More channels, more problems. It's a tale of two digital systems, and it may lead to more than a few headaches for both MSOs and TV broadcasters.
As TV stations make the switch to digital broadcasting and cable operators keep busy growing their own DTV services, a technical snarl looms: There's a difference in the way digital channel and program information is sent to viewers over the air and through the wire.
And in a world in which broadcasters send multiple video streams down their 6-megahertz channel, some screens could be left blank on a digital TV set near you.
The problem emanates from an opportunity posed by digital broadcasting. Broadcasters are eyeing delivery of multiple video streams via their single broadcast channel. That allows stations to simulcast in high-definition and standard-resolution digital formats, and affords them the ability to add channels for auxiliary programming.
The problem is that broadcasters use the Program and Service Information Protocol (PSIP) to direct information about channel lineups and programming — including changes in feeds — for over-the-air broadcasts. But cable's digital set-top boxes and control systems don't recognize PSIP, so there's a potential for disconnection.
It's an issue that's front and center for PBS. To date, 85 public-TV stations are broadcasting digitally and have started to experiment with multiplexing. PBS's digital affiliates reach 59 percent of U.S. TV households.
PBS now is distributing a four-channel multicast and a single high-definition feed that runs 24 hours per day. Many member stations have opted to alternate the feeds, switching from the digital quartet aired during the day to the high-definition feed at night, as well as adding some local-programming options, according to PBS vice president and chief engineer John Tollefson.
But therein lies the problem — if nothing is done at the cable headend, the channels that are discontinued at night simply fade to black.
"Over-the-air doesn't matter," said Tollefson. "Over the air, the PSIP they are transmitting will inform the television receiver what to do, and when a station changes from a multicast to a single channel, the PSIP actually will direct it back to the actual channel, so you don't have this blank-channel problem.
"But when you are in cable, that doesn't work because cable has its own channel-mapping information and does not use the broadcast PSIP. Consequently, that information that comes from the TV station doesn't end up directing the set-top boxes.
"That set-top box doesn't know what to do with that information so it is generally not even there," Tollefson said.
Since blank channels aren't exactly desirable in television, PBS's engineering advisory committee has come up with an alternative — a barker channel. This low-bitrate static screen simply informs viewers the channel is not active and directs them to the primetime channels.
That's what viewers see when they tune into PBS affiliate WETA in Washington.
WETA initiated its digital multiplex broadcasts on Nov. 1, proffering four standard-definition channels from midnight to 8 p.m. each day, including a simulcast of the station's analog channel; WETA Prime, a channel featuring some of the more popular library and primetime programs; a rebranded version of dignet PBS Kids; and a home improvement and crafts channel.
Between 8 p.m. and midnight, WETA drops the kids and home-improvement channels and keeps WETA Prime, along with an HD simulcast of its analog lineup.
That multiplex is delivered to a Comcast Corp. system in Arlington, Va., that also covers Alexandria, Va.; Prince Georges, Calvert, Charles and Anne Arundel counties, Md.; and Washington, D.C. WETA is also preparing to offer the multiplex on Cox Communications Inc.'s system in Fairfax County, Va.
During evening hours, viewers see a barker display on the dormant kids' and home-improvement channels, while the WETA and HD channel stay active.
"That way we maintain a presence, and at least people know there is a four-channel multiplex," said Ed Kennedy, WETA's director of engineering and technology.
Still, the system bypasses the PSIP information, including programming descriptions, so WETA must manually deliver the programming information to Comcast. Given the amount of time that takes, the process lags somewhat, according to Kennedy.
"Someone here has to take our program guide and deliver a file to them that they can convert into their system," he said. "We're still working on how that is going to work. It would be good if we could automate that."
Indeed, the issue has not escaped cable operators' attention. While cable operators are also not wild about blank screens in the channel lineup, it will take time for the technology to catch up to the problem, according to Cox vice president of multimedia technology John Hildebrand.
"In the ideal world, I think what we would like to see is a tie-in between the guide data that we provide and the PSIP information that describes the multiplex that we get from the broadcaster," he said. "In real time, you'd like for your guide to subtract or add channels based on what channels are actually available on the plant. But that kind of functionality doesn't exist today."
Set-top box makers, including Scientific-Atlanta Inc. and Motorola Inc., are also aware of the problem.
"That's actually something we are looking at right now — what is the best way to work within that situation?" said Jim Kiker, director of Explorer set-top development for S-A's products and strategy group.
While the barker channel solution is more immediately available, Kiker didn't rule out creating a system to incorporate the PSIP information directly onto cable's channel map.
"I think it is safe to say we are evaluating all types of solutions for this, and PSIP … I wouldn't eliminate that as a potential solution," he said.
Incorporating PSIP signaling at the headend "is another thing that has to be done," said Lou Mastrocola, senior director of product marketing for the digital media systems group of Motorola Broadband Communications Sector. "None of this stuff is mind-boggling — it's just another thing that the operators have to be aware of when they set up the system.
"They have to coordinate, especially at the local level with the broadcasters. They have to understand exactly what the broadcasters are doing."
Broadcasters looking to port PSIP information on the boxes have approached Motorola, said Mastrocola, but the company has referred those queries to cable operators.
"We don't want to spend a lot of time developing an application only to see it get filtered out in the headend and never get down to the box," Mastrocola said.
On the other hand, it is even less likely that over-air broadcasters can — or would — switch to cable's system.
"PSIP is now the standard for broadcasters," Tollefson said. "That is a very fundamental change, and it would probably be counterproductive for over-the-air.
"Although there is a recognition that three-fourths or perhaps more of the viewing is done in cable households on cable, you've got still a lot of TV sets even in cable homes that are not connected to cable. So broadcasters would probably be ill-advised to do so."
So for the near term, the two systems will have to cohabitate in the digital-TV world, with fixes such as barker channels offered to ward off viewer confusion. Eventually, engineers from the cable and broadcast world may be able to forge a better link, but it will take a while.
"I think over time, at the technical level — and this is generally where there is a little bit of freedom or license to come to some commonality in spite of the higher-level, cutthroat nature of the competition — that there will be a resolution to this," Kennedy said. "But it is probably going to take a couple of years."