Electronics project notes/Audio notes - basic audio hacks and DIY

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The physical and human spects dealing with audio, video, and images

Vision and color perception: objectively describing color · the eyes and the brain · physics, numbers, and (non)linearity · color spaces · references, links, and unsorted stuff

Image: file formats · noise reduction · halftoning, dithering · illuminant correction · Image descriptors · Reverse image search · image feature and contour detection · OCR · Image - unsorted

Video: format notes · encoding notes · On display speed · Screen tearing and vsync


Audio physics and physiology: Sound physics and some human psychoacoustics · Descriptions used for sound and music

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 · (tape) noise reduction


Digital sound and processing: capture, storage, reproduction · on APIs (and latency) · programming and codecs · some glossary · Audio and signal processing - unsorted stuff

Music electronics: device voltage and impedance, audio and otherwise · amps and speakers · basic audio hacks · Simple ADCs and DACs · digital audio · multichannel and surround
On the stage side: microphones · studio and stage notes · Effects · sync


Electronic music:

Electronic music - musical terms
MIDI · Some history, ways of making noises · Gaming synth · microcontroller synth
Modular synth (eurorack, mostly):
sync · power supply · formats (physical, interconnects)
DAW: Ableton notes · MuLab notes · Mainstage notes


Unsorted: Visuals DIY · Signal analysis, modeling, processing (some audio, some more generic) · Music fingerprinting and identification

For more, see Category:Audio, video, images

Combining multiple signals (to one input)

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)

You may want this when you want to

connect multiple inputs to e.g. a speaker set with only one input plug
wire an AUX port to somethat that doesn't have it


Manual switch

A switch connects one source, or the other.

The sources are isolated from each other, mechanically, and you will only ever hear one.

The simplest version is a switch (DPDT for stereo). Could do this in a cable, or make a small box with some sockets.

You'll get a pop whenever you switch. There are quick but dirty ways around this.

But often you don't want to have to do this manually in the first place, so...


Y splitter

A wire with three plugs, that connects everything to everything.

The listening device will be high-impedance, so is fine with this (and most other things). Worst case the combination is so loud that it distorts.


However, the two (or more) outputs will interact. Which is not intended, and which will usually affect audio quality, which is already enough reason not to do this.

If passive (e.g. two guitars pickups) this wouldn't be so bad.

If the output is active, that means they have a small amp at the end, and you are connecting multiple of those directly together - which means they treat each other as loads. This can push in enough current to break things, so this is probably a bad idea.


In other words

  • a splitter is fine to put one source into multiple listeners
  • a splitter that puts multiple sources together, intended for one listener, may be trouble.


Y splitter with resistors

The most basic fix to the last-mentioned issue is a passive summing circuit, which puts a resistor on an input lines before you join them.

It doesn't completely isolate the signals, but you can dimension the resistors so that it can't put out enough current to damage another of itself.

(again, instrument pickups are a little different - you generally wouldn't do this)

It will lower volume by a bunch of dB, so you'll turn the volume up, bringing up the existing noise floor.

In some situations you may not have input gain, just the amp's main volume, and the total possible sound produced is also lower. Whether this is an issue depends on context.

See/read:


Passive potmeter mixer

Instead of just a resistor, you could add a potmeter so that you can fade between the two channels (or mix each input into the output).

This mostly the same as the previous, except that you can change the volume. It's arguably a little worse, because you can turn the pot to zero ohm for each channel, and probably fix that by adding a little more resistance in series.

You may want a two-channel pot, to separately do the same thing with both stereo channels.

Same notes on less signal and more noise as before.


Buy active mixer

Adding more channels would attenuate down some more, and the noise starts to come closer, so you'll soon want an active summing circuit instead (recognizable by that they take DC power input).

Your cheap "I could probably make this" level $20 active mixers are not the best quality and are often they're two-channel (or 4-channel mono, almost the same thing).

Halfway decent ones are easily $50, and if you find that a little expensive for what they are, you can consider soldering your own (see below).


Build active mixer

The "passive adder plus potmeter" option is functionally half of a real mixer - the most important next step is a buffer and a little amplification right after, e/g/ so that you avoid attenuating the signal into the next device's noise floor. So, an op amp.

This does require a power supply.

Note that adding more channels to this is fairly simple and cheap. (planning for a number of channels, and/or stereo, is still handier to do up front though)

http://www.learningaboutelectronics.com/Articles/How-to-build-your-own-audio-mixer

http://web.archive.org/web/20150220140529/http://www.mattkrass.com/?page_id=880

http://forum.allaboutcircuits.com/showthread.php?t=26654


Multiple headphones on one output

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)

A Y-cable puts two loads in parallel.


This does two things:

Makes the headphones interact

...because they are not simple loads. However, they won't do so very much, you probably won't notice or care, and it can't damage the headphones.


doubles the load

(...if equal impedance. If not, one will be louder than another.)

If the output can handle that, it's perfectly fine.

Most headphone amp will be fine, as they're typically designed to cope with real-world variation in headphones (impedances vary).

Computers, mobile phones, and such will often be fine for similar reasons.


Things that can't deal will distort the sound, in extreme cases overheat.

You can usually avoid that by keeping the volume low. Half is safest, more may be okay depending on the combination of devices.


If there e.g. is a distinction between line out and headphone out, then think twice and read the manual - but as far as I can tell this is rare.


http://boards.straightdope.com/sdmb/showthread.php?t=624492


Audio amplifier

Adding a line/aux in

Line out from speaker out

Avoiding pops

Car audio

Tape related

Cassette speed mod

Tape loops