Electronics project notes/Audio notes - noise reduction: Difference between revisions
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Latest revision as of 18:49, 22 April 2024
Dolby noise reduction
Dolby itself is a company and brand.
It is probably most associated with its noise reduction (aimed at reducing noise on tape), and with Dolby surround.
For context, tape hiss refers (roughly) to unavoidable hiss due to the nature of the tape medium.
That hiss is also also a few dB stronger above a few kHz.
Dolby Noise Reduction addressed this with some selective companding.
It does this to higher frequencies, because the hiss is stronger there, and because the signal at higher frequencies is typically much quieter so could be amplified without causing any saturation / distortion.
For example, a Dolby B encoder (recording stage) boosts the frequency band about ~7kHz (with a well defined defined curve) by 10dB, and a decoder (player) lowers it the same way and by just as much. (So yes, it's much like a well defined EQ, and if you have a deck without Dolby, you can imitate it with EQ)
Playing a Dolby B recording on a dolby B player means the end-to-end frequency response is the same, but the higher frequency parts of the signal is ~10dB stronger on tape, so the tape-introduced noise is effectively ~10dB less end-to-end.
For completeness,
- playing a recording without noise reduction 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 noise reduction or with it disabled will sound unnaturally bright
- because it's playing booster high high frequencies
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 strong enough that playback on systems without Dolby noise reduction sounds wrong. Though, because it's roughly two B passes (strength-wise, anyway), playback on decks set to B sounds good enough.
Dolby HX Pro is not actually noise suppression
For context, higher frequencies act like tape bias anyway, so a constant added bias would actually suppress recorded signal - roughly speaking, strong high frequency signal would suppress low frequencies.
So HX (Headroom eXtension) lessens the bias it adds based on the presence of high frequency in the signal, making for slightly more accurate recording in the high frequencies. (The details are actually more involved, but this is the gist)
It's not a huge difference, but it's a good idea (apparently more so on cheaper tape(verify)). It is also not something players need to worry about. It's purely about doing the recording step a little better.
HX Pro is the only real version out there (there was an original HX, but it was a flawed first implementation, an basically not used).
Variant list
There are more than most of us have probably heard of:
- 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 Pro - 1980, not reduction at all
- Dolby SR - 1986, ten-band, sort of a refinement on A
- Dolby S - 1989, roughly something inbetween B or C and SR
Unsorted:
- 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 fancier types, with more expensive hardware, rarely made it to consumer players.
- Note that B and C do not particularly deal with transients.
- Some Dolby variants introduced adaptive gaining and frequency masking (verify).
- 'Dolby HX' and 'HX Professional' is apparently not exactly the same thing
https://www.soundonsound.com/sound-advice/q-what-different-about-varieties-dolby-noise-reduction