Audio Visualization for less than $10!

TODO: introduction

TODO: Block diagram

Input signal: "Line level"

First, let’s get a view of the signal we are accepting on the circuit’s input.

Magnitude

With some variation, it's common knowledge (by means of human experience) that most devices which accept a set of headphones through a 3.5mm "TRS" audio jack will output roughly the same maximum volume through that set of headphones.


A typical, well-loved 3.5mm TRS jack besides a laptop line-out port.

Don't believe me? Grab a 3.5mm cable meant to connect an output, like a phone, to a speaker, and use a multimeter to measure the unloaded RMS AC output voltage of a 60Hz tone. Be sure to use the same software on multiple devices to get a normalized comparison of different hardware.


We generated this 60Hz tone using Audacity, under Generate > Tone > Sine, 60 Hz, Amplitude 1 (100%)

Experiment pitfalls:
- Q: Uhh ... which conductors am I measuring?
  
  If you're unsure which conductors to measure between: the sleeve is usually a common reference ground, but in doubt, use your multimeter in continuity mode and see if you can find contuinty to the computer case or, for example, the outer shield of a USB connector.
- Q: Why 60Hz?
  
  Not all multimeters are equal, but some things don't change: any portable multimeter equipped to probe the 60Hz, 120VAC signal provided by your household electrical socket is also suited to calculate RMS AC voltages for 60Hz sinusoidal signals. The Asian-market Fluke 12E+ used here was able to measure the highest frequencies our sound cards could produce -- around 20 kHz -- but a cheaper Centech multimeter struggled past a perfectly audible 1 kHz tone.
We tried this experiment on a Thinkpad and a Dell desktop, using the mpv media player on two Fedora Workstation systems:


Laptop, 100%	Laptop, 130%	Desktop, 100%	Desktop, 130%

The signal was identical at 100% volume on each system ... but when we asked mpv to overamplify the volume to "130%", the laptop and desktop began to deviate.

I guess that's why I start to get tinnitis after a stressful day on my laptop. 😂

Frequencies

Depending on what type of music you listen to, you'll find that the frequencies that are most pronounced vary widely. If you already have a piece of audio in mind that you want to visualize using your board, you can use this neat online tool published by Academo to find some frequencies at which the audio peaks.

I've already made my choice, of course. Folks who grew up with a first-generation PlayStation (or who had older siblings who did, in my case) might find my online handle oddly familiar -- as a kid, I always liked watching the rain, so when I encontered the stormy puzzle-platform level Hurricos in my first playthrough of Insomniac's Spyro 2: Ripto's Rage, I felt right at home.


Concept art for Hurricos, produced by John Loren for the Spyro: Reigited Trilogy.

The soundtrack for this level shows pronounced frequencies around G_2 and G_4; as it continues, the composer Stuart Copeland (yes, that Copeland) integrates some sort of electric arc sound effect reminiscent of either a hi-hat or a brush-slapped snare -- the tone centers around G_8.


Hurricos, as seen by a custom `mpv` visualizer

Converting those notes back from A440 under 12-tone equal temperment into Hz is easy. If you count the number of downward keyboard half-steps n from your note down to A_4, your frequency in Hertz will be


f_{\text{Hz}} = 440 \times 2^{\left(\frac{n}{12}\right)}

For example, G_2 = 440 \times 2^{\left(\frac{-26}{12}\right)} \approx 98 \ \text{Hz}.

Pitfalls

How precise should I be?

Because of the quality factor (Q) of the band-pass filters we're going to construct, the precision of your measurement here is not important past the first one or two significant figures. If you can't tell between 800 Hz and 810 Hz, don't fret it. (Get it? Fret?)

Our values

A440, note	Frequency, Hz	Frequency, rad./s
`G_2`	98	616
`G_4`	392	2463
`G_8`	5588	35108

An aside about the attributes of (passive) bandpass filters

TODO: Band-pass filters .,.,.

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