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 audio tape recording. The first was Dolby A, a professional broadband noise reduction system for recording studios 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. 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. Other solutions are to 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 (ips) in a simple plastic shell when 15 or 7.5 ips was for high fidelity and 3 3/4 ips was of 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. Within 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 "Dolby B" noise reduction scheme 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 or all 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.
Dolby noise reduction is a form of dynamic preemphasis employed during recording, plus a form of dynamic deemphasis used during playback, that work in tandem to improve the signal-to-noise ratio. While Dolby A operates across the whole spectrum, the other systems specifically emphasize the audible frequency range where background tape hiss, an artifact of the recording process that is similar to white noise, is most noticeable (usually above 1 kHz, or two octaves above Middle C).
The Dolby preemphasis boosts the recorded level of the quieter audio signal at these higher frequencies during recording, effectively compressing the dynamic range of that portion of the signal, so that quieter sounds above 1 kHz receive a proportionally greater boost. As the tape is recorded, the relative amplitude of the signal above 1 kHz is used to determine how much pre-emphasis to apply — a low-level signal is boosted by 10 dB (Dolby B) or 20 dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby Level" (+3 VU), no signal modification is performed.
The sound is thus recorded at a higher overall level on the tape relative to the tape's overall noise level, requiring the tape formulation to preserve this specially recorded signal without distortion. On playback, the opposite process is applied (deemphasis), based on the relative signal component above 1 kHz. Thus as this portion of the signal decreases in amplitude, the higher frequencies are progressively more sharply attenuated, which also filters out the constant background noise on the tape when and where it would be most noticeable.
The two (pre- and de-emphasis) processes are intended to cancel each other out as far as the actual recorded program is concerned. Only de-emphasis is applied to the incoming signal and noise during playback. After playback de-emphasis is complete, apparent noise in the output signal is reduced, and this process should not produce any effect noticeable to the listener (other than reduced noise of course). Playback without noise reduction produces a noticeably brighter sound, however.
The calibration of the recording and playback circuitry is therefore critical for faithful reproduction of the original program content, and this is easily offset by poor quality tape, dirty recording/playback heads, or using inappropriate bias levels/frequency for the tape formulation, as well as tape speed, when recording or duplicating. This can manifest itself as muffled-sounding playback, or "breathing" of the noise level as the signal varies.
On some high-end consumer equipment, Dolby calibration control is included. For recording, a reference tone at Dolby Level may be recorded for accurate playback level calibration on another transport. At playback, the same recorded tone should produce the identical output, as indicated by a Dolby logo marking at approximately +3 VU on the VU meter(s). In consumer equipment, Dolby Level is defined as 200 nWb/m, and calibration tapes were available to assist with the task of correct level setting. For accurate off-the-tape monitoring during recording on 3-head decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the rubric "Double Dolby".