Information AboutDolby Digital |
| CATEGORIES ABOUT DOLBY DIGITAL | |
| consumer electronics | |
| audio codecs | |
| film sound production | |
| high-definition television | |
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VERSIONS Dolby Digital includes several similar technologies, which include Dolby Digital, Dolby Digital EX, Dolby Digital Live, Dolby Digital Surround EX, Dolby Digital Plus, and Dolby TrueHD. Dolby Digital Dolby Digital, or AC-3, is the common version containing up to six discrete channels of sound, with five channels for normal-range speakers (20 Hz – 20,000 Hz) (Right front, Center, Left Front, Right Rear and Left Rear) and one channel (20 Hz – 120 Hz) for the Subwoofer driven Low Frequency Effects . Mono and Stereo modes are supported, as well. AC-3 supports audio sample-rates up to 48KHz. Batman Returns was the first film to use the Dolby Digital technology when it premiered in theaters in Summer 1992. The LaserDisc version of Clear And Present Danger featured the very first Home Theater Dolby Digital mix in 1995. This Codec has several aliases, which are different names for the same codec:
Dolby Digital EX Dolby Digital EX is similar in practice to Dolby's earlier Pro-Logic format, which utilized Matrix technology to add a center and single rear surround channel to stereo soundtracks. EX adds an extension to the standard 5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating 6.1 or 7.1 channel output. However, the format is not considered a true 6.1 or 7.1 channel codec because it lacks the capability to support a discrete 6th channel unlike the competing DTS -ES codec. Dolby Digital Live Dolby Digital Live (DDL) is a real-time encoding technology for interactive media such as 2006 ALC888DD and ALC888H. A similar technology known as DTS Connect is available from competitor DTS . An important benefit of this technology is that it enables the use of digital multichannel sound with consumer sound cards, which are otherwise limited to PCM stereo or multichannel analog. Dolby Digital Surround EX Dolby Digital Surround EX was co-developed by ''. It provides an economical and backwards-compatible means for 5.1 soundtracks to carry a sixth, center back surround channel for improved localization of effects. The extra surround channel is matrix encoded onto the discrete Left Surround and Right Surround channels of the 5.1 mix, much like the front center channel on Dolby Surround encoded stereo soundtracks. The result can be played without loss of information on standard 5.1 systems, or played in 6.1 or 7.1 on systems equipped with Surround EX decoding and additional speakers. Dolby Digital Surround EX has since been used for the Star Wars prequels on the DVD versions and also the remastered original Star Wars trilogy. A number of DVDs have Dolby Digital Surround EX audio option. Dolby Digital Plus See Also: Dolby Digital Plus E-AC-3, more commonly known as Dolby Digital Plus, is an enhanced coding system based on the AC-3 codec. It offers increased bitrates (up to 6.144 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce Compression Artifacts , and backward compatibility with existing AC-3 hardware. Dolby TrueHD See Also: Dolby TrueHD 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 8 channels (although HD DVD and Blu-ray Disc standards currently limit their maximum number of audio channels to eight, Dolby TrueHD can output up to 14 sound channels simultaneously). It also supports extensive metadata, including dialog normalization and Dynamic Range Control. 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:
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. APPLICATIONS OF DOLBY DIGITAL , a competing system.]] Dolby Digital SR-D Cinema Soundtrack s 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 part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and also considered the successor to MP3 . AAC outperforms AC-3 at any bitrate, but is more complex. The advantages of AAC become clearly audible at less than 400 kbit/s for 5.1 channels, and at less than 180 kbit/s for 2.0 channels. Dolby Digital Plus (DD-Plus) is supported in HD DVD , as a mandatory codec, and in Blu-ray Disc , as an optional codec. DOLBY TECHNOLOGIES IN PACKAGED MEDIA FORMATS SEE ALSO
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. REFERENCES EXTERNAL LINKS
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