Procedural Composer

Procedural Composer: Pure-Code Audio Synthesis & Chiptune/Game Sound Engine Guide

This skill provides complete mathematical, musical, and software architecture patterns for generating high-quality sound effects (SFX) and polyphonic background music (BGM) purely in code—without relying on external .wav, .mp3, or .ogg audio files.

Available scripts

• scripts/sound.go: Sound engine driver and declarative JSON audio definition parser.
• scripts/sound_test.go: Go unit tests and example BGM/SFX composition recipes.
• scripts/play.go: CLI audio player and WAV export utility.

1. Core Architectural Principles & DSP Foundations

1.1 Zero-Asset Engine Strategy

Procedural audio synthesizes audio samples on-the-fly or bakes them into PCM buffers in memory at application startup:

• Zero disk I/O: Eliminates asset loading failures and missing file errors.
• Minimal footprint: Thousands of sounds and complex soundtracks require only kilobytes of code.
• Dynamic runtime control: Real-time manipulation of pitch, tempo, filter cutoff, vibrato, and volume based on game state.

1.2 Output Format Standard

Standard audio output uses 16-bit signed Little-Endian PCM stereo at 44,100 Hz (or 48,000 Hz):

• Sample Rate ($f_s$): $44,100 \text{ Hz}$ (44,100 samples per second per channel).
• Channels: 2 (Stereo: Left [bytes 0–1], Right [bytes 2–3]).
• Bits Per Sample: 16-bit signed integer range $[-32,768 \text{ to } +32,767]$.

2. SECTION 1: Background Music (BGM) Composition & Architecture

2.1 Yamaha YM2612 Benchmark & Multi-Channel Richness Standard

To achieve rich, professional game soundtrack quality, procedural music generators should mimic or exceed the complexity of classic 16-bit sound chips such as the Yamaha YM2612 (Sega Genesis) and SNES SPC700.

The YM2612 6-Channel Standard

A music track should feature at least 6 distinct polyphonic channels/instrument layers mixed simultaneously:

1. FM Channel 1 (Lead Melody): Primary lead line (sawtooth or low-duty pulse wave with LFO vibrato).
2. FM Channel 2 (Counter-Melody): Secondary counterpoint or call-and-response lead line.
3. FM Channel 3 (Harmony Pad / Strings): Sustaining chord pad holding harmonic progression.
4. FM Channel 4 (Bassline): Driving octave or walking bassline (square, saw, or triangle sub-bass).
5. FM Channel 5 (Arpeggiator / Motion): Rapid 16th or 32nd note arpeggio runs for movement.
6. FM Channel 6 / Noise (Drums / Percussion / SFX Stinger): Filtered noise bursts for snare/hi-hat or kick transients.

2.2 Dual Music Playback Architecture: One-Off vs. Looped Playback

The audio playback subsystem (SoundSystem in scripts/sound.go) explicitly supports two distinct playback modes:

• Looped Playback (Play(pcm, true)): For stage themes, boss battles, title screens, and menus where music must loop continuously without audible gaps using an infinite reader wrapper (audio.NewInfiniteLoop).
• One-Off / Single Play (Play(pcm, false)): Default mode for game over music, stage clear fanfares, calamity alerts, and victory stingers that play once to completion and then stop without looping.

2.3 Style-Based Reference Baselines for Duration & Tempo (BPM)

Tempo (BPM) and track length are heavily dictated by game style, genre, narrative mood, and scene context (e.g., bullet-hell shmup vs. ambient puzzle vs. epic RPG). The table below offers flexible reference baselines rather than rigid rules:

BPM to Note Duration Calculation

$$t_{\text{beat}} = \frac{60}{\text{BPM}}$$

• Quarter Note ($1/4$): $t_{\text{beat}}$
• Eighth Note ($1/8$): $t_{\text{beat}} / 2$
• Sixteenth Note ($1/16$): $t_{\text{beat}} / 4$
• Measure ($4/4$ time): $240 / \text{BPM}$ seconds

Flexible Scene Reference Table

3. SECTION 2: JSON Sound Format & CLI Player (play.go)

To test sound effects and multi-track compositions outside game runtimes, use the declarative JSON Sound Format parsed natively by scripts/sound.go.

3.1 Declarative JSON Sound Specification

Sound Effect JSON (sfx_laser.json)

{
  "title": "Laser Bow Shot",
  "type": "sfx",
  "sequence": [
    {
      "wave_type": "square",
      "duration": 0.15,
      "start_freq": 800.0,
      "end_freq": 150.0,
      "duty_cycle": 0.25,
      "volume": 0.3,
      "attack": 0.01,
      "decay": 0.05,
      "sustain": 0.2,
      "release": 0.09,
      "pan": 0.0
    }
  ]
}

Multi-Track BGM Song JSON (song_boss.json)

{
  "title": "Boss Battle Theme",
  "type": "song",
  "bpm": 150,
  "time_signature": "4/4",
  "tracks": [
    {
      "name": "Ch1 Sawtooth Lead",
      "pan": -0.2,
      "notes": [
        {
          "wave_type": "sawtooth",
          "duration": 0.2,
          "start_freq": 659.25,
          "end_freq": 659.25,
          "vibrato_freq": 8.0,
          "vibrato_depth": 10.0,
          "volume": 0.12,
          "attack": 0.02,
          "decay": 0.05,
          "sustain": 0.7,
          "release": 0.05
        }
      ]
    },
    {
      "name": "Ch4 Driving Bass",
      "pan": 0.0,
      "notes": [
        {
          "wave_type": "square",
          "duration": 0.2,
          "start_freq": 110.0,
          "end_freq": 110.0,
          "duty_cycle": 0.5,
          "volume": 0.14,
          "attack": 0.01,
          "decay": 0.1,
          "sustain": 0.5,
          "release": 0.05
        }
      ]
    }
  ]
}

3.2 Using the CLI Player Tool (play.go)

The CLI player scripts/play.go allows developers and testing agents to play or export audio definitions:

# 1. Play JSON sound effect or song (one-off by default)
go run ./scripts play sound.json

# 2. Play JSON sound effect or song in a loop
go run ./scripts play -loop song_stage.json

# 3. Play built-in 6-channel Genesis style demo soundtrack (one-off or -loop)
go run ./scripts demo
go run ./scripts demo -loop

# 4. Synthesize JSON definition and export directly to a 16-bit 44.1kHz Stereo .wav file
go run ./scripts export song_stage.json stage_theme.wav

4. Mandatory Quality Directives for AI Generation

When an AI agent uses this skill to compose audio or generate sound driver code:

1. NEVER Generate Short 1-Bar or 2-Bar Musical Loops:
   • Any BGM generated MUST be a complete composition spanning at least 8 to 16 bars with chord progressions, theme development, and harmonic transitions.
2. MANDATORY 6-Channel Polyphony:
   • Every background soundtrack MUST feature at least 6 distinct polyphonic instrument tracks (Lead, Counter-Melody, Harmony Pad, Bass, Arpeggiator, Drums/Percussion).
3. MANDATORY Expressive Parameterization:
   • Every note/track MUST specify tailored DutyCycle ($0.125 - 0.5$), Pan (stereo field distribution), VibratoFreq/VibratoDepth for lead instruments, and distinct ADSR envelope ramps.
4. MANDATORY 32-Bit Summation & Clamping:
   • Multi-track audio mixing MUST sum in 32-bit integers (int32) and hard-clamp to [-32768, +32767] to eliminate wrap-around distortion.
5. No Monophonic Beeps:
   • Sound effects must use frequency modulation sweeps, noise filters, or 2-stage note sequences (GenerateSequencePCM) to sound crisp, punchy, and retro-console authentic.

5. Gotchas & Engineering Best Practices

• Integer Overflow Wrap-Around: Summing track samples directly in int16 causes violent digital clipping and speaker crackle. Always sum tracks in int32 and hard-clamp to [-32768, +32767].
• Audio Popping & Clicks: Instantly stopping a waveform oscillator creates a steep DC offset jump that sounds like a loud "pop" or click. Always apply a release envelope ramp (at least $5\text{--}10\text{ ms}$).
• Memory & Frame GC Spikes: Avoid instantiating or generating PCM slices inside main frame rendering functions (Update/Draw). Pre-render all audio buffers during system initialization.
• Audio Channel Choking: High-frequency user events (e.g. clicking 50 times/sec) will choke audio players. Implement cooldown rate limiters ($30\text{--}50\text{ ms}$) on interactive triggers.

6. Summary Checklist for Procedural Audio Quality

1. Pre-render SFX & Loops: Generate PCM byte buffers on boot to keep execution overhead at zero during gameplay frames.
2. Mimic YM2612 6-Channel Polyphony: Layer at least 6 distinct instrument channels (Lead, Counterpoint, Pad, Bass, Arpeggio, Percussion/Noise) to achieve retro console soundtrack depth.
3. Support Dual Playback Modes: Provide both Play(pcm, false) (one-off) for stingers and Play(pcm, true) (infinite loop) for BGM.
4. Hard-Clamp Mixed PCM: Always sum in int32 and clamp to [-32768, +32767] when mixing audio tracks to avoid harsh digital overflow wrap-around distortion.
5. Declarative JSON & CLI Testing: Use JSON sound definitions and go run . export to validate audio quality and generate .wav previews.
6. Enforce AI Generation Quality Directives: Enforce 16-bar length, 6-channel polyphony, and expressive parameterization on all agent outputs.

7. State-Based Adaptive BGM Composition Rules

When composing code-synthesized multi-track audio for dynamic game states:

• Exploration / Ambient: Low BPM ($60\text{--}90$), sparse instrumentation, soft pads, subtle woodwinds, key of C Major.
• Tension / Stealth: Medium BPM ($90\text{--}110$), staccato strings, muted sub-bass, ticking percussion.
• Combat / Action / Boss: High BPM ($120\text{--}160+$), driving drums, heavy bassline, intense brass/synths, key of D minor.