AC-3 supports audio sample-rates up to 48 kHz. Mono and stereo modes are also supported. The most elaborate mode in common usage involves five channels for normal-range speakers (20 Hz – 20,000 Hz) (right front, center, left front, rear right, rear left) and one channel (20 Hz – 120 Hz allotted audio) for the subwoofer driven low-frequency effects. Dolby Digitalĭolby Digital is the common version containing up to six discrete channels of sound. This delay can be adjusted in steps of the time between perforations, (approximately 10.4msec) Versionsĭolby Digital has similar technologies, included in Dolby Digital EX, Dolby Digital Live, Dolby Digital Plus, Dolby Digital Surround EX, Dolby Digital Recording, Dolby Digital Cinema, Dolby Digital Stereo Creator and Dolby Digital 5.1 Creator. If a penthouse soundhead is used then the data must be delayed in the processor for the required amount of time, around 2 seconds. To allow for the dual-soundhead arrangement the digital data is recorded 26 frames ahead of the picture. However for new projectors it made sense to use dual analogue/digital soundheads in the normal optical soundhead position under the projector head.

The simplest way of converting existing projectors is to add a so-called 'penthouse' digital soundhead above the projector head. Almost all current release cinema prints are of this type and will probably also include SDDS data and a timecode track to synchronise CDROMs carrying DTS soundtracks.Ī photo of a 35 mm film print featuring all four audio formats (or 'quad track')- from left to right: SDDS (blue area to the left of the sprocket holes), Dolby Digital (grey area between the sprocket holes labelled with the Dolby 'Double-D' logo in the middle), analog optical sound (the two white lines to the right of the sprocket holes), and the DTS time code (the dashed line to the far right.) 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.

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. This data is finally decoded into a 5.1 channel audio source.

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. A constant bit rate of 320kbit/s is used. 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. 5 Dolby technologies in packaged media formatsīatman Returns was the first film to use Dolby Digital technology when it premiered in theaters in Summer 1992.

Dolby digital recording logo