Dolby SR: Difference between revisions

Via the  [[https://en.wikipedia.org/wiki/Dolby_Stereo 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[1]) 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 A noise reduction is a process in which the dynamic range of a recording is compressed, and then expanded upon playback via a Dolby sound processor. In basic terms, during noise reduction a compressor boosts the quiet parts and reduces the loud parts while an expander makes the loud parts louder and quiet parts quieter. [[Dolby SR]] (spectral recording) applies a different compression and expansion characteristic to each frequency band. Dolby A applies the same one to all the bands. This is part of why it sounds bad when you playback an A-type track with Dolby SR noise reduction. You (or rather the Dolby processor) is applying the incorrect expansion characteristic. The expander does not match the compressor!  

[[File:Dolby stereo SR .JPG|right|thumb|400px|Dolby logo that may appear in the end credits of some films recorded using Dolby SR.]]

Dolby SR is difficult to distinguish from [[Dolby A]] by sight. Like with [[aspect ratios]] it is best to use multiple sources in order to help inform your decision, including things like the production year of the print, any labels identifying format on the print itself, on the leader, or in the credits. Country of origin, whether the film was a hollywood production or low-budget independent film. Here are some tips on telling them apart:

* First, make sure it's not a [[mono]] print with two [[variable area]] tracks. This is very common. It's safe to say that any film made before 1975 will be a mono print. There are some cases where mono films were re-released later on with remastered Dolby tracks, but these usually are fairly well labeled and easy to identify. There are many films released in mono post-1975, especially foreign titles or films made with smaller budgets. Look at the two tracks closely, with mono prints they will be identical. With Dolby A or SR prints the tracks will be the same in some spots and differ in others. Check areas that have music in them, the credits at the end of the film are often a good spot.