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Dolby NR is a noise reduction system developed by Dolby Laboratories for use in analogue magnetic tape recording. It works by companding, i.e. reducing the dynamic range of the sound during recording and expanding it during playback. Several types of Dolby NR were developed, including A (1966), B (1968), C (1980), S, and SR. Most widely used in consumer products is the B type, which allows for acceptable playback on devices without noise reduction. Dolby A and Dolby SR were developed for professional use. Dolby A operates in four bands providing 10 dB noise reduction across the full frequency spectrum. The Dolby SR system 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 it is much more expensive to implement than Dolby B or C, but is capable of providing up to 25 dB noise reduction in the high frequency range. Dolby SR is only found on professional recording equipment.