Sinclair's suggestion for the U.S. DTV
transmission standard to be modified to include COFDM in order to
address these reception issues is not totally unwarranted. Recently, tests
with off-the-air DTV signals in highly populated urban locations
demonstrated reception problems with first generation DTV receivers,
which were significant enough for some broadcasters such as Sinclair to
declare that the FCC's 8-VSB standard is unworkable.
The FCC studied the issue, before its Office of Engineering and
Technology prepared a report on the latest technical assessment of VSB.
Many key players involved in DTV, including Motorola and NxtWave,
confirmed this week that they met with the FCC officials in recent weeks to
share their view on the improvement of the 8VSB modulation scheme.
Chip vendors armed with the newest generation of VSB demodulation ICs
designed to combat with dynamic multipath issues said that their chips are
currently being field tested. Matt Miller, president and chief executive of
NxtWave Communications Inc. (Newtown, Pa.), said, "We are proving
that our chip does what it says it does."
Meanwhile, Motorola has also completed field tests of its 8-VSB
demodulation chip in 48 sites here and in Philadelphia, according to Bob
Stokes, director of digital television operations at Motorola. Motorola
shared its field test results with its key customers and some broadcasters on
the DTV market under non disclosure agreements.
As it continues its field testing in multiple terrain and geographical areas,
Motorola plans to publish a comprehensive report on their field test results
in November, Stokes added. Volume production of Motorola's chips is
slated in January, while DTV systems incorporating their chips to be
displayed by major CE vendors at the Consumer Electronics Show early
next year.
While it was generally known that 8-VSB could be susceptible to multipath
interference (interference from multiple signals arriving at an antenna at
different times due to obstacles such as buildings or automobiles) in
extreme circumstances, this was not judged to be an impeding factor,
according to Motorola.
Unlike the U.S. DTV broadcasting system based on the ATSC standard,
the European terrestrial DVB system (DVB-T), which uses COFDM
modulation, is inherently resistant to multipath. But COFDM systems
requires more than twice the transmitter output power for equivalent
coverage and have less data capacity than 8-VSB. The DVB-T system is
also less tolerant of impulse noise interference, which is commonly
produced by home electrical appliances, automotive ignition systems and
high-voltage power transmission lines.
In sum, 8-VSB and COFDM serve different purposes, and each has
advantages. "For this situation [DTV in the Americas], 8-VSB performs
better overall," said Stokes, due to its strengths in increased bandwidth for
video, audio and auxiliary data as well as lower power transmission power.
Stokes also added that in designing a DTV system capable of receiving
both cable and terrestrial broadcast signals, building parts that demodulate
both VSB and QAM can be done with very little additional cost. Doing
both COFDM and QAM — that are mathematically so far apart —
requires a double conversion tuner which could result in as high as $10
additional cost, he explained. |
