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Dolby Surround Sound

Dolby Surround was the earliest consumer version of Dolby's multichannel analog film sound decoding format Dolby Stereo. It was introduced to the public in 1982 during the time home video recording formats (such as Betamax and VHS) were introducing Stereo and HiFi capability. The term Dolby Surround is used so as not to confuse theater surround (Dolby SR, which has four channels of audio) with home stereo, which has only two. Dolby Surround is the earliest domestic version of theatrical Dolby Stereo. The term also applies to the encoding of material in this sound format.

Dolby Digital is the name for audio compression technologies developed by Dolby Laboratories. It was originally named Dolby StereoDigital until 1994. Except for Dolby TrueHD, the audio compression is lossy. The first use of Dolby Digital was to provide digital sound in cinemas from 35mm film prints. It is now also used for other applications such as HDTV broadcast, DVDsBlu-ray Discs and game consoles.

Dolby Digital audio is 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 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. 35mm film prints use a fixed rate of 320 kbit/s, which is the same as the maximum bit rate for 2-channel MP3. 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). HD DVD limits AC-3 to 448 kbit/s. ATSC and digital cable standards limit AC-3 to 448 kbit/s. Blu-ray Disc, the PlayStation 3 and the Xbox game console can output an AC-3 signal at a full 640 kbit/s. Some Sony PlayStation 2 console games are able to output AC-3 standard audio as well, primarily during pre-rendered cutscenes.

Dolby is 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.

Dolby Digital Plus (DD-Plus) and TrueHD are supported in HD DVD, as mandatory codecs, and in Blu-ray Disc, as optional codecs.

Batman Returns was the first film to use Dolby Digital technology when it premiered in theaters in the summer of 1992. Dolby Digital 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 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 are finally 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 current release 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 are 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).

As of 2014, Dolby Digital in the cinema is being gradually replaced with Dolby Surround 7.1, a process that may take up to the year 2016 to complete.

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