By Lisa Johnston
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Dolby Laboratories has developed a more efficient backward-compatible extension to its Dolby Digital 5.1-channel audio codec for use by digital-cable and satellite-TV operators who want to compress more TV channels into their existing bandwidth.
The extension, called Dolby Digital Plus, has also been proposed by the Advanced Television Systems Committee (ATSC) as an optional backup to the current Dolby Digital codec for over-air DTV transmissions. In future DTV sets, the lower bit-rate Dolby Digital Plus format would kick in to deliver uninterrupted audio even if the picture freezes or blocks up during temporary dips in signal strength, which can be caused by multipath, rain or other atmospheric conditions.
For their part, cable and DBS operators are looking at high-compression audio and video codecs "to provide more channels, services and functions on demand," said Craig Eggers, Dolby's consumer technology marketing director. Cable and DBS operators, he noted, are considering MPEG-4 and H.264 compression technologies to deliver four to five times more content with the same picture quality as the MPEG-2 standard that they currently use, he explained.
To deliver 5.1-channel audio with new video codecs, operators will look for a more efficient 5.1-channel audio codec, and Dolby Digital Plus fits the bill, Eggers continued.
The existing Dolby Digital codec supports bit rates of 320Kbps to 640Kbps for 5.1-channel audio and 128Kbps to 640Kbps for stereo. Digital broadcasters typically transmit 5.1 audio at 384Kbps, although ATSC supports up to 448Kbps.
Dolby Digital Plus, Eggers said, will enable broadcasters to deliver "a highly credible" 5.1 experience down to 192Kbps, and a stereo experience down to 96Kbps. "I am not a golden ear, and so for me it's difficult to hear the difference between 192 [Dolby Digital Plus] and 384 [Dolby Digital]," he noted.
The new codec, however, also supports lower and higher bit rates, up to 6Mbps, in fact. Dolby Digital Plus could therefore theoretically support high-quality audio in more than 5.1 channels on future prerecorded HD discs, said Dolby senior technical staff member Craig Todd.
"DD+ is convenient for consumers and broadcasters alike," a spokesman added. "It offers an end-to-end solution that brings compatibility to the mix. Because it's a format that is familiar to broadcasters, it will be simpler to implement in the station and in the set-top box."
Future ATSC TVs could also be built with ATSC tuners that up-convert the DD Plus stream to full Dolby Digital, Todd noted.
Backward-compatibility with existing DD decoders was necessary because almost one-third of U.S. households are already equipped with a home theater system, Eggers said.
Dolby also sees possible Dolby Digital Plus applications for DVD players. While playing a movie, for example, an interactive DVD player with a modem could log on to a studio Web site to stream interactive audio content, such as artist commentary, over bandwidth-constricted phone lines, Dolby said.
For future HD disc formats, Dolby has provided a "very flexible proposal" to the DVD Forum, Eggers said. The proposal could deliver more than 5.1 channels while being backward-compatible with existing 5.1-channel home theater systems, he noted. He declined to comment on whether the rival Blu-ray Group has been approached.
For future ATSC DTV sets, ATSC selected Dolby Digital Plus as a "candidate standard" that is now open for public comment and testing before it goes out for balloting. ATSC president Mark Richer expects it to become a formal standard late in the third quarter or early in the fourth.
The new audio codec could also be used by broadcasters with future ancillary video services that could be available under a planned Enhanced-VSB standard, Richer noted. E-VSB, which might be approved late in the fourth quarter, would enable broadcasters to send a secondary lower bit-rate video stream that would be more robust than typical DTV streams in low-signal conditions, he said. In one potential application, broadcasters could send news clips to laptops and PCs equipped with a DTV receiver.
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