Dolby A: Difference between revisions

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 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!
-===Dolby A vs. [[Dolby SR]]===
+===Visual Identification===
 Dolby A is difficult to distinguish from [[Dolby SR]] by sight. Here are some tips on telling them apart:
 * Many studio films made between 1975 and 1985 will have Dolby A tracks.
 * 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 A tracks will display the words "Stereo" on the edges of the film [[leader]].
-* A Dolby A track played in Dolby SR will usually sound harsh or blown out. A Dolby SR track played in Dolby A will sound acceptable, so if you really cannot figure it out Dolby A is the better choice.
+===Auditory Identification==
+If you are still unsure, the best way to identify whether you need to play something as Dolby A or Dolby SR is to run a reel and listen. Stand in the room where the film is being played, close enough that you can hear well. It's harder to do this using a booth monitor. Have someone else switch between A and SR if you cannot do it yourself while in the auditorium. Follow the below tips and it should become fairly obvious what is correct.
+* A played as SR: the expander does not match the compressor. It will sound bright, tinny, and sibilant.
+* SR played as A: Dolby indended SR to be backwards compatible ("reasonably well"). Though the expander and compressor don't match, so it will not sound very good. It's a little harder to identify than A played as SR. It will sounds dull and flat, because you're not getting the full dynamic range of the recorded track. This likely will be more obvious in scenes with music or a lot of sound, rather than dialogue only.
+* Mono as A or SR: you're putting it through an expander (it was never compressed, recorded with 30db dynamic range). This will sound pretty obviously incorrect. Sounds exaggerated, loud, and very harsh.
 <gallery widths=250px heights=250px>
 File:Dolby stereo.jpg|Dolby Stereo logo.