Dolby Spectral Recording, commonly called Dolby SR,is a sound format that incorporates a Dolby SR-Type noise reduction process. It is used in many aspects of analog recording but for the purposes of Sprocket School we will be talking exclusively about its use with 35mm stereo optical track prints and 35mm/70mm film prints with magnetic soundtracks. It is a 4 channel format- left, center, right and surround.
It is the most common optical sound format on films made after 1986, when it began to replace Dolby A
It is important for projectionists to be able to distinguish between Dolby A and Dolby SR because different circuitry within the sound processor is required to play them back correctly. If you playback Dolby A as SR or vice versa, it will not sound correct.
Via the [Dolby Stereo] Wikipedia page: "The optical soundtrack on a Dolby Stereo encoded 35 mm film carries not only left and right tracks for stereophonic sound, but also—through a matrix decoding system (Dolby Motion Picture matrix or Dolby MP) similar to that developed for "quadraphonic" or "quad" sound in the 1970s—a third center channel, and a fourth surround channel for speakers on the sides and rear of the theater for ambient sound and special effects. This yielded a total of four sound channels, as in the 4-track magnetic system, in the track space formerly allocated for one mono optical channel. Dolby also incorporated its A-Type noise reduction into the Dolby Stereo system."
Dolby SR vs. Dolby A
Dolby SR is difficult to distinguish from Dolby A by sight. Here are some tips on telling them apart:
- Some (but not all) films made after 1986 will have Dolby SR tracks, though A-type was still in use well into the 90s.
- Some (but not all) films with Dolby SR tracks will display the words "Dolby SR" with the Dolby logo in the end credits. Some (but not all) films with Dolby A tracks will display the word "Dolby" - or "Dolby Stereo" rather than "Dolby SR" - with the Dolby logo in the end credits. But be careful, just because you see one of these in the credits of the film does not guarantee that it is A Type or SR. Some films were released as both mono and stereo prints, or a film may have been re-mastered and released as SR but was originally mono. Seeing the logo in the credits is just another clue to be used with alongside other sources of information.
- Some (but not all) films with Dolby SR tracks will display the words "Dolby SR" on the edges of the film leader.
- A Dolby SR track played in Dolby A will usually sound fine, but will have more noise than you would expect from a soundtrack with Dolby Noise Reduction.