Dolby Stereo

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Different audio formats on one 35mm film. The two analog Dolby sound tracks can be seen clearly in the right half of the picture.

Dolby Stereo is an analog optical sound format for cinema films introduced by the Dolby company in 1975 . The first film released with this sound standard was A Star Is Born .

In the age of analogue film reproduction, every movie produced, regardless of the digital soundtracks, contained an analogue optical soundtrack, whereby the soundtrack was not only available in stereo, but also with noise suppression according to the Dolby SR compander system , and Dolby A in older copies . (There is a clear distinction between the stereo method and the noise suppression method, since pure mono soundtracks can also be noise-suppressed by means of Dolby SR.) In this, actually two-track soundtrack, four channels (left, center, right, surround sound) are matrixed which are scanned by a light sound device when the film copy is played back on the projector and dematrized by the sound processor. The processor processes these two audio tracks - by adding, subtracting and phase shifting the signals - in such a way that audio information that only occurs in the left or right audio track is reproduced on the left or right, and information that occurs equally in both is reproduced in placed in the middle and faded out left and right. The same audio information that appears in the left audio track with a time delay to the right is faded out in the stage channels and placed in the surround sound channel. Sound information that occurs proportionally is divided accordingly between the relevant channels. All types of optical soundtracks are downwardly compatible, so (e.g. in the event of interference) an SR soundtrack can also be played back with devices that only perform noise suppression in accordance with Dolby-A or not at all, and also dispense with de-matrixing in which case only the left and right stage channel or the one existing mono channel is recorded. In return, the playback of a mono soundtrack in Dolby A or a Dolby A soundtrack in Dolby SR can lead to undesirable playback effects, which is why the projectionist should carefully check the sound format when preparing for a film screening.

If the cinema does not have the necessary technical equipment (sound equipment and processors) to reproduce digital sound, or if the digital sound fails due to a fault or excessive error rates, the analogue sound can still be reproduced. Depending on the quality and condition of the film copy, and depending on the high quality of the cinema's sound system installed and maintained, when switching from analogue to digital sound there are at best barely or at worst noticeable differences in the playback quality.

Dolby Stereo is also used as Dolby Surround in the home, although this process uses the same matrixing technique, but does not include noise suppression due to the lower volume and amplification. The digital advancement is called Dolby Digital , which has also found its way into the home. In regular cinema operations, all film-based, whether with analog or digital signal processing, have lost a lot of relevance and play due to the digitization of projection and sound reproduction, like most other components of classic cinema technology, in favor of uncompressed, 24-bit PCM sounds with a maximum of 16 channels at most it still plays a role where film copies are archived, in museums or in the context of re-performances or repertoire performances.

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