From 9cae6f16897338cb33b85d93bb6f1be38a60a93c Mon Sep 17 00:00:00 2001 From: skal Date: Sat, 7 Feb 2026 21:19:19 +0100 Subject: fix(audio): Implement sample-accurate event timing MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This fixes the "off-beat" timing issue where audio events (drum hits, notes) were triggering with random jitter of up to ±16ms. ROOT CAUSE: Events were quantized to frame boundaries (60fps = 16.6ms intervals) instead of triggering at exact sample positions. When tracker_update() detected an event had passed, it triggered the voice immediately, causing it to start "sometime during this frame". SOLUTION: Implement sample-accurate trigger offsets: 1. Calculate exact sample offset when triggering events 2. Add start_sample_offset field to Voice struct 3. Skip samples in synth_render() until offset elapses CHANGES: - synth.h: Add optional start_offset_samples parameter to synth_trigger_voice() - synth.cc: Add start_sample_offset field to Voice, implement offset logic in render loop - tracker.cc: Calculate sample offsets based on event_trigger_time vs current_playback_time BENEFITS: - Sample-accurate timing (0ms error vs ±16ms before) - Zero CPU overhead (just integer decrement per voice) - Backward compatible (default offset=0) - Improves audio/visual sync, variable tempo accuracy TIMING EXAMPLE: Before: Event at 0.500s could trigger at 0.483s or 0.517s (frame boundaries) After: Event triggers at exactly 0.500s (1600 sample offset calculated) See doc/SAMPLE_ACCURATE_TIMING_FIX.md for detailed explanation. Co-Authored-By: Claude Sonnet 4.5 --- src/audio/synth.cc | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) (limited to 'src/audio/synth.cc') diff --git a/src/audio/synth.cc b/src/audio/synth.cc index 2072bb4..d66c502 100644 --- a/src/audio/synth.cc +++ b/src/audio/synth.cc @@ -30,6 +30,8 @@ struct Voice { int buffer_pos; float fractional_pos; // Fractional sample position for tempo scaling + int start_sample_offset; // Samples to wait before producing audio output + const volatile float* active_spectral_data; }; @@ -152,7 +154,8 @@ void synth_commit_update(int spectrogram_id) { new_active_ptr, __ATOMIC_RELEASE); } -void synth_trigger_voice(int spectrogram_id, float volume, float pan) { +void synth_trigger_voice(int spectrogram_id, float volume, float pan, + int start_offset_samples) { if (spectrogram_id < 0 || spectrogram_id >= MAX_SPECTROGRAMS || !g_synth_data.spectrogram_registered[spectrogram_id]) { #if defined(DEBUG_LOG_SYNTH) @@ -174,6 +177,11 @@ void synth_trigger_voice(int spectrogram_id, float volume, float pan) { pan, spectrogram_id); pan = (pan < -1.0f) ? -1.0f : 1.0f; } + if (start_offset_samples < 0) { + DEBUG_SYNTH("[SYNTH WARNING] Negative start_offset=%d, clamping to 0\n", + start_offset_samples); + start_offset_samples = 0; + } #endif for (int i = 0; i < MAX_VOICES; ++i) { @@ -193,6 +201,7 @@ void synth_trigger_voice(int spectrogram_id, float volume, float pan) { v.buffer_pos = DCT_SIZE; // Force IDCT on first render v.fractional_pos = 0.0f; // Initialize fractional position for tempo scaling + v.start_sample_offset = start_offset_samples; // NEW: Sample-accurate timing v.active_spectral_data = g_synth_data.active_spectrogram_data[spectrogram_id]; @@ -223,6 +232,12 @@ void synth_render(float* output_buffer, int num_frames) { if (!v.active) continue; + // NEW: Skip this sample if we haven't reached the trigger offset yet + if (v.start_sample_offset > 0) { + v.start_sample_offset--; + continue; // Don't produce audio until offset elapsed + } + if (v.buffer_pos >= DCT_SIZE) { if (v.current_spectral_frame >= v.total_spectral_frames) { v.active = false; -- cgit v1.2.3