Dolby Laboratories have developed a series of noise reduction systems for use in analogue magnetic tape recording, which are named as Dolby Noise Reduction Systems (Dolby NR). The technology employs a form of dynamic preemphasis during recording and deemphasis during playback that work in tandem for the improvisation of signal-to-noise ratio.
The noise reduction systems may operate across the entire spectrum or specifically emphasize on the audible frequency range where the background tape hiss is most noticeable. The background hiss is similar to white noise, an artifact of the recording process that is usually noticeable above 1 kHz. At these higher frequencies during recording, the Dolby preemphasis boosts the recorded level of the audio signal that compresses the dynamic range of that portion of the signal. This provides a proportionally greater boost to the quieter sounds above 1 kHz.
The amplitude of the signal higher than 1 kHz is used to calculate the amount of preemphasis that is to be applied, as the tape is recorded. When the signal rises in amplitude, the amount of preemphasis to be applied is reduced. Low level signals are boosted by 10 dB or 20 dB and no signal modification is performed at the “Dolby level” (+3 VU). Thus, recording is done at higher overall level on the tape, relative to its noise level. This requires preservation of the signal without distortions.
Playback employs the opposite process known as deemphasis, on the basis of relative signal component above 1 kHz. This decreases the amplitude of the signal and higher frequencies get sharply attenuated, filtering out the background noise on the tape. These two processes cancel out each other. Deemphasis is applied to the incoming signal and noise during playback and once it is complete, the noise in the output signal is reduced.
The noise reduction systems include Dolby A, Dolby B, Dolby C, Dolby SR and Dolby S. Dolby A was the professional broadband noise reduction for recording studios in 1966, Dolby B was introduced in 1968 that is a sliding band system which helped in promoting fidelity on cassette tapes and is used with stereo tape players and recorders till date, Dolby C was developed to improve the cassette medium with twice noise reduction as compared to Dolby B type, Dolby SR (Spectral Recording) is another professional noise reduction system that maximizes the recorded signals with the use of a series of filters and Dolby S provides more noise reduction and reduces low-frequency noise.