Dolby Digital
Dolby TrueHD, developed by Dolby Laboratories, is an advanced lossless audio codec based on Meridian Lossless Packing. Support for the codec is mandatory for HD DVD and optional for Blu-ray Disc hardware. TrueHD supports 24 bit, 96 kHz audio channels at up to 18 Mbit/s over 14 channels (HD DVD and Blu-ray Disc standards currently limit the maximum number of audio channels to eight). It also supports extensive metadata, including dialog normalization and Dynamic Range Control.
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Channel configurations
Although most commonly associated with the 5.1 channel configuration, Dolby Digital allows a number of different channel selections. The full list of available options is:
- Mono (Center only)
- 2-channel stereo (Left + Right), optionally carrying matrixed Dolby Surround
- 3-channel stereo (Left, Center, Right)
- 2-channel stereo with mono surround (Left, Right, Surround)
- 3-channel stereo with mono surround (Left, Center, Right, Surround)
- 4-channel quadrophonic (Left, Right, Left Surround, Right Surround)
- 5-channel surround (Left, Center, Right, Left Surround, Right Surround)
All of these configurations can optionally include the extra Low Frequency Effect (LFE) channel. The last two with stereo surrounds can optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.
Many Dolby Digital decoders are equipped with downmixing functionality to distribute encoded channels to available speakers. This includes such functions as playing surround information through the front speakers if surround speakers are unavailable, and distributing the center channel to left and right if no center speaker is available. When outputting to separate equipment over a 2-channel connection, a Dolby Digital decoder can optionally encode the output using Dolby Surround to preserve surround information.
The '.1' in 5.1, 7.1 etc. refers to the LFE channel, which is also a discrete channel.
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Applications of Dolby Digital
Dolby Digital SR-D cinema soundtracks are optically recorded on a 35 mm release print using sequential data blocks placed between every perforation hole on the sound track side of the film. A CCD scanner in the projector picks up a scanned video image of this area, and a processor correlates the image area and extracts the digital data as an AC-3 bitstream. These data are finally decoded into a 5.1 channel audio source.
Dolby Digital audio is also used on DVD-Video and other purely digital media, like home cinema. In this format, the AC-3 bitstream is interleaved with the video and control bitstreams.
The system is used in many bandwidth-limited applications other than DVD-Video, such as digital TV. The AC-3 standard allows a maximum coded bit rate of 640 kbit/s. 35 mm film prints use a fixed rate of 320 kbit/s. HD-DVD and DVD-Video discs are limited to 448 kbit/s, although many players can successfully play higher-rate bitstreams (which are non-compliant with the DVD specification). ATSC and Digital cable standards limit AC-3 to 448 kbit/s. Blu-ray Disc, the Sony PlayStation 3 and the Microsoft Xbox game console can output an AC-3 signal at a full 640 kbit/s. Some Sony PlayStation 2 console games are also capable to output AC-3 standard audio as well.
Dolby is also part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and considered the successor to MP3. AAC outperforms AC-3 at any bitrate, but is more complex.[8]
Dolby Digital Plus (DD-Plus) is supported in HD DVD, as a mandatory codec, and in Blu-ray Disc, as an optional codec.
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Dolby Technologies in packaged media formats
| HD DVD | Blu-ray | DVD | DVD-Audio | Laserdisc | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Codec | Player support | Channels (max) | Max Bit Rate | Player support | Channels (max) | Max Bit Rate | Player support | Channels (max) | Max Bit Rate | Player support | Channels (max) | Max Bit Rate | Player support | Channels (max) | Max Bit Rate |
| Dolby Digital | Mandatory | 5.1 | 504 kbit/s | Mandatory | 5.1 | 640 kbit/s | Mandatory | 5.1 | 448 kbit/s | Optional in video zone for playback compatibility on DVD-Video players | 5.1 | 448 kbit/s | Optional | 5.1 | 384 kbit/s |
| Dolby Digital Plus | 7.1 | 3 Mbit/s | Optional | 7.1 | 1.7 Mbit/s |
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| Dolby TrueHD | 8 | 18 Mbit/s | 8 | 18 Mbit/s | |||||||||||
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Technical details
The data layout of AC-3 is described by simplified "C-like" language in official specifications. An AC-3 stream is made up by a series of synchronization frames, which are composed of six audio blocks. Each audio block contains 256 audio samples per channel. Note 6×256 = 1536 = Audio frame size. Below is a simplified AC-3 header intended to give an introduction into the data syntax. A detailed description of the header can be found in the ATSC "Digital Audio Compression (AC-3) Standard", section 5.4.
| Field Name | # of bits | Description |
|---|---|---|
| syncword | 16 | 0x0B77 Transmission of data is left bit first also known as Big Endian. |
| CRC | 16 | |
| Sampling frequency | 2 | '11'=reserved '10'=32 kHz '01'=44.1 '00'=48 |
| Frame Size Code | 6 | |
| Bit Stream Identification | 5 | |
| Bit Stream Mode | 3 | '000'=main audio service |
| Audio Coding Mode | 3 | '010'=left, right channel ordering |
| Center Mix level | 2 | |
| Surround Mix Level | 2 | |
| Dolby Surround Mode | 2 | '00'=not indicated '01'=Not surround encoded '10'=Yes, surround encoded. |
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See also
- Dialnorm – Dolby Digital metadata parameter controlling decoder gain.
- Dolby Laboratories – company history and technology development.
- Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
- Dolby Stereo – first cinema analogue surround sound system
- Dolby Pro Logic – consumer version of the Dolby Stereo analogue surround sound system.
- Dolby TrueHD – lossless codec for HD DVD and Blu-ray Disc
- Home cinema
- Loudspeaker
- SoundStorm – a real-time AC3 encoder for nForce2-based computers.
- C-Media – Producer of DDL audio chipsets used in many sound cards and motherboards.
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References
- ^ Dolby Digital Live – Dolby.com
- ^ Montego DDL – Turtlebeach.com
- ^ [1] – Htomega.com
- ^ HDA X-Plosion 7.1 DTS Connect – Auzentech.com
- ^ [2] – techreport.com
- ^ NVIDIA nForce 500 motherboard reviews – AnandTech.com, June 8, 2006
- ^ Daniel_K, Who Fixed Creative's Broken Vista Drivers, Speaks Out Wired.com
- ^ http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf
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External links
- Dolby Laboratories
- Dolby Chart
- AC3Filter - open source DirectShow AC3 decoder
- Open source liba52 dolby digital decoder
- ATSC standards, from which the A/52B "Digital Audio Compression (AC-3) Standard" may be downloaded
- A52Codec QuickTime codec, enables QuickTime to play back AC3 streams on PPC and Intel Macs
- Dolby Digital 5.1 television programs
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