Technology Spotlight: Dolby's Enhanced Dolby Digital

Apr 1, 2004 12:00 PM, By George Petersen

The Future of Digital Audio Encoding?


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One of the biggest announcements at this month's NAB show will not be a product, per se, but the debut of a new audio compression protocol. Representing the second generation of Dolby Digital, the unnamed process will be unveiled in ongoing demonstrations to the broadcast and production communities.

Given the real-life limitations in bandwidth that seem to follow every signal delivery chain — traditional terrestrial broadcasting to Internet streaming — the need to maximize the number of available channels into ever-smaller data pipelines is great. And with emerging services such as video-on-demand and DTV bringing broadcasters the capacity for alternate channels, there is no chance that this trend will slow down — ever.

“Squeezing more services over the same bandwidth requires more efficient codecs, both in audio and video,” says Dolby's broadcast product manager, Jeff Riedmiller. “On the video side, most people are familiar with protocols such as H.264. On the audio side, we've developed the next generation of Dolby Digital.”

An enhanced version of Dolby Digital makes sense for consumers, hardware manufacturers, software developers, content producers and broadcasters. Consumers are hardly warm to the idea of having to change hardware every few years due to the debut of new — and incompatible — playback schemes, so backward compatibility was deemed essential.

“In considering the development of this, we focused on four key points,” Riedmiller explains. “The first is compatibility. This new system must have the ability to work with the 37 million existing multichannel decoders in use in people's homes. Obviously, any enhancements we make to the system must be compatible with those users, and this new format does provide a path to those existing Dolby Digital decoders. We also wanted to be more efficient in terms of perceptual audio coding itself, yet at the same time, offer a known quality at a lower bit rate for spectral efficiency. Everybody's looking for the ability to send the same quality picture with audio but with less bits, particularly in broadcast, using limited satellite bandwidth. This enhanced Dolby Digital addresses that need.

“Another factor is cost,” he continues, “and as we can maintain compatibility with existing Dolby Digital decoders, there's a cost savings to any new implementer. via the enhanced decoder's ability to decode both legacy and enhanced Dolby Digital bitstreams, as well as provide a seamless Dolby Digital bitstream for carriage over S/PDIF or Toslink interfaces to multichannel home theater systems.”

But equally important is the ability of any proposed standard to work with formats that are on the horizon. “Four or five years ago, some of the premium services on cable [as well as terrestrial TV] started offering 5.1 programming because viewers were used to watching DVD in 5.1 surround and they wanted that on television, as well,” says Riedmiller. “We also envision this technology being used on the high-definition fixed media that will be coming in the future, such as high-definition DVD and things like that, which have always driven the other side of the business.”

The whole point of perceptual audio coding is to reduce bandwidth and storage requirements while maintaining quality. What data savings would this new codec offer over existing Dolby Digital files? “It depends on the content, but it's also governed by the quality that the broadcaster wants to convey,” says Riedmiller. “We give our customers the ability to scale the content any way they'd like. In Dolby Digital today, we recommend that people encode stereo content at 192 kbits/second. There are people running the bit rate quite a bit lower than that in stereo and have great results with it. But with this enhanced format, we're confident that in stereo, you could get that 192k rate down to 96k.”

Bandwidth aside, does this enhanced system also support all of the cool, creative control features in the audio metadata, such as dynamic range control, dialog normalization and downmixing? According to Riedmiller, all of the Dolby Digital audio metadata is preserved in the new system, and the next generation of decoders will be able to play both existing Dolby Digital and the enhanced format with all of metadata intact.

“This new process is still in the early stages, so we won't be showing hardware systems, but we will be doing technology demonstations for existing service providers, cable systems and satellite services worldwide who may be looking at their next-generation set-top box,” explains Riedmiller. “Certainly, local broadcasters won't need to worry about this right away, but if you're a satellite provider who wants to carry more channels of high-definition video, you'll need a new video coding system. We're offering the new audio coding system to complement this new video codecs such as H.264.

“We're excited about this and look forward to telling the world about this at NAB,” says Riedmiller, who added that Dolby would be providing active listening demonstrations of the system at its booth during the show. “We're unveiling the capabilities of enhanced Dolby Digital for the broadcast market at NAB. Applications for the enhanced version could include satellite broadcast, cable television, video-on-demand, terrestrial broadcasting and fixed media [DVD, etc.]; pretty much anywhere you'd find AC-3 today, we'd expect to find the enhanced version. It could also apply to toys, games and wireless, as well as going into the other spectrums where there are no bandwidth constraints. The system is also capable of going beyond 5.1, and there are provisions for that.”

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