Dolby Digital Live Encoding

[Marketta Filipovich]

Weare consistently asked about connecting a PC to a home theater system. It's not as easy as it sounds. For example, while many sound cards on the market can pass Dolby Digital and DTS "raw" signals for decoding by the receiver, most EAX/A3D surround games are only able to be output via the sound card's analogue outputs. Dolby Digital and DTS have both offered solutions to allow for a real-time, digital 5.1 encoded signal from a PC to be made to your home theater system.

Dolby Digital specifically solved this dillemma recently by announcing its Dolby Digital Live. This technology has been integrated into select 915 chipset-based Intel Desktop Boards featuring Intel High Definition Audio and is also available on select audio cards. Sound cards currently supporting Dolby Digital LIVE technology include BlueGears' HDA Digital X-Mystique 7.1 Gold Sound Card and Turtle Beach's Montego DDL.

A real-time encoding technology, Dolby Digital Live converts any audio signal into a Dolby Digital bitstream for transport and playback through a home theater system. With it, your PC or game console can be hooked up to your Dolby Digital-equipped audio/video receiver or digital speaker system via a single digital connection, eliminating the need for multiple cables and ensuring the integrity of the audio signal.

The real-time interactive capabilities of Dolby Digital Live technology are ideally suited to PCs which, until now, only output multi-channel EAX/A3D audio via analogue multi-channel outputs. Video game consoles are also able to utilize this technology to provide real-time surround output.

In addition to select sound cards, Dolby Digital Live technology is also integrated into the motherboard audio chipsets of many high-performance PCs. These PCs provide an S/PDIF connector and use a digital cable for one-step connection to a home theater system. Dolby Digital Live can also enable other future entertainment capabilities on the PC because of its ability to deliver any audio source via a single digital interface to an existing home theater system.

Dolby Digital, originally synonymous with Dolby AC-3 (see below), is the name for a family of audio compression technologies developed by Dolby Laboratories. Called Dolby Stereo Digital until 1995, it is lossy compression (except for Dolby TrueHD). The first use of Dolby Digital was to provide digital sound in cinemas from 35 mm film prints. It has since also been used for TV broadcast, radio broadcast via satellite, digital video streaming, DVDs, Blu-ray discs and game consoles.

Dolby AC-3 was the original version of the Dolby Digital codec. The basis of the Dolby AC-3 multi-channel audio coding standard is the modified discrete cosine transform (MDCT), a lossy audio compression algorithm.[1] It is a modification of the discrete cosine transform (DCT) algorithm, which was proposed by Nasir Ahmed in 1972 for image compression.[2] The DCT was adapted into the MDCT by J.P. Princen, A.W. Johnson and Alan B. Bradley at the University of Surrey in 1987.[3]

Dolby Laboratories adapted the MDCT algorithm along with perceptual coding principles to develop the AC-3 audio format for cinema. The AC-3 format was released as the Dolby Digital standard in February 1991.[4][5] Dolby Digital was the earliest MDCT-based audio compression standard released, and was followed by others for home and portable usage, such as Sony's ATRAC (1992), the MP3 standard (1993) and AAC (1997).[6]

Batman Returns was the first film to be announced as using Dolby SR-D (Spectral Recording-Digital) technology when it premiered in all selected movie theaters in the summer of 1992.[7] Dolby Digital cinema soundtracks are optically recorded on a 35 mm release print using sequential data blocks placed between every perforation hole on the soundtrack side of the film. A constant bit rate of 320 kbit/s is used. A charge-coupled device (CCD) scanner in the image 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. The data is then decoded into a 5.1 channel audio source. All film prints with Dolby Digital data also have Dolby Stereo analogue soundtracks using Dolby SR noise reduction and such prints are known as Dolby SR-D prints. The analogue soundtrack provides a fall-back option in case of damage to the data area or failure of the digital decoding; it also provides compatibility with projectors not equipped with digital soundheads. Almost all modern cinema prints are of this type and may also include SDDS data and a timecode track to synchronize CD-ROMs carrying DTS soundtracks.

The simplest way of converting existing projectors is to add a so-called penthouse digital soundhead above the projector head. However, for new projectors it made sense to use dual analogue/digital soundheads in the normal optical soundhead position under the projector head. To allow for the dual-soundhead arrangement the data is recorded 26 frames ahead of the picture. If a penthouse soundhead is used, the data must be delayed in the processor for the required amount of time, around 2 seconds. This delay can be adjusted in steps of the time between perforations, (approximately 10.4 ms).

Dolby Digital[8] has similar technologies, included in Dolby Digital EX,[9] Dolby Digital Live,[10] Dolby Digital Plus,[11] Dolby Digital Surround EX,[12] Dolby Digital Recording,[13] Dolby Digital Cinema,[14] Dolby Digital Stereo Creator[15] and Dolby Digital 5.1 Creator.[16]

In 1991, a limited experimental release of Star Trek VI: The Undiscovered Country in Dolby Digital played in 3 US theatres.[19] In 1992, Batman Returns was the first movie to be released in Dolby Digital.[21][22] In 1995, the LaserDisc version of Clear and Present Danger featured the first home theater Dolby Digital mix, quickly followed by True Lies, Stargate, Forrest Gump, and Interview with the Vampire among others.[23][24]

Dolby Digital EX is similar to Dolby's earlier Pro Logic format, which utilized matrix technology to add a center surround channel 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.

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 Pro Logic 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 with Surround EX decoding and added speakers. A number of DVDs have a Dolby Digital Surround EX audio option.

Dolby Digital Live (DDL) is a real-time encoding technology for interactive media such as video games. It converts any audio signals on a PC or game console into a 5.1-channel 16-bit/48 kHz Dolby Digital format at 640 kbit/s and transports it via a single S/PDIF cable.[26] 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 digital PCM stereo or analog multichannel sound because S/PDIF over RCA, BNC, and TOSLINK can only support two-channel PCM, Dolby Digital multichannel audio, and DTS multichannel audio. HDMI was later introduced, and it can carry uncompressed multichannel PCM, lossless compressed multichannel audio, and lossy compressed digital audio. However, Dolby Digital Live is still useful with HDMI to allow transport of multichannel audio over HDMI to devices that are unable to handle uncompressed multichannel PCM.

Dolby Digital Live is available in sound cards using various manufacturers' audio chipsets. The SoundStorm, used for the Xbox game console and certain nForce2 motherboards, used an early form of this technology. DDL is available on motherboards with codecs such as Realtek's ALC882D,[27] ALC888DD and ALC888H. Other examples include some C-Media PCI sound cards and Creative Labs' X-Fi and Z series sound cards, whose drivers have enabled support for DDL.

NVIDIA later decided to drop DDL support in their motherboards due to the cost of involved royalties, leaving an empty space in this regard in the sound cards market.Then in June 2005 came Auzentech, which with its X-Mystique PCI card, provided the first consumer sound card with Dolby Digital Live support.

While they forgot about the plan, programmer Daniel Kawakami made a hot issue by applying Auzentech Prelude DDL module back to Creative X-Fi cards by disguising the hardware identity as Auzentech Prelude.[29]

Eventually Creative struck an agreement with Dolby Laboratories regarding the Dolby license royalty by arranging that the licensing cost be folded into the purchase price of the Creative X-Fi PCI cards rather than as a royalty paid by Creative themselves.[28] Based on the agreement, in September 2008 Creative began selling the Dolby Digital Live packs enabling Dolby Digital Live on Creative's X-Fi PCI series of sound cards. It can be purchased and downloaded from Creative. Subsequently, Creative added their DTS Connect pack to the DDL pack at no added cost.[33]

E-AC-3 (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 even more audio channels (up to 15.1 discrete channels[35] in the future), and improved coding techniques (only at low data rates) to reduce compression artifacts, enabling lower data rates than those supported by AC-3 (e.g. 5.1-channel audio at 256 kbit/s). It is not backward compatible with existing AC-3 hardware, though E-AC-3 codecs generally are capable of transcoding to AC-3 for equipment connected via S/PDIF. E-AC-3 decoders can also decode AC-3 bitstreams. The fourth generation Apple TV supports E-AC-3.[36] The discontinued HD DVD system directly supported E-AC-3. Blu-ray Disc offers E-AC-3 as an option to graft added channels onto an otherwise 5.1 AC-3 stream, as well as for delivery of secondary audio content (e.g. director's commentary) that is intended to be mixed with the primary audio soundtrack in the Blu-ray Disc player.

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