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Problem: test_demo was "flashing a lot" - visual effects triggered ~400ms
before audio was heard, causing poor synchronization.
Root Causes:
1. Beat calculation used physical time (platform_state.time), but audio
peak measured at playback time (400ms behind due to ring buffer)
2. Peak decay too slow (0.7 per callback = 800ms fade) relative to beat
interval (500ms at 120 BPM)
Solution:
1. Use audio_get_playback_time() for beat calculation
- Automatically accounts for ring buffer latency
- No hardcoded constants (was considering hardcoding 400ms offset)
- System queries its own state
2. Faster decay rate (0.5 vs 0.7) to match beat interval
3. Added inline PeakMeterEffect for visual debugging
Changes:
- src/test_demo.cc:
- Added inline PeakMeterEffect class (red bar visualization)
- Use audio_get_playback_time() instead of physical time for beat calc
- Updated logging to show audio time
- src/audio/backend/miniaudio_backend.cc:
- Changed decay rate from 0.7 to 0.5 (500ms fade time)
- src/gpu/gpu.{h,cc}:
- Added gpu_add_custom_effect() API for runtime effect injection
- Exposed g_device, g_queue, g_format as non-static globals
- doc/PEAK_METER_DEBUG.md:
- Initial analysis of timing issues
- doc/AUDIO_TIMING_ARCHITECTURE.md:
- Comprehensive architecture documentation
- Time source hierarchy (physical → audio playback → music)
- Future work: TimeProvider class, tracker_get_bpm() API
Architectural Principle:
Single source of truth - platform_get_time() is the only physical clock.
Everything else derives from it. No hardcoded latency constants.
Result: Visual effects now sync perfectly with heard audio.
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