Electronics project notes/Audio notes - noise reduction

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This is for beginners and very much by a beginner / hobbyist.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect


And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes


Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes


Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · power supply considerations · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.


Dolby noise reduction

This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)

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

Other noise reduction