Electronics project notes/Audio notes - noise reduction
Dolby noise reduction
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Dolby itself is a company/brand.
Perhaps most associated with its noise reduction, aimed at reducing noise on tape.
Very roughly, tape hiss refers to the medium's unavoidable hiss, that is a few dB stronger above a few kHz. Dolby Noise Reduction addressed this with some selective companding.
It does this to higher frequencies not only because the hiss is stronger there, but also because higher frequencies are almost always much quieter, so it's unlikely this will cause any saturation / distortion.
For example, a Dolby B encoder (recording stage) boosts the frequencies about ~7kHz by 10dB (in a well defined curve), and a decoder (player) lowers it by just as much. (And yes, you could imitate Dolby B with EQs on both ends. That's more or less what it amounts to)
Playing a Dolby B recording on a dolby B player means the end-to-end frequency response is the same, but the part of the signal is ~10dB stronger on tape, so the noise is effectively ~10dB lower.
For completeness, yes,
- playing a recording without NR on a player set to Dolby B will sound dull
- because it lowers the higher frequencies for no good reason
- playing a Dolby B recording on a player without NR or with it disabled will sound unnaturally bright
- because it's playing with the high frequencies still boosted
Dolby B was popular, in part because it's simple, and probably in part because it's subtle enough that playing B-recorded tapes in players without dolby would sound bright - brighter than intended but not exactly a bad thing, particuarly if you had an EQ.
Dolby C is the same idea, but is ~10dB stronger and starts to come in two octaves lower.
This is more noise reduction, but also strong enough that playback without NR sounds wrong. Though, because it's roughly two B passes (strength-wise, anyway), playback on decks without C still sound decent when set to B.
Note that B and C do not particularly deal with transients.
Some variants introduced adaptive gaining and frequency masking (verify).
Dolby A - 1965, four-band, and embedding a warble tone that could be used to identify this type, and assist alignment. Used in some pro gear, cinema, and more
Dolby B - 1968, consumer, basically one-band, mainly used in cassettes
Dolby C - 1980, consumer, stronger than B
Dolby HX - 1980, not reduction at all, just a way to void over-biasing while recording.
Dolby SR - 1986, ten-band, sort of a refinement on A
Dolby S - 1989, roughly something inbetween B or C and SR
The more professional variants take more care, to e.g. make the level production a little more truthful to the original.
That, and the warble tone stuff, was not so interesting to home recording so the more expensive hardware rarely made it to consumer players.