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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 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!  


===Visual and Contextual Identification===
===Optical track Identification===
Dolby A is difficult to distinguish from [[Dolby SR]] 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:
Dolby A is difficult to distinguish from [[Dolby SR]] by sight. Like with [[aspect ratios]] it is best to use multiple sources of information in order to help inform your decision. Such sources include the production year of the print, any identifying labels or text on the leader, or in the credits, country of origin, and production information like whether the film was a Hollywood production or low-budget independent film. Do not blinding trust labels added to the print by other projectionists or even from the distributor!
* 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.   
 
====Production Date====
*It's safe to say that any film made before 1975 will be a mono print. However there are many films released in mono _after_ 1975, especially foreign titles or films made with smaller budgets.
**Exceptions: 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.  
 
====Visual Inspection====
* First, make sure it's not a [[mono]] print with two [[variable area]] tracks. This is very common. 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.   
* Many studio films made between 1975 and 1985 will have Dolby A tracks. If the tracks look different at any point, you can rule out mono.  
* Many studio films made between 1975 and 1985 will have Dolby A tracks. If the tracks look different at any point, you can rule out mono.  
* 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 made after 1986 will have Dolby SR tracks, though A-type was still in use well into the 90s.
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