P · Music theory for acid (a noob's prologue)

Section 1Notes & the keyboard

Western music slices pitch into 12 notes that repeat forever, higher and lower. Their names: C C# D D# E F F# G G# A A# B — seven letters plus five "sharps" (the black keys). After B it wraps back to C, one octave higher. Play the keyboard — click, or use your computer keys A W S E D F T G Y H U J K:

MIDI number is just an integer name for each key (A4 = 69). The whole keyboard is one formula: freq = 440 · 2^((midi − 69) / 12). That's the only equation in this lesson.

Section 2Octaves, semitones & equal temperament

Two facts do most of the work:

• Octave = ×2 frequency. A note and the note 12 keys up sound "the same but higher" because the higher one vibrates exactly twice as fast (A4 = 440 Hz, A5 = 880 Hz). Our ears hear ratios, not differences — that's why pitch is logarithmic (and why the sliders in lesson 00 were too).
• Semitone = one key. The smallest step in Western music: adjacent keys (e.g. C→C#). Twelve of them make an octave.

Equal temperament is the deal Western instruments struck: split the octave into 12 equal steps, each a frequency ratio of 2^(1/12) ≈ 1.0595. Multiply by that 12 times and you've doubled — a perfect octave. The trade-off: every other interval is very slightly "out of tune" versus pure whole-number ratios, but in exchange you can play in any key on the same 12 keys. Almost all acid/house/techno lives in this 12-note equal-tempered world. Many Eastern traditions did not take this deal — section 7.

Section 3Harmonics → harmony · why some notes agree

Remember the spectrum in lesson 00 — a note is secretly a stack of overtones at 1×, 2×, 3×, 4×… its frequency. Here's the payoff: those overtones are the good intervals. Play the harmonic series of a low note and listen to what each overtone is:

Two notes sound consonant when their overtones line up (simple ratios like 2:1, 3:2). They clash (dissonant) when the ratios are complex and overtones beat against each other. So harmony isn't arbitrary — it falls out of the physics you already saw on the scope.

Section 4Intervals — the distance between two notes

An interval is just "how many semitones apart". Name them once and you'll hear them everywhere. Each button plays the two notes together (root + the interval):

The perfect fifth (7 semitones, 3:2) and octave (12, 2:1) are rock-solid — that's your low harmonics again. The tritone (6) is the tense, "evil" one. The minor third (3) is what makes a chord/scale sound sad — and it's the backbone of acid's minor mood.

Section 5 ★Scales — the index of classics

A scale is a chosen subset of the 12 notes — a mask that says "these belong together". Stay on a scale's notes and almost anything you play sounds intentional. Pick a root (the home note) and a scale; the keyboard lights up its notes and plays it. This is your reference — steal freely.

Section 6Modes — same notes, different centre of gravity

Take the white keys C→C and you get the major scale (bright, happy). Play the same white keys but treat A as home and you get natural minor (dark). Nothing changed but the home note — that's a mode. The seven modes of the major scale are just seven different starting points:

They're all in the scale index above — pick a root and audition them. For acid, you'll mostly live in Aeolian (minor) and Phrygian, and reach for harmonic minor / Phrygian dominant when you want that exotic, "eastern" edge.

Section 7West vs East — the notes between the keys

Everything so far assumed 12 equal semitones. That's a Western choice. Other traditions divide pitch differently — and crucially, many use pitches that sit between our piano keys (microtones / quarter tones). Hear it: this plays C, then a note halfway to C# (a "quarter tone", +50 cents — impossible on a piano), then C#.

Same 7-note "major-ish" scale, two worlds. Western major uses exact semitones; Arabic Maqam Rast flattens the 3rd and 7th by a quarter tone into "neutral" notes that don't exist on a keyboard. Play both back to back — that half-flat is the whole flavour:

Acid is firmly Western 12-TET — but it borrows the flavour: the "exotic/eastern" vibe in a lot of dark acid & psytrance comes from Phrygian dominant / harmonic minor, which mimic maqam Hijaz using only the 12 keys. You get 80% of the mystery with none of the microtones.

Section 8Chords & rhythm (a peek — more later)

Chords = several notes at once, usually stacked in thirds. A triad is three notes: minor = root + 3 + 7 semitones (sad); major = root + 4 + 7 (bright). Acid is mostly monophonic — one note at a time — so chords matter less; but your bassline implies a chord by outlining its notes.

Rhythm gets its own module (10–12), but the vocabulary: music is counted in bars of 4 beats (4/4 time). Split each beat into 4 and you get 16th notes — the grid of a classic 303 line and every step sequencer. Tempo is BPM (beats per minute); acid house sits around 120–130 BPM. That's all you need until Module 3.

Section 9The acid cheat-sheet

If you remember nothing else, remember this and you'll make convincing acid lines:

• Key: pick a root, use natural minor or Phrygian. A minor is the friendliest start (all white keys). For darker/eastern, Phrygian dominant or harmonic minor.
• Safe notes: when in doubt, minor pentatonic (5 notes) — nearly impossible to hit a wrong note. Perfect for the game's auto-generated melodies.
• Motion: acid lines lean on the root and octave, walk up/down by semitone (chromatic passing notes are welcome), and land back home. Repetition is the point.
• Feel: 4/4, 16th-note grid, ~125 BPM. The groove comes from accents & slides (Module 2), not from lots of notes.

That's the whole theory budget. Next: Lesson 00 → build the oscillator whose harmonics you just met.