Noise Reduction Systems
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 portions of the signal coming into the tape and boosts their volume during recording. Then, it identifies these same frequencies on the tape during playback and lowers their volume. The music then sounds as it did before it was boosted, but the volume of the annoying hiss, which is a high-frequency signal, is also reduced. The Dolby circuit also has other refinements to keep hiss to a minimum. Dolby B, an improved version of the original Dolby system, was introduced in 1970, reducing tape hiss by 10 decibels. In addition to Dolby, there are other noise reduction systems, such as DBX, that operate on different principles. However DBX recorders had Dolby circuits incorporated within them, and eventually disappeared from the consumer market.
Another way to combat tape hiss is by improving the tape itself. Some tapes are produced with special oxide layers that can be more strongly magnetized than regular tapes. For instance, some "premium" quality tapes utilize chromium compounds in their formulation. Another innovation is the introduction of a pure metal coating, which allows for professional quality recording of compact disks.
A relatively new recording technology, digital audiotape (DAT), eliminates the problem of tape noise entirely. DAT encodes the music on the tape in the form of impulses representing binary digits (ones and zeros), the same way data is stored on a computer. These impulses are then decoded to produce music when the tape is played. Because the advanced circuitry of this system cannot mistake tape hiss for the digital data, no tape noise is produced upon playback. However, Dolby Laboratories introduced Dolby C in the late 1980s, then Dolby S in 1990 as analog technologies that reduce noise to a level comparable to DAT. Dolby C reduced tape hiss by 20 decibels. Based on Dolby's Spectral Recording system, commonly found in non-digital recording studios, Dolby S eventually replaced Dolby C and reduces noise by up to 24 decibels in high frequencies and 10 in lower frequencies. Dolby S works slightly differently from other Dolby systems by reacting to the signal to which it is applied, conforming to the signal's characteristics. This allows for a wider range of noise reduction. As of the late 1990s, neither DAT or Dolby S has eliminated the other, though the sale of cassettes has continued to fall in the face of the compact discs.
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