Technology Corner: Randy Hoffner
PSIP Data Often Incorrect, Missing
On the long list of all that is wrong with DTV implementation
are the problems involving the broadcasting of incorrect –
or simply missing – system information and other data that
is necessary to facilitate the correct operation of the DTV receiver.
Some time ago, the DTV Station Project conducted a survey in which
it studied Transport Stream Identifiers (TSID) and PSIP channel
identification data broadcast by the DTV stations broadcasting in
our nation's capital, and the results were less than encouraging.
Six months later, the survey was repeated, with results similar
to those found in the first survey.
The surveys revealed that several D.C. area DTV stations were
broadcasting incorrect or no PSIP and/or TSID data. In some cases,
incorrect major channel numbers were intentionally being broadcast
as part of a station's marketing efforts. The station was transmitting
its DTV station’s RF channel number rather than the NTSC station
RF channel number.
Although there may be a reasonable marketing rationale for this,
DTV receivers in the PSIP standard are not designed to consider
that marketing rationale, and it can cause them to malfunction.
Neither PSIP nor TSID is mandated by the FCC DTV rules. But without
the correct navigation information, DTV receivers often function
incorrectly. It is a fact that if a DTV station wishes to be correctly
received and displayed by all DTV receivers (and in some cases,
to be received at all), there is a minimum set of system information
that must be accurately transmitted in the DTV transport stream.
ERRORS ON FILE
Recently, Triveni offered to analyze transport streams supplied
by U.S. DTV broadcasters. The company analyzed transport streams
from some 20 DTV stations and that analysis revealed a number of
errors within various aspects of the transport streams.
Transport stream errors included errors in video and audio buffer
usage, as well as jitter and frequency errors in the Program Clock
Reference (PCR). The decoder uses the PCR information to synchronize
its 27 MHz system clock with that of the encoder. Errors in frequency
or excessive jitter in the PCR signal can have a devastating effect
on the decoder's operation. Buffer errors can degrade the quality
of video or audio (depending on which buffer is affected), and can
cause lip-sync errors, but PCR errors can cause even more serious
problems, such as not being able to decode the data at all.
A second major problem discovered in the analysis was in program
and system information, more familiarly known as PSIP. Errors included
incorrect intervals for PSIP table transmissions, missing PSIP tables
or syntax errors, and lack of PSIP metadata.
Efforts are under way to address the problems of DTV in general
– and transport stream and PSIP errors specifically. One big
contributor to solving problems between encoders and receivers is
the continuing series of interoperability "plugfests" – cosponsored
by ATSC, CEA and the DTV Station Project – that occur throughout
the year. The next one is scheduled for October 9 and 10.
A variety of test materials are encoded into ATSC bitstreams using
standard encoders; these bitstreams are then transmitted by the
DTV Station Project. Receiver and decoder manufacturers wishing
to participate receive the signals at a Washington, D.C. area hotel.
In this way, receiver manufacturers are able to determine how their
receivers and prototype products react to various DTV signals. No
reports are solicited or made on performance; rather, this is a
method for manufacturers on both the encoding and receiving sides
to determine how their products interoperate.
Such things as lip-sync, 608 (NTSC) captioning encapsulated in
708 (DTV captioning) packets and other issues have been tested.
In the future, tests will be conducted on things like true 708 captioning,
data broadcasting and directed channel change.
The ATSC, on its own and in partnership with organizations like
the National Institute of Standards and Technology (NIST), the Society
of Broadcast Engineers (SBE) and SMPTE, has mounted efforts to educate
broadcasters on the various aspects of DTV.
DTV implementation, like DTV itself, is work-in-progress. There
is much to do before it becomes as familiar and smooth functioning
as NTSC, but efforts are under way to move toward that goal.
Randy Hoffner is manager of technology and strategic planning
at ABC, New York, N.Y. The views expressed in his column are his
own, and not necessarily those of ABC. Write to him c/o TV Technology.
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