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.
