[1] The first was Dolby A, a professional broadband noise reduction system for recording studios that was first demonstrated in 1965, but the best-known is Dolby B (introduced in 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, although Dolby has as of 2016 ceased licensing the technology for new cassette decks.

As a result of their narrow tracks and slow speed, cassettes make tape hiss a very severe problem.

This basic concept, increasing the volume to overwhelm inherent noise, is known as pre-emphasis, and is found in a number of products.

Thus, as this portion of the signal decreases in amplitude, the higher frequencies are progressively increasingly attenuated, which also reduces in level the constant background noise on the tape when and where it would be most noticeable.

The correct calibration of the recording and playback circuitry is critical in order to ensure faithful reproduction of the original program content.

For accurate off-the-tape monitoring during recording on 3-head tape decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the "Double Dolby" label.

In 2004, Dolby A-type noise reduction was inducted into the TECnology Hall of Fame, an honor given to "products and innovations that have had an enduring impact on the development of audio technology.

It consisted of a single sliding band system providing about 9 dB of noise reduction (A-weighted), primarily for use with cassette tapes.

Without the de-emphasis effect of the decoder, the sound will be perceived as brighter as high frequencies are emphasized, which can be used to offset "dull" high-frequency response in inexpensive equipment.

The width of the noise reduction band is variable, as it is designed to be responsive to both the amplitude and the frequency distribution of the signal.

This continued in some record labels and hardware manufacturers even after Dolby C had been introduced, during the period when the new standard was relatively little-known.

In 1971 WFMT started to transmit programs with Dolby NR,[11] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.

A similar system named High Com FM was evaluated in Germany between July 1979 and December 1981 by IRT,[15] and field-trialed up to 1984.

As a result, recordings are cleaner and crisper with a much improved high-frequency response that the cassette medium heretofore lacked.

With a good quality tape, the Dolby C response could be flat to 20 kHz at the 0 dB recording level, a previously unattainable result.

An A-weighted signal-to-noise ratio of 72 dB (re 3% THD at 400 Hz) with no unwanted "breathing" effects, even on difficult-to-record passages, was possible.

Dolby C was also used on professional video equipment for the audio tracks of the Betacam and Umatic SP videocassette formats.

[1][21] In the motion picture industry, as far as it concerns distribution prints of movies, the Dolby A and SR markings refer to Dolby Surround which is not just a method of noise reduction, but more importantly encodes two additional audio channels on the standard optical soundtrack, giving left, center, right, and surround.

Dolby S mostly appeared on high-end audio equipment and was never widely used, although it did filter down to several mid-tier models manufactured during the mid to late 1990s.

It is basically a cut-down version of Dolby SR optimized specifically for cassette recording, and uses many of the same noise reduction techniques.

If the audio signal contains strong high-frequency content (in particular from percussion instruments such as hi-hat cymbals), this adds to the constant bias causing magnetic saturation on the tape.

Bang & Olufsen continued work in the same direction, which resulted in a 1981 patent (EP 0046410) by Jørgen Selmer Jensen.

[24] Bang & Olufsen immediately licensed HX-Pro to Dolby Laboratories, stipulating a priority period of several years for use in consumer products, to protect their own Beocord 9000 cassette tape deck.