Track Generators

This document details each track generator in MIDI Sketch.

Track Overview

MIDI Sketch generates 8 tracks across different MIDI channels:

Channel Assignment

Track	Channel	Program	Role
Vocal	0	Piano (0)	Main melody
Aux	5	Pad (89)	Sub-melody support
Chord	2	E.Piano (4)	Harmonic backing
Bass	3	E.Bass (33)	Harmonic foundation
Motif	4	Synth (81)	BackgroundMotif style
Arpeggio	5	Synth (81)	SynthDriven style
Drums	9	GM Drums	Rhythm
SE	15	-	Section markers

Vocal Track

Source: src/track/vocal.cpp (~470 lines), src/track/melody_designer.cpp (~520 lines)

The vocal system uses a template-driven melody designer for predictable, stylistically-accurate melody generation.

Architecture

The vocal generation is split into two components:

1. MelodyDesigner (melody_designer.cpp) - Template-driven pitch selection
2. Vocal Generator (vocal.cpp) - Section structure and coordination

Melody Templates

7 melody templates define melodic characteristics:

ID	Name	Plateau	Max Step	Use Case
0	Auto	-	-	VocalStyle-based selection
1	PlateauTalk	0.65	2	NewJeans, Billie Eilish style
2	RunUpTarget	0.20	4	YOASOBI, Ado style
3	DownResolve	0.30	3	B-section, pre-chorus
4	HookRepeat	0.40	3	TikTok, K-POP hooks
5	SparseAnchor	0.50	2	Official髭男dism, ballad
6	CallResponse	-	-	Duet patterns
7	JumpAccent	-	-	Emotional peaks

• Plateau ratio: Probability of staying on the same pitch (higher = more repetitive)
• Max step: Maximum interval in semitones (lower = smoother)

Generation Flow

Pitch Selection (4 Choices Only)

The MelodyDesigner limits pitch selection to 4 options:

enum class PitchChoice {
    Same,       // Stay on current pitch (plateau_ratio)
    StepUp,     // +1 semitone
    StepDown,   // -1 semitone
    TargetStep  // ±2 toward target (if template has target)
};

This constrained approach produces more natural, singable melodies.

Vocal Attitudes

Attitude	Description	Implementation
Clean	Conservative, singable	Chord tones only, on-beat
Expressive	Emotional, dynamic	Tensions allowed, timing variance
Raw	Edgy, unconventional	Non-chord tones, boundary breaking

Phrase Caching

Phrases are cached by section type to ensure musical coherence:

std::map<SectionType, std::vector<Phrase>> phraseCache_;

// A section uses same/similar phrases when repeated
// Chorus maintains its melodic identity

Range Constraints

struct VocalRange {
    uint8_t low = 60;   // C4
    uint8_t high = 79;  // G5
};

---

Aux Track

Source: src/track/aux_track.cpp (~440 lines)

The Aux (auxiliary) track provides sub-melody support for the main vocal. It's not a counter-melody, but a "perceptual control layer" that enhances the main melody.

Purpose

Role	Description
Addictiveness	Pulse loops create repetitive, catchy patterns
Physicality	Groove accents add body movement feel
Stability	Phrase tails provide resolution
Structure	Helps listeners perceive section boundaries

Aux Functions

5 auxiliary functions are available:

ID	Function	Description
A	PulseLoop	Repetitive same-pitch or fixed-interval patterns
B	TargetHint	Hints at vocal target with chord tones
C	GrooveAccent	Rhythmic accents with staccato
D	PhraseTail	End-of-phrase descending resolution
E	EmotionalPad	Long sustained chord tones

Template → Aux Mapping

Each melody template automatically selects appropriate aux functions:

Template	Aux Functions	Reason
PlateauTalk	A (PulseLoop)	Ice Cream / minimal style
RunUpTarget	B + D	YOASOBI ascending then resolving
HookRepeat	A + C	TikTok repetitive hooks
SparseAnchor	E + D	Ballad emotional support

Generation Constraints

• Always generated after vocal (to avoid collisions)
• Narrower range than vocal (50-70% of vocal range)
• Lower velocity (0.5-0.8× vocal velocity)
• Uses HarmonyContext to avoid dissonance with vocal

---

Chord Track

Source: src/track/chord_track.cpp (~820 lines)

Generates harmonic voicings with voice leading optimization.

Voicing Types

Voice Leading Algorithm

int voiceLeadingDistance(Voicing& prev, Voicing& next) {
    int distance = 0;
    for (int i = 0; i < 4; i++) {
        distance += abs(prev.notes[i] - next.notes[i]);
    }
    return distance;
}

// Select voicing that minimizes distance
Voicing selectBestVoicing(Voicing& prev, vector<Voicing>& candidates) {
    return min_element(candidates, [&](auto& a, auto& b) {
        return voiceLeadingDistance(prev, a) < voiceLeadingDistance(prev, b);
    });
}

Bass Coordination

Uses BassAnalysis to avoid doubling:

if (bassAnalysis.hasRootOnBeat1) {
    // Use rootless voicing - bass provides root
    voicing = generateRootlessVoicing(chord);
} else {
    // Include root in chord voicing
    voicing = generateFullVoicing(chord);
}

Register Constraints

constexpr uint8_t CHORD_LOW = 48;   // C3
constexpr uint8_t CHORD_HIGH = 84;  // C6

---

Bass Track

Source: src/track/bass.cpp (~450 lines)

Generates the harmonic foundation with root-focused patterns.

Pattern Types

Pattern	Description	Rhythm
Sparse	Minimal, ballad-style	Beat 1 only
Standard	Pop/rock baseline	Beats 1, 3 with fills
Driving	Energetic, forward	Eighth notes throughout

Generation Logic

Approach Notes

Beat 4 may use chromatic approach to next root:

// If next chord root is C
// Beat 4 could be B (half step below) or Db (half step above)
uint8_t approachNote = nextRoot - 1; // chromatic approach

---

Drums Track

Source: src/track/drums.cpp (~680 lines)

Generates drum patterns with fills and dynamics.

GM Drum Map

constexpr uint8_t KICK = 36;
constexpr uint8_t SNARE = 38;
constexpr uint8_t SIDE_STICK = 37;
constexpr uint8_t CLOSED_HH = 42;
constexpr uint8_t OPEN_HH = 46;
constexpr uint8_t RIDE = 51;
constexpr uint8_t CRASH = 49;
constexpr uint8_t TOM_HIGH = 50;
constexpr uint8_t TOM_MID = 47;
constexpr uint8_t TOM_LOW = 45;

Pattern Styles

Fill Types

enum class FillType {
    TomDescend,    // High → Mid → Low tom
    TomAscend,     // Low → Mid → High tom
    SnareRoll,     // Rapid snare hits
    Combo          // Mixed elements
};

Fills are inserted at:

• Section transitions
• Every 4 or 8 bars
• Before chorus

Ghost Notes

Velocity-reduced snare articulations for groove:

// Main snare: velocity 100
// Ghost note: velocity 40-60

---

Motif Track

Source: src/track/motif.cpp (~470 lines)

For BackgroundMotif composition style. Creates repeating patterns.

Parameters

struct MotifParams {
    MotifLength length;           // TwoBars, FourBars
    RhythmDensity rhythm_density; // Sparse, Medium, Driving
    MotifMotion motion;           // Stepwise, GentleLeap
    RepeatScope repeat_scope;     // FullSong, PerSection
    MotifRegister register_;      // Mid, High
};

Pattern Generation

Register Ranges

Register	Range
Mid	C3 (48) - C5 (72)
High	C4 (60) - C6 (84)

---

Arpeggio Track

Source: src/track/arpeggio.cpp (~200 lines)

For SynthDriven composition style. Creates arpeggiated patterns.

Parameters

struct ArpeggioParams {
    ArpeggioPattern pattern;  // Up, Down, UpDown, Random
    ArpeggioSpeed speed;      // Eighth, Sixteenth, Triplet
    uint8_t octave_range;     // 1-3 octaves
    float gate;               // Note length ratio (0.0-1.0)
    bool sync_chord;          // Follow chord changes
};

Pattern Types

Speed Conversion

Tick getNoteDuration(ArpeggioSpeed speed) {
    switch (speed) {
        case Eighth:    return TICKS_PER_BEAT / 2;    // 240
        case Sixteenth: return TICKS_PER_BEAT / 4;    // 120
        case Triplet:   return TICKS_PER_BEAT / 3;    // 160
    }
}

---

SE Track

Source: src/track/se.cpp (~15 lines)

Minimal track for section markers (text events only).

void generateSE(Song& song) {
    for (auto& section : song.arrangement.sections) {
        MidiEvent marker;
        marker.tick = section.start_tick;
        marker.type = MidiEventType::Text;
        marker.text = section.name;
        song.se.addEvent(marker);
    end
}

---

Velocity Calculation

Common velocity formula across tracks:

uint8_t calculateVelocity(
    uint8_t baseVelocity,
    int beat,
    SectionType section,
    float trackBalance
) {
    float beatAdjust = getBeatAccent(beat);      // Strong beats: +10
    float sectionMult = getSectionEnergy(section); // Chorus: 1.2

    return clamp(
        baseVelocity * beatAdjust * sectionMult * trackBalance,
        1, 127
    );
}

Track Balance

Track	Balance	Notes
Vocal	1.00	Lead instrument
Aux	0.50-0.80	Sub-melody support
Chord	0.75	Supporting
Bass	0.85	Foundation
Drums	0.90	Timing driver
Motif	0.70	Background
Arpeggio	0.85	Mid-level