Engineers lobby manufacturers to build DTV gear they'll need

Originally published in Current, Aug. 4, 1997

By Steve Behrens

As electronics firms rush to string together a working end-to-end digital TV system, public broadcasters are looking and lobbying for capabilities they expect to need.

The PBS Engineering Committee, for instance, will soon see responses from a request for information sent to about 30 companies in June. The RFI asked whether they plan to make master control equipment that will permit pass-through of a ready-to-air compressed DTV signal from the satellite to the local transmitter, according to committee Chairman Bruce Jacobs.

It's one of several RFIs making the rounds of manufacturers. WETA, Washington, D.C., sent one out earlier in the spring, asking how they'll deal with multiple programs on a single DTV channel. And the broadcasting industry's official model station--at WRC in Washington--sent out its own RFI.

"There's a lot to be figured out," says Ed Williams, a veteran engineer working on DTV planning at PBS. Practical DTV technology is so young that engineers still have to invent a way to prevent a visible glitch when they switch between video sources, he says. "It's like starting off in the early 1950s, when we didn't know how to do that with analog technology," says Williams. "Eventually, we learned how to make synchronous switches for a clean transition."

Information about planned DTV master-control equipment will help answer a basic question for PBS network operations, says Jacobs: how much should PBS compress its DTV programs before feeding them by satellite to the stations?

Compressing the feeds all the way to the broadcast bit rate of 19.4 megabits-per-second will not only save satellite bandwidth but also give stations a ready-to-air signal that won't require expensive processing at the station, according to Jacobs. In these early days of DTV, a HDTV compressor is selling for $600,000, he says.

But the tradeoff is that stations won't be able to add a "super" such as a station i.d. "bug" unless they go to the expense of decompressing the signal, fiddling with it and then recompressing it.

The plan now is that--in the first years, when few stations and viewers are into DTV--PBS will fully compress its satellite feeds, says Williams. Later, he speculates, DTV will grow in popularity, equipment will get cheaper, and the network can change over to a less-compressed data stream on the satellite. Stations wanting to superimpose a phone number could decompress the feed, add the super and then recompress it down to the broadcast rate. That would minimize degradation by putting the feed through the full compression only once, he says. CBS, according to Williams, is talking about feeding its network at 45 mb/sec.

The advance of equipment design eventually may make the compression issue moot. So far, no one has demonstrated DTV technology to insert a voiceover or a bug or accomplish a fade with compressed material, says Mark Richer, a former PBS engineering v.p. who is now running Comark Digital Services, a systems integration arm of Comark Communications, a major transmitter maker.

The high cost of equipment for decompression and compression isn't the only reason to do those things centrally at the network level, says Jacobs. Another reason is that human expertise will be needed to do the most efficient job of multiplexing program streams into a signal for transmission.

When several programs will be uplinked together, the operator decides how much of the data capacity to allot to each one of them depending on their needs, according to Jacobs. Pictures with fast motion and important detail require more of the data stream. This is how DirecTV packs so many channels into its DBS service.

"One half-hour you may be transmitting four SDTV programs and 3 mb/sec of data, and the next half-hour may be an HDTV movie and one SDTV program and 1 mb/sec of data," says Richer. "And next, you may have HDTV baseball, and it takes all your bits. It's going to be very dynamic."

Questions about compression rates may look easy someday, compared to the struggles ahead over the set-up for channel naming and navigation. Richer says those issues are being discussed now by committees of the Advanced Television Systems Committee, where he was executive director before moving to Comark.

It isn't even clear yet how many programs a station will be able to air simultaneously, but Jacobs says it won't be a simple either/or choice between a single HDTV picture or several lesser pictures.

One option may be two "really good" 480-line interlaced pictures--with the same number of picture lines as today's analog TV, but with separately transmitted brightness and color components, he says. Or the station may choose to air one 480-line picture, plus two or more of lower quality.



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