Dolby SR (Spectral Recording) noise reduction system was developed by Dolby Laboratories. It is a four-channel noise reduction format that improves the dynamic range of analog recordings and transmissions by 25 dB. The system has been in common use in professional audio since 1986 and in cinema since the early 1990s. Dolby SR is a revised version of the earlier formats and merges the aspects of Dolby A, B and C for an improved dynamic range of analogue recordings and transmissions.
The system is used by recording and post production engineers, broadcasters and audio professionals in professional audio analog recording. Almost all 35 mm film release prints use it as the optical analog format. The system is invariably an accompanying optical analog format, when Dolby Digital is present. The SR track is used in cinemas incapable of digital playback for films with digital soundtracks. In this case, it serves as a backup to deal with problems related to the digital track.
Dolby’s Cat.280 card was originally used for the implementation of Dolby SR that was pin-compatible with the Cat.22 A-type noise reduction card. This helped in upgrading the devices that used Cat. 22 card from A to SR, by replacement of Cat.22 with the Cat.280 card. This card functions in many Dolby devices with the requirement of Cat.280T for decoding only.
The spectral recording process mostly relies on actual signal improvements, but another complicated masking technique is used for better results than a digital recording. This makes Dolby SR score in terms of apparent noise level, in comparison to digital recording.