Electronics project notes/Audio notes - noise reduction

⚠ 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 and video notes: See avnotes Platform specific: : Microcontroller and computer platforms ·· Arduino and AVR notes · ESP series notes · STM32 series notes · Teensy series notes · RP2040 and RP2350 notes Less sorted: USB notes · Ground · device voltage and impedance (+ audio-specific) · electricity and humans · Soldering · landline phones · pulse modulation · PLL · multimeter notes · signal reflection · Project boxes · resource metering · Radio and SDR · vacuum tubes · Unsorted stuff · 'E-fuse' 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 — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Dolby itself is a company, and a brand, but there are probably two main associations:

• for a few decades, it was mostly associated with noise reduction (aimed at reducing noise on tape), and with
• and later mostly known for Dolby surround (and perhaps Dolby Digital)

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 boosted 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 earlier.

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, but dynamic tape bias.

(there was also HX without the pro, which was basically a first flawed first implementation, basically never used; HX Pro is the only real version out there and when we say HX we often mean HX Pro(verify)).

HX (Headroom eXtension) lessens the bias it adds when there is more high frequency in the signal.

The details why are more involved.

The idea is that this makes for slightly more accurate recording in the high frequencies (because it is less likely to saturate the medium?(verify) and then mostly for strong high frequency content such as hihats).

It's not a huge difference, but it's a good idea (apparently more so on cheaper tape(verify)).

It's purely about doing the recording step a little better, so not not something players need to worry about.

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

https://en.wikipedia.org/wiki/Dolby_noise-reduction_system