"Dolby's first development under the aegis of his new company was called Dolby A-type® noise reduction. It was a sophisticated new form of audio compression and expansion that dramatically reduced the background hiss inherent in professional tape recording without discernible side effects on the material being recorded. Among the new concepts incorporated into the system was the treatment of soft signals only, leaving the loud signals that naturally mask noise unprocessed, and dividing the spectrum into multiple bands to prevent the pumping (noise modulation) inherent with conventional wideband companders."
"While at first glance noise reduction (NR) appears to be an esoteric invention with limited applications, its effects on the audio industry have been profound. The multitrack recording techniques that blossomed in the late 1960s and early 1970s, for example, came about only because of Dolby A-type NR. Without it, the high tape hiss resulting from the combination of narrow tracks and multiple mixdowns would have been intolerable. And when applied to consumer formats and motion picture sound, the results were to be even more far-reaching. "