When audio cassette tapes were introduced, they quickly became more popular than open-reel and eight-track tapes because of their small size. However, their miniature size also created a new problem. On a length of tape, the narrower the recording track, the larger, relatively, are the iron oxide particles which carry the magnetized sound impulses on the surface of the tape. Thus, there is less "room" for the musical signal in relation to the inherent noise on the tape, caused by the random magnetization in the oxide particles. As a result, unwanted hiss may be heard on cassette tapes, especially during quiet musical passages.

There have been several systems introduced to combat this tape hiss. The first and most popular was the one created by an American, Ray Dolby, in 1967. Originally called the S/N Stretcher, the Dolby system employs complex circuitry that detects high-frequency...