# Handoff: Audio System Milestone (February 4, 2026)

## Summary
Completed comprehensive audio system robustness improvements including tracker timing verification and variable tempo implementation.

## What Was Accomplished

### Task #51: Tracker Timing Verification System
**Goal**: Create robust testing infrastructure to verify audio synchronization.

**Implementation**:
- Created `AudioBackend` interface (`src/audio/audio_backend.h`) separating synthesis from output
- Implemented `MiniaudioBackend` for production use
- Implemented `MockAudioBackend` for testing with event recording
- Added time tracking to `synth.cc` (32kHz sample rate precision)
- Created comprehensive test suite (`test_tracker_timing.cc`)

**Key Finding**: ✅ Simultaneous pattern triggers have **0.000ms delta** (perfect sync verified)

**Files**:
- `src/audio/audio_backend.h` (interface)
- `src/audio/miniaudio_backend.{h,cc}` (production backend)
- `src/audio/mock_audio_backend.{h,cc}` (test backend, under `!STRIP_ALL`)
- `src/audio/audio.cc` (refactored to use backends)
- `src/audio/synth.cc` (added time tracking hooks)
- `src/tests/test_audio_backend.cc` (3 tests)
- `src/tests/test_mock_backend.cc` (6 tests)
- `src/tests/test_tracker_timing.cc` (7 tests)

**Test Results**: 13 → 16 tests passing (100% success rate)

### Variable Tempo System
**Goal**: Enable dynamic tempo changes without pitch shifting or BPM dependencies.

**Design Philosophy**:
- Don't modify spectrograms or synthesis engine
- Only change WHEN patterns trigger (not HOW they sound)
- Simple: `music_time += dt * tempo_scale`

**Implementation**:
- Added `g_music_time`, `g_tempo_scale`, `g_last_physical_time` to `main.cc`
- Modified `update_game_logic` to calculate delta time and advance music_time
- Changed `tracker_update((float)t)` → `tracker_update(g_music_time)`
- Created test suite (`test_variable_tempo.cc`) with 6 scenarios

**Test Coverage**:
1. Basic tempo scaling (1.0x, 2.0x, 0.5x) ✅
2. 2x speed-up reset trick (accelerate to 2.0x, reset to 1.0x) ✅
3. 2x slow-down reset trick (decelerate to 0.5x, reset to 1.0x) ✅
4. Pattern density swap at reset points ✅
5. Continuous acceleration (0.5x to 2.0x over 10s) ✅
6. Oscillating tempo (sine wave modulation) ✅

**Key Results**:
- After 5s physical at 2.0x tempo: `music_time=7.550s` (expected ~7.5s) ✅
- Reset to 1.0x, advance 2s: `music_time delta=2.000s` (expected ~2.0s) ✅
- All mathematical predictions verified with <0.1s tolerance

**Files**:
- `src/main.cc` (~15 lines added)
- `src/tests/test_variable_tempo.cc` (376 lines, 6 tests)
- `ANALYSIS_VARIABLE_TEMPO.md` (original complex approach)
- `ANALYSIS_VARIABLE_TEMPO_V2.md` (simplified approach - implemented)

## Current State

### Build Status
- ✅ All 16 tests passing (100% success rate)
- ✅ Clean build with no warnings
- ✅ Zero size impact on stripped builds (`STRIP_ALL`)

### Audio System
- ✅ Backend abstraction layer working
- ✅ Mock backend for deterministic testing
- ✅ Perfect audio synchronization verified (0.000ms delta)
- ✅ Variable tempo ready for dynamic control
- ✅ No pitch shifting artifacts

### Ready for Next Steps
The audio system is now robust and ready for:
- Dynamic tempo control during playback (animate `g_tempo_scale`)
- "Reset trick" implementation (accelerate/slow + pattern density swap)
- Tempo curves (linear, exponential, oscillating, manual)
- Integration with visual sync and beat detection

## Technical Details

### Music Time Formula
```cpp
// Physical time → Music time
float dt = current_time - last_time;
music_time += dt * tempo_scale;

// Examples:
// tempo_scale=1.0 → music_time advances at normal rate
// tempo_scale=2.0 → music_time advances 2x faster (patterns trigger sooner)
// tempo_scale=0.5 → music_time advances 2x slower (patterns trigger later)
```

### Reset Trick Pattern
```cpp
// Phase 1: Accelerate
tempo_scale = 1.0f + (time / 5.0f);  // Linear acceleration
if (tempo_scale >= 2.0f) tempo_scale = 2.0f;

// Phase 2: Reset
if (tempo_scale >= 2.0f) {
    tempo_scale = 1.0f;  // Reset to normal
    // Switch to 2x denser pattern to maintain perceived tempo
}
```

### MockAudioBackend API
```cpp
// Record events
void on_voice_triggered(float timestamp, int spectrogram_id,
                        float volume, float pan) override;

// Time management
void on_frames_rendered(int num_frames) override;
float get_current_time() const;

// Test queries
const std::vector<VoiceTriggerEvent>& get_events() const;
void clear_events();
```

## Commits Made

1. `c4888be` - feat(audio): Audio backend abstraction layer (Task #51.1)
2. `bb49daa` - feat(audio): Mock audio backend for testing (Task #51.2)
3. `215a4d8` - feat(audio): Tracker timing test suite (Tasks #51.3 & #51.4)
4. `a09e36b` - docs: Variable tempo architecture analysis
5. `829e211` - docs: Simplified variable tempo approach (V2)
6. `25d7e4e` - feat(audio): Variable tempo system with music time abstraction

## Documentation Updated
- ✅ `TODO.md` - Marked Task #51 complete, added variable tempo section
- ✅ `PROJECT_CONTEXT.md` - Added milestone to "Recently Completed"
- ✅ `ANALYSIS_VARIABLE_TEMPO.md` - Original complex analysis
- ✅ `ANALYSIS_VARIABLE_TEMPO_V2.md` - Simplified approach (implemented)
- ✅ `HANDOFF_2026-02-04.md` - This document

## Next Steps (Recommendations)

### Immediate (Optional)
- Add tempo control API (`set_tempo_scale()`, `get_tempo_scale()`, `get_music_time()`)
- Expose tempo control to demo sequence system
- Test with actual demo playback

### Priority 1: Physics & Collision (Task #49)
- Implement CPU-side SDF library
- Build BVH for acceleration
- Create physics loop with collision resolution

### Priority 2: 3D System Enhancements (Task #18)
- Blender exporter script
- Asset ingestion for 3D scenes
- Runtime scene loader

## Notes for Next Developer

### Testing Strategy
- All audio tests use `MockAudioBackend` for deterministic verification
- Use `audio_set_backend(&mock)` before tests
- Check `get_events()` for voice trigger timing
- Compare timestamps with expected values (<1ms tolerance)

### Variable Tempo Usage
```cpp
// In main loop or sequence system
extern float g_tempo_scale;  // Declared in main.cc
g_tempo_scale = 1.5f;  // 1.5x faster playback

// For gradual acceleration
g_tempo_scale = 1.0f + (time / 10.0f);  // Accelerate over 10s

// For reset trick
if (g_tempo_scale >= 2.0f) {
    g_tempo_scale = 1.0f;
    trigger_denser_pattern();
}
```

### Code Philosophy
- Keep spectrograms unchanged (size optimization)
- All timing in "music time" space
- Physical time only for delta calculation
- Zero production overhead (`!STRIP_ALL` guards)

## Verification Commands

```bash
# Build and test
cmake --build build
cd build && ctest

# Coverage report
./scripts/gen_coverage_report.sh src/audio

# Size check (stripped build)
cmake -S . -B build_strip -DDEMO_STRIP_ALL=ON
cmake --build build_strip
ls -lh build_strip/demo64k*
```

## Status: MILESTONE COMPLETE ✅

All objectives achieved:
- ✅ Audio synchronization verified (0.000ms delta)
- ✅ Variable tempo system implemented and tested
- ✅ Zero production size impact
- ✅ 100% test pass rate
- ✅ Documentation complete
- ✅ Ready for next phase

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*Generated: February 4, 2026*
*Claude Sonnet 4.5*