Melody Evaluation System

This document explains the candidate selection and evaluation mechanism used in melody generation.

Overview

MIDI Sketch generates multiple melody candidates and selects one through an evaluation system.

Candidate Generation

Section-Specific Candidate Counts

Different sections use different candidate counts:

Section	Candidates	Notes
Chorus	100	Hook section
B (Pre-chorus)	50	Transition section
Bridge / Chant	30	Contrast section
A (Verse) / Intro / Outro	20	Stable sections

Generation Process

For each candidate:

1. Rhythm Pattern - Generate note positions and durations
2. Pitch Selection - Apply melody template (PlateauTalk, RunUpTarget, etc.)
3. Constraint Application - Apply singability and range limits
4. Embellishment - Add passing tones, neighbor tones

Two-Stage Evaluation

Evaluation occurs in two stages: Culling (filter candidates) and Scoring (rank candidates).

Stage 1: Culling

Candidates are filtered using penalty-based evaluation:

Penalties

Penalty	Range	Detection Target
High Register	0.0-1.0	Consecutive high notes (above D5)
Leap After High	0.0-1.0	Large jump followed by high note
Rapid Direction Change	0.0-1.0	Zigzag patterns
Monotony	0.0-1.0	Repeated notes without variation
Breathless	0.0-0.3	Consecutive short notes without breaks
Gap Ratio	0.0-1.0	Scattered notes with silence between

Gap Ratio

The Gap Ratio penalty targets scattered, disconnected note patterns. Higher gap ratio indicates more silence between notes.

Bonuses

Bonus	Range	Detection Target
Clear Peak	0.0-0.2	Single high point in the phrase
Motif Repeat	0.0-0.2	AAAB repetition pattern
Phrase Cohesion	0.0-1.0	Notes forming coherent groups

Phrase Cohesion Criteria

• Stepwise motion runs (connected notes)
• Consistent rhythm patterns
• 3-gram cell repetition (interval + duration motifs)

Stage 2: Scoring

Candidates that pass culling are scored on 6 dimensions:

Singability Score

Measures interval distribution:

Interval Type	Target Range
Step (1-2 semitones)	40-50%
Same pitch	20-30%
Small leap (3-4 semitones)	15-25%
Large leap (5+ semitones)	5-10%

Chord Tone Ratio

Measures chord tone frequency on strong beats:

• Strong beat: Beats 1 and 3 in 4/4 time (every 2 beats, tick % 960 == 0)
• Higher ratio indicates more harmonically grounded melodies

Contour Shape

Detects melodic shapes:

• Arch: Rise then fall
• Wave: Oscillating pattern
• Descending: Gradual descent

Surprise Element

Measures large leaps (5+ semitones) per phrase. Target: 1-2 leaps.

AAAB Pattern

Detects repetition with variation - same phrase repeats 3 times then varies.

Rhythm-Interval Correlation

Measures how well note durations match interval sizes:

Combination	Score	Reason
Long note + large leap	High	Singers need time for large jumps
Short note + step	High	Quick passages work best stepwise
Short note + large leap	Low	Difficult to sing

Based on pop vocal theory: singers need preparation time for large pitch changes. This scoring rewards melodies that are naturally singable.

Style-Specific Weights

Different vocal styles use different evaluation weights:

Style	Singability	Surprise	Plateau Bias	High Register
Standard	0.25	0.15	1.0	1.0
Idol	0.30	0.12	1.2	1.0
Rock	0.20	0.20	0.8	1.2
Ballad	0.40	0.10	1.1	0.9
Anime	0.25	0.25	0.9	1.3
Vocaloid	0.10	0.25	0.6	1.1

Parameter Definitions

• Singability: Weight for interval-based scoring
• Surprise: Weight for large leap detection
• Plateau Bias: Preference for same-pitch continuation
• High Register: Preference for higher pitches

Style-Specific Cohesion Thresholds

Different styles require different levels of melodic cohesion:

Style	Cohesion Threshold	Notes
Ballad	Higher	Needs smooth, connected lines
CityPop	Higher	Legato phrases preferred
Vocaloid	Lower	Tolerates angular melodies
Rock	Lower	Accepts disconnected patterns

Melodies below the cohesion threshold receive penalties during culling.

Style-Specific Gap Thresholds

Style	Gap Threshold	Notes
Ballad	Higher	More silence tolerated
Idol/Rock	Lower	Higher note density expected

Post-Generation Analysis

The Dissonance Analyzer checks harmonic issues after generation.

Issue Types

Type	Description	Example
Simultaneous Clash	Two notes with dissonant interval	Bass E + Melody F = minor 2nd
Non-Chord Tone	Note not in current chord	D over C major chord
Sustained Over Chord Change	Note became non-chord after change	C sustained over F chord
Non-Diatonic Note	Note not in the key's scale	F# in C major

Severity Levels

Severity	Intervals	Notes
High	Minor 2nd (1), Major 7th (11)	Strong dissonance
Medium	Tritone (6), Strong beat non-chord	Context-dependent
Low	Weak beat non-chord (passing tone)	Often acceptable

CLI Usage

# Generate and analyze
./midisketch_cli --seed 42 --analyze

# Analyze existing MIDI
./midisketch_cli --input song.mid --analyze

Output example:

=== Dissonance Analysis ===
Total issues: 3
  Simultaneous clashes: 1 (high: 1, medium: 0)
  Non-chord tones: 2 (medium: 1, low: 1)

Pipeline Summary

Summary

• Multiple candidates are generated per section (20-100)
• Two-stage evaluation: culling then scoring
• Style-specific weights adjust evaluation criteria
• Post-generation dissonance analysis available