As more broadcast TV stations make the transition to digital, cable operators are faced with some complex technical tangles. The latest to rear its head is a problem with packet identification (PID) numbers.
The problem has arisen as cable operators look to local stations' multiplex digital signals before retransmitting them over the plant.
Melding multiple signals on one 6-megahertz channel provides a great advantage in bandwidth management, but the process is complicated by a commitment the National Cable & Telecommunications Association made in February 2001.
In a letter sent to the Federal Communications Commission, the NCTA pledged that its member cable operators would pass through the Program and Service Information Protocol (PSIP) information attached to local broadcasters' unencrypted digital signals.
While the cable systems themselves ignore the information, the pass-through allows off-the-air TV tuners to read the PSIP data on digital channel assignments and basic program information.
But when operators set two off-air digital signals side-by-side on a channel, they often reassign the signal's PSIP information a different PID number, which designates the position the information occupies in the video-signal stream. Separating the PSIP information onto two PID numbers ensures that the information doesn't get jumbled as it passes through the cable plant.
The problem is end devices such as digital and high-definition TVs are programmed to look for PSIP information on only one PID number. So if the number is changed, the TV set doesn't see the second signal.
Cable operators do recognize the problem, but it is largely up to technologists to develop the necessary software to keep PSIP information separate when channels occupy the same PID, according to John Hildebrand, vice president of multimedia technology at Cox Communications Inc.
"We're really early in this whole area, of taking off-air digital signals, be they digital TV or HDTV, and putting them on the cable plant," Hildebrand said. "While we are doing that today and working to honor the NCTA commitment to the FCC to carry certain amounts of the PSIP, the vendors that we are using for multiplexing really haven't caught up with that. So there is still some work to be done."
That work is already under way at Terayon Communication Systems Inc. Its DM 6400 multiplexer, now readying for shipment, has a somewhat limited capability to manipulate PSIP information, but plans are to increase that capacity in the product's future generations, according to Jay Cox, Terayon's senior field application engineer.
"We've already begun in our next-generation software coming out to have the ability to do some manipulation of the PSIP information — basically convert the virtual channel table to a cable-friendly virtual channel table," he said. "From Terayon's perspective, it's only another step in the software, and we have already begun basic manipulation of it."
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