Dolby SR
The Dolby SR (Spectral Recording) system, introduced in 1986, was the company's second professional noise reduction system. It is a much more aggressive noise reduction approach than Dolby A. It attempts to maximise the recorded signal at all times using a complex series of filters that change according to the input signal. As a result, Dolby SR is much more expensive to implement than Dolby B or C, but Dolby SR is capable of providing up to 25 dB noise reduction in the high frequency range. It is only found on professional recording equipment.
In the motion picture industry, as far as it concerns distribution prints of movies, the Dolby A and SR markings refers to Dolby Surround which is not just a method of noise reduction, but more importantly encodes two additional audio channels on the standard optical soundtrack, giving left, center, right, and surround.
SR prints are fairly well backward compatible with old Dolby A equipment. The Dolby SRD marking refers to Dolby Digital and its variants.
Read more about this topic: Dolby Noise-reduction System