Generation Pipeline

This document explains the step-by-step music generation process in MIDI Sketch.

Pipeline Overview

Phase 1: Structure Building

The generator first creates the song structure based on StructurePattern:

void Generator::buildStructure() {
    arrangement_ = StructureBuilder::build(params_.structure);
}

Structure Patterns

Pattern	Bars	Sections
StandardPop	24	A(8)-B(8)-Chorus(8)
BuildUp	28	Intro(4)-A(8)-B(8)-Chorus(8)
DirectChorus	16	A(8)-Chorus(8)
RepeatChorus	32	A(8)-B(8)-Chorus(8)-Chorus(8)
FullPop	56	Intro-A-B-Chorus-A-B-Chorus-Outro
FullWithBridge	52	Intro-A-B-Chorus-Bridge-Chorus-Outro
Ballad	56	Intro(8)-A-B-Chorus-Interlude-B-Chorus-Outro
ExtendedFull	90	Full form with bridge and extended sections

Section Types

Each section has properties that affect generation:

struct Section {
    SectionType type;         // Intro, A, B, Chorus, Bridge, Interlude, Outro
    uint8_t bars;             // Length in bars
    VocalDensity vocal_density;    // Full, Sparse, None
    BackingDensity backing_density; // Normal, Thin, Thick
};

Phase 2: Rhythm Section

Bass Generation

Bass is generated first as the harmonic foundation:

Bass Patterns:

• Sparse: Quarter notes on beats 1 and 3 (ballad, chill)
• Standard: Quarter note rhythm with occasional eighths
• Driving: Eighth note patterns with approach notes

Chord Generation

Chord voicing uses bass analysis for coordination:

void Generator::generateChord() {
    BassAnalysis bassAnalysis = analyzeBass(song_.bass);
    // Use rootless voicing when bass has root
    if (bassAnalysis.hasRootOnBeat1) {
        useRootlessVoicing();
    }
}

Voice Leading Algorithm:

1. Calculate distance between consecutive voicings
2. Minimize movement (sum of semitone distances)
3. Maximize common tone retention
4. Apply inversions to optimize transitions

Drums Generation

Drum patterns are selected based on mood:

Style	Characteristics	Used By
Sparse	Half-time feel, minimal	Ballad, Chill
Standard	8th hi-hat, 2&4 snare	StraightPop
FourOnFloor	4-on-floor kick	ElectroPop, IdolPop
Upbeat	Syncopated, 16th hi-hat	BrightUpbeat
Rock	Ride cymbal, crash accents	LightRock
Synth	Tight 16th hi-hat	Yoasobi, Synthwave

Fill Generation:

• Tom descend/ascend patterns
• Snare rolls
• Combination fills at section transitions

Phase 3: Melody Generation

Vocal Track

The most complex generator with phrase caching:

Vocal Attitudes:

Attitude	Characteristics
Clean	Chord tones only, on-beat rhythms
Expressive	Tensions with delayed resolution, slight timing deviation
Raw	Non-chord tones, phrase boundary breaking

Non-Chord Tones:

• 4-3 suspensions
• Anticipations
• Passing tones
• Neighbor tones

Motif Track (BackgroundMotif style)

Generates repeating patterns:

MotifParams params {
    .length = MotifLength::TwoBars,    // 2 or 4 bars
    .rhythm_density = RhythmDensity::Medium,
    .motion = MotifMotion::Stepwise,
    .repeat_scope = RepeatScope::FullSong
};

Arpeggio Track (SynthDriven style)

Generates arpeggiated patterns:

ArpeggioParams params {
    .pattern = ArpeggioPattern::UpDown,
    .speed = ArpeggioSpeed::Sixteenth,
    .octave_range = 2,
    .gate = 0.5f  // Note length ratio
};

Phase 4: Polish

Transition Dynamics

Automatically applies energy transitions:

Section Energy Multipliers:

Section	Multiplier
Intro	0.75
A	0.85
B	1.00
Chorus	1.20
Bridge	0.90
Outro	0.80

Humanization

Adds natural variation to timing and velocity:

void applyHumanization(Song& song, float intensity) {
    // Timing: random offset ±ms
    // Velocity: random ±value
    // Not applied to drums
}

MIDI Output

Finally, the Song is converted to SMF Type 1:

Track Mapping:

Track	Channel	Program
Vocal	0	0 (Piano)
Chord	1	4 (E.Piano)
Bass	2	33 (E.Bass)
Motif	3	81 (Synth Lead)
Arpeggio	4	81 (Saw Lead)
Drums	9	GM Drums
SE	15	Text events

Key Transposition

All generation happens in C major. Final transposition is applied at output:

uint8_t MidiWriter::transposePitch(uint8_t pitch, Key key) {
    return pitch + static_cast<uint8_t>(key);
}