Digital Surround Sound Formats
SRD
  SRD, also known as "SR Digital" is a 6-track format where the sound is mixed in six discrete channels: Left, Center, Right, Left Surround, Right Surround and Subwoofer. The 5.1 discrete 6-track mix is encoded and compressed digitally through a process called AC-3 and stored as tiny bits of information between the perforations of the film. The 6-track Printmaster is laid down on the dubbing stage directly onto a Magneto Optical Disc provided by Dolby, as well as a 6-track DA88 for backup. We now have near CD quality sound superior to the "optical" track and all its inherent problems. A matrixed 2-track LT/RT printmaster is still made so that if a fault occurs with the 6-track digital tracks on the film, the projection system will automatically switch to the analog LT/RT optical tracks.
DTS
  DTS, also known as "Digital Theater Systems" is sold by a competitor of Dolby, and is similar to SRD in that it is a 6-track digital system. The main difference is that only a "code" is put onto the film. This coding controls a CD player that contains the 6-track audio information. Like Dolby, a 2-track LT/RT printmaster is also present on the film. The main drawback of this system is that the projectionists in the field may not load the disc.
SDDS
  Sony Corporation's "Sony Dynamic Digital Sound" is yet another competitor with the same 6-track setup as SRD and DTS but with two additional channels (Left Extra, Right Extra) making it an 8-track system. All the information is on the film, in addition to the analog optical track, on the same print. SDDS does have a Digital back up track.