page_title: Dolby Digital
section_title: Cinema
hierarchical_section_title: Dolby Digital / History / Cinema
caption_attribution_description: English: Dolby Digital track reader mounted on a Kalee model 20 projector.
context_page_description: Dolby Digital, also known as Dolby AC-3, is the name for audio compression technologies developed by Dolby Laboratories. Originally named Dolby Stereo Digital until 1995, except for Dolby TrueHD, the audio compression is lossy, based on the modified discrete cosine transform algorithm. The first use of Dolby Digital was to provide digital sound in cinemas from 35 mm film prints; today, it is now also used for other applications such as TV broadcast, radio broadcast via satellite, digital video streaming, DVDs, Blu-ray discs and game consoles.
context_section_description: Batman Returns was the first film to be announced as using Dolby SR-D 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 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 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 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).
  As of 2015, Dolby Digital in film sound mixing is being gradually replaced with Dolby Surround 7.1, with the more advanced Dolby Atmos technology also gaining in popularity. While majority of movie theaters currently utilize Dolby Digital, virtually all films released today are mixed in Dolby Surround 7.1 and Dolby Atmos.
caption: A Dolby Digital Penthouse Soundhead mounted on a mid-1950s vintage Kalee model 20 projector
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