Making the Transition

According to figures released by the National Association of Broadcasters in mid-March, there are now 1175 DTV stations on-air in 205 markets serving 99.6 percent of U.S. TV households. A large proportion of those households are in markets that are served by five or more digital broadcast channels, while in 30 markets all of the local stations have now gone on-air with digital signals.

But the transition to total digital broadcasting, mandated by the FCC for completion by 2007, has been hampered by a variety of issues. One significant factor, the high cost of installing a digital transmission plant, has funneled funds away from other equipment, including audio, an area that has long been near the bottom of the priority list in the visual medium of television broadcasting.

There are a number of barriers to surmount before broadcasters can go on-air with surround sound to accompany their hi-def pictures. In addition to the equipment costs, there are new audio production technologies and techniques to be learned, and, as Fred Engel of Chicago’s WTTW in Chicago explains, from ingestion to distribution, the entire process can be challenging.

Engel, director of engineering operations at the Midwest PBS station, offers the example of Soundstage, the acclaimed music series last seen in 1985 that was reincarnated last year and starts its second season in June. Recorded at WTTW’s Chicago Production Center Studios, the show is posted by production partner HD Ready.

“The program gets edited on an Avid DS system then mixed off-site,” explains Engel. “They put a 5.1 mix through a Dolby E encoder and put that onto one of the AES tracks on a Sony HDCAM tape. They also give us a stereo mix on the other track.

“If we’re going to air it directly from videotape, we take the Dolby E track and run it through a Dolby E decoder, and then run it through a Dolby Digital encoder. We take that into our MPEG encoder for our HDTV transmission.”

For other shows, a 19.3 megabit pre-recorded hi-def stream from PBS is ingested via satellite onto WTTW’s Sencore server, which automates playout of 100 hours of programming, without the need for decodes or re-encodes. “In essence, we’re leaving the program encoded in Dolby all the way through from satellite to server to playout.”

He elaborates, “If it’s a surround sound show, and if we route it correctly, we can recover the 5.1. We do have to decode the video down to base band video. If it is a surround sound show we also recover the AC-3 stream and take that into our MPEG encoder for transmission.”

Experiments with JVC’s Pro DVHS machine have allowed that stream to be recorded onto inexpensive VHS tape. “But when we play that back we have to take it down to base-band analog, component video, and analog audio, then re-encode it. In that process we go through three encode/decodes of the original material, and you’re not supposed to do that.”

There are potential pitfalls, he observes. “We had an issue once with a program where we got the channels mixed up. Our plan eventually, whether it is surround sound or not, is to store everything on the server in Dolby Digital and just make sure, on the non-surround shows, that we get the narration tracks on the left and right.”

WTTW’s broadcast center is wired with two channels of AES audio, allowing simultaneous routing of stereo and Dolby E. “Some shows only come in stereo, others are Dolby Digital, so you have to be able to easily switch from one method to the other in order to be able to get audio to come up correctly.”

Keeping sound and picture in sync can be a headache, he notes, often because of the equipment. “There were these devices that took the MPEG streams and decoded them down to base-band video and audio. You could watch the audio drift out. You shut the box off, counted to 10, switched it back on, and it would be fine for the next two days. But every now and then they would get unglued from each other.”

Then there’s digital video processing, which adds latency. “In the video world, frame synchronizers are wonderful products. You can take all these non-synchronous feeds and synchronize them to the house,” says Engel. “So you can do a fade and not have color shifts or horizontal jumps.”

But, there’s a cost. Multiple frame synchronizers and digital video effects units all take time to process. “It all adds up to video delay and your audio is now several frames ahead of the video,” he observes. “Thank goodness Dolby came along, because their stuff takes time to process, so it brings it a little closer!”

WTTW hopes to bring 5.1 production fully in-house, starting with a new jazz series later this year. The facility has been multitracking shows for 30 years, and is now equipped with a surround-capable Pro Tools system, 5.1 Genelec monitor setup, and a soon-to-be-commissioned Sony Oxford.

“We have the infrastructure in place, but we have a lot of things to learn,” admits Engel, noting that his engineers will need to become acquainted with the necessary techniques for producing 5.1 music. “We have all the pieces to the puzzle, we just need a little more time to perfect it for ourselves.”

And while there is a passionate group of DTV viewers in the market, he says, the public television station hasn’t received any additional funding for their hi-def broadcasts. “We put in a $2 million transmitter plant — we pay $1 million-plus a year to lease the space at Sears tower. We pay $8000 a month for the electricity just to run that thing.”

But Engel appears more than happy to meet the challenges and keep PBS at the forefront of the transition to HDTV. “PBS will probably continue to be the leader in High Definition programming and surround sound, with Soundstage, Austin City Limits, and more shows coming down the pike.”

Surround Professional Magazine