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SR-D

From Chace Audio

SR-D refers to Dolby's proprietary method of including Dolby® Digital audio, in addition to analog audio, on an optical sound track for theatrical playback. The Dolby Digital track can be up to 6 channels and typically contains a 5.1 surround mix. “SR” refers to the Dolby SR noise reduction used in the analog sound track, and the “D” refers to the digital track. The digital audio information is stored between the sprocket holes, and the analog Lt/Rt track is located in the standard sound track area between the sprocket holes and the film frames. The source of this audio is an M.O.D. (master optical disc) that is provided by and mastered at Dolby Labs.

If the digital audio area on the film becomes damaged, the audio will default to the analog Lt/Rt track. This change can be heard in theaters most noticeably as a loss of sub-bass and surround information. It is common now that most studios encode release prints not only with SR-D, but also with the other digital surround formats DTS and SDDS. This is so all prints will be compatible with all theaters no matter what audio format they have installed.

The four standard soundtracks found on optical film are highlighted in this drawing. The digital SR-D track is illustrated in blue.

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Macro of 35mm film audio tracks, from left to right: Sony SDDS, Dolby® Digital, analog Optical, and DTS time code. Image courtesy of Wikipedia Commons.

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