Dolby Digital - Dolby Digital in The Cinema

Dolby Digital in The Cinema

Batman Returns was the first film to use Dolby Digital technology when it premiered in theaters in Summer 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. This data is 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 digital data 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 will probably 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 digital data is recorded 26 frames ahead of the picture. If a penthouse soundhead is used then 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 msec)

As of August 2012, Dolby Digital in the cinema is being gradually replaced with Dolby Surround 7.1, process that may take up to the year 2016 to complete.