Dolby C

A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog magnetic tape recording. The first was Dolby A, a professional broadband noise reduction for recording studios in 1965, but the best-known is Dolby B (introduced 1968), a sliding band system for the consumer market, which helped make high fidelity practical on cassette tapes which used a relatively noisy tape size and speed. It is common on high fidelity stereo tape players and recorders to the present day. Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use. Dolby B, C, and S were designed for the consumer market. Aside from Dolby HX, all the Dolby variants work by companding, or compressing the dynamic range of the sound during recording and expanding it during playback.

When recording a signal on magnetic tape, there is a low level of "noise" in the background which sounds like hissing. One solution is "low-noise" tape which records more signal, and less "noise", run tape at a higher speed, or use a wider tape. Cassette tapes were originally designed to trade off fidelity for convenience of recording voice by using a very narrow tape run at a very slow speed of 1 7/8 inches per second in a simple plastic shell when 15 or 7 1/2 ips was for high fidelity and 3 and 3/4 was lower fidelity. When Advent designed one of the first cassette tape decks for high fidelity, noise reduction was one way to fix problems with the format, along with chrome and high-bias tapes with extended high frequency response. With 10 to 20 years, the original reel-to-reel high fidelity tape recorders which did not need noise reduction were eventually replaced by cassette decks for home use. The dominant noise reduction scheme "Dolby B" was widely accepted because if an inexpensive cassette player lacked the switch, they would just sound brighter which often offset the dull sounds of cheap players.

The signal to noise ratio is simply how large the music signal is compared to the low level of the "noise" with no signal. When the music is loud, the low hiss is not noticeable, but when the music is soft or in silence, most of what can be heard is the noise. If the recording level is adjusted so that the music is always loud, then it could in theory be turned down later, and the noise volume would also be turned down. The idea is for electronics to automatically increase the recording volume when it is soft, but reduce the volume on playback. Some schemes like Dolby B concentrate only on the high frequencies so that the "hiss" sound of noise will be masked when volume is turned down for playback.

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