Dolby noise reduction
The family of compander systems that tamed tape hiss — quiet treble is boosted on recording and pulled back down on playback, taking the hiss with it.
Ray Dolby founded Dolby Laboratories in London in 1965 and started with Dolby A, a four-band system for studio reel-to-reel machines. What made the name famous was Dolby B in 1968, a simplified version that only works on the treble and conquered the cassette deck. Dolby C followed in 1980 with roughly twice the noise reduction, and Dolby SR arrived for studios in 1986.
The trick is to compress quiet high frequencies on the way onto tape and expand them on the way off. That only works if both ends are calibrated alike: a tape recorded at the wrong level or played on a deck with badly aligned heads comes out dull or pumping. Play a B-encoded tape without decoding and you get the opposite — bright, hissy treble, which plenty of cassette owners mistook for hi-fi.
Rival dbx compressed the whole band and delivered more noise reduction, but sounded worse when calibration was off.