Dolby A is a sound format that incorporates a Dolby A-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 was in use from approximately 1975 to 1986, when it was largely replaced by Dolby SR.

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[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!

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 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 blindly trust labels added to the print by other projectionists or even from the distributor!

Production Date

A new print of American Graffiti with a remastered Dolby track. Released in 1973, original prints would have been mono.
  • 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.
  • Many studio films made between 1975 and 1985 will have Dolby A tracks.
  • Some (but not all) films made after 1986 will have Dolby SR tracks, though A-type was still in use well into the 90s.

Visual Inspection

Lab leader identifying a print as Dolby A.
  • 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 different in others. You may need to check multiple spots along the soundtrack before you encounter one where they are different. Check areas that have music in them, the credits at the end of the film are often a good spot to do so.
  • 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" or "Dolby SR Spectral Recording" with the Dolby logo in the end credits.
  • Some (but not all) films with Dolby A tracks will display the words "Stereo" on the edges of the film leader.
  • Some (but not all) films with Dolby SR tracks will display the words "Dolby SR" on the edges of the film leader.
    • Note: If you use this information first make sure the leader is original to the film


If you cannot be 100% certain by using the information above, the best way to identify whether you need to play something as Dolby A or Dolby SR is just 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.

See Also

External Links