Design Article
Optimizing the Implementation of Dolby Digital Plus in SoC Designs
Chris Cavigioli and Brett Miller MIPS Technologies, Inc.; Roger Dressler and Rob Hislop Dolby Laboratories
1/28/2007 10:28 AM EST
Dolby Digital Plus, the newest generation of Dolby Digital, is making its way into next-generation home entertainment applications, bringing superior sound quality, more efficient audio compression, and an improved user experience to the home.
Dolby Digital Plus, also known as Enhanced AC-3 (E-AC-3), has been developed to address the changing requirements of two burgeoning consumer markets. For the emerging HD DVD and Blu-ray Disc optical disc players, the data compression capabilities of Dolby Digital Plus allow movie studios to combine a superior audio experience with high-definition video. The technology also enhances the latest digital TV set-top boxes (STBs), enabling broadcasters to deploy offerings with lower bit rates, reducing costs and adding flexibility and value for consumers.
While there are tremendous advantages to using Dolby Digital Plus, there are also design considerations that system-on-a-chip (SoC) teams need to understand. The rigorous approval testing that is required at Dolby Laboratories before a chip or a system can use the Dolby Digital Plus logo is substantial. Working with Dolby Laboratories, MIPS Technologies has developed an optimized, tested version of Dolby Digital Plus that will run on any of its 32-bit synthesizable processor cores—significantly slashing audio subsystem development time. This approach also lowers the overall cost of the SoC by eliminating the need for an expensive digital signal processor (DSP).
Emerging Standards for the Home Entertainment Market HD DVD and Blu-ray Disc
Dolby Digital Plus is mandatory in the HD DVD specification and is defined as optional in Blu-ray Disc. The specifications also allow for multiple independent streams so a movie can be shipped with multiple independent soundtracks or support multiple languages.
For the first time, HD DVD and Blu-ray Disc players allow external streams from the Internet to be mixed with the native audio on the disc after the disc has been shipped to the consumer. For example, a high-definition video stream can be accompanied by an audio track mastered on the disc that is simultaneously mixed with a more recent director's commentary streamed from a studio website.
Next-Generation Set-Top Boxes
Dolby Digital Plus is also vital to the next generation of set-top boxes. High-definition TV depends on high-quality sound for a complete experience. Broadcasters and network operators are all looking at ways to dramatically improve their offerings. The greater compression of Dolby Digital Plus allows 5.1 surround sound using as few as 224 kbps, a significant reduction from the current 384-448 kbps data rate typically used for Dolby Digital (AC-3). This provides an opportunity for broadcasters to enhance their services by allowing value-added services for the hearing or visually impaired. For those broadcasters using MPEG stereo at 160-256 kbps, they can upgrade to 5.1-channel surround sound using essentially the same bandwidth (see Figure 1).
Figure 1: The advantages of upgrading to Dolby Digital Plus for broadcasters.
Dolby Digital Plus maintains backward compatibility with the worldwide installed base of more than 36 million Dolby Digital audio/video receivers using an S/PDIF connection (Figure 2a). Fully decoding the stream to linear PCM and then re-encoding it to Dolby Digital would require significant processing power and would reduce the audio quality. Using an efficient low-latency, low-complexity conversion process, Dolby Digital Plus streams are converted into Dolby Digital streams more easily and with less impact on audio quality.
Figure 2a: The S/PDIF connection from a set-top box or optical disc player to an A/V receiver today requires that a Dolby Digital Plus stream be converted to a 640 kbps Dolby Digital bitstream.
While there are tremendous advantages to using Dolby Digital Plus, there are also design considerations that system-on-a-chip (SoC) teams need to understand. The rigorous approval testing that is required at Dolby Laboratories before a chip or a system can use the Dolby Digital Plus logo is substantial. Working with Dolby Laboratories, MIPS Technologies has developed an optimized, tested version of Dolby Digital Plus that will run on any of its 32-bit synthesizable processor cores—significantly slashing audio subsystem development time. This approach also lowers the overall cost of the SoC by eliminating the need for an expensive digital signal processor (DSP).
Emerging Standards for the Home Entertainment Market HD DVD and Blu-ray Disc
Dolby Digital Plus is mandatory in the HD DVD specification and is defined as optional in Blu-ray Disc. The specifications also allow for multiple independent streams so a movie can be shipped with multiple independent soundtracks or support multiple languages.
For the first time, HD DVD and Blu-ray Disc players allow external streams from the Internet to be mixed with the native audio on the disc after the disc has been shipped to the consumer. For example, a high-definition video stream can be accompanied by an audio track mastered on the disc that is simultaneously mixed with a more recent director's commentary streamed from a studio website.
Next-Generation Set-Top Boxes
Dolby Digital Plus is also vital to the next generation of set-top boxes. High-definition TV depends on high-quality sound for a complete experience. Broadcasters and network operators are all looking at ways to dramatically improve their offerings. The greater compression of Dolby Digital Plus allows 5.1 surround sound using as few as 224 kbps, a significant reduction from the current 384-448 kbps data rate typically used for Dolby Digital (AC-3). This provides an opportunity for broadcasters to enhance their services by allowing value-added services for the hearing or visually impaired. For those broadcasters using MPEG stereo at 160-256 kbps, they can upgrade to 5.1-channel surround sound using essentially the same bandwidth (see Figure 1).
Figure 1: The advantages of upgrading to Dolby Digital Plus for broadcasters.
Dolby Digital Plus maintains backward compatibility with the worldwide installed base of more than 36 million Dolby Digital audio/video receivers using an S/PDIF connection (Figure 2a). Fully decoding the stream to linear PCM and then re-encoding it to Dolby Digital would require significant processing power and would reduce the audio quality. Using an efficient low-latency, low-complexity conversion process, Dolby Digital Plus streams are converted into Dolby Digital streams more easily and with less impact on audio quality.
Figure 2a: The S/PDIF connection from a set-top box or optical disc player to an A/V receiver today requires that a Dolby Digital Plus stream be converted to a 640 kbps Dolby Digital bitstream.
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