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-# Analysis: Variable Tempo / Unified Music Time
-
-## Current Architecture Analysis
-
-### 1. BPM Dependencies (Problems Found)
-
-#### Location 1: `TrackerScore` (tracker.h:32)
-```cpp
-struct TrackerScore {
-  const TrackerPatternTrigger* triggers;
-  uint32_t num_triggers;
-  float bpm;  // ❌ FIXED BPM - Cannot change dynamically
-};
-```
-
-#### Location 2: `tracker_update()` (tracker.cc:57)
-```cpp
-float beat_to_sec = 60.0f / g_tracker_score.bpm;  // ❌ Used to convert beats→seconds
-```
-
-**Critical Issue (tracker.cc:84):**
-```cpp
-int frame_offset = (int)(event.beat * beat_to_sec * 32000.0f / DCT_SIZE);
-```
-
-**Problem:** The BPM is **baked into the spectrogram** at pattern trigger time!
-- Events within a pattern are composited once with fixed spacing
-- The resulting spectrogram frames encode the tempo
-- Once triggered, the pattern plays at the encoded tempo
-- **Cannot speed up/slow down dynamically**
-
-#### Location 3: `main.cc` (lines 99, 166)
-```cpp
-// Bass pattern timing
-if (t - last_beat_time > (60.0f / g_tracker_score.bpm) / 2.0) {  // ❌ BPM dependency
-
-// Visual beat calculation
-float beat = fmodf((float)current_time * g_tracker_score.bpm / 60.0f, 1.0f);  // ❌ BPM dependency
-```
-
----
-
-## Current Time Flow
-
-```
-Physical Time (hardware clock)
-    ↓
-platform_state.time (real seconds elapsed)
-    ↓
-current_time = platform_state.time + seek_offset
-    ↓
-tracker_update(current_time)  ← checks if patterns should trigger
-    ↓
-Pattern compositing: event.beat * (60.0 / BPM) * 32kHz
-    ↓
-Spectrogram generated with FIXED frame spacing
-    ↓
-synth_trigger_voice() → registers spectrogram
-    ↓
-Audio callback @ 32kHz pulls samples via synth_render()
-    ↓
-Voice advances through spectral frames at FIXED rate (one frame per DCT_SIZE samples)
-```
-
-**The Fundamental Problem:**
-- Music time = Physical time (1:1 mapping)
-- Patterns are pre-rendered at fixed tempo
-- Synth playback rate is locked to 32kHz sample clock
-
----
-
-## What Needs to Change
-
-### Architecture Requirements
-
-1. **Separate Music Time from Physical Time**
-   - Music time advances independently: `music_time += dt * tempo_scale`
-   - `tempo_scale = 1.0` → normal speed
-   - `tempo_scale = 2.0` → double speed
-   - `tempo_scale = 0.5` → half speed
-
-2. **Remove BPM from Pattern Compositing**
-   - Events should use abstract "beat" units, not seconds
-   - Pattern generation should NOT convert beats→frames using BPM
-   - Keep events in "music time" space
-
-3. **Variable Playback Rate in Synth**
-   - Synth must support dynamic playback speed
-   - As tempo changes, synth advances through spectral frames faster/slower
-
----
-
-## Proposed Solution
-
-### Phase 1: Introduce Unified Music Time
-
-#### Change 1: Update `TrackerScore`
-```cpp
-struct TrackerScore {
-  const TrackerPatternTrigger* triggers;
-  uint32_t num_triggers;
-  // REMOVE: float bpm;
-};
-```
-
-#### Change 2: Update `TrackerPatternTrigger`
-```cpp
-struct TrackerPatternTrigger {
-  float music_time;  // Renamed from time_sec, now abstract units
-  uint16_t pattern_id;
-};
-```
-
-#### Change 3: Update `TrackerEvent`
-```cpp
-struct TrackerEvent {
-  float beat;  // Keep as-is, already abstract
-  uint16_t sample_id;
-  float volume;
-  float pan;
-};
-```
-
-#### Change 4: Update `tracker_update()` signature
-```cpp
-// OLD:
-void tracker_update(float time_sec);
-
-// NEW:
-void tracker_update(float music_time);
-```
-
-**Note:** Pattern compositing still needs tempo for now (see Phase 2).
-
----
-
-### Phase 2: Variable Tempo Synth Playback
-
-The hard part: **Dynamic playback speed without pitch shifting.**
-
-#### Option A: Naive Approach (Pitch Shifts - NOT GOOD)
-```cpp
-// Simply advance through frames faster/slower
-v.current_spectral_frame += playback_speed;  // ❌ Changes pitch!
-```
-
-**Problem:** Advancing through spectral frames faster/slower changes pitch.
-- 2x speed → 2x frequency (octave up)
-- 0.5x speed → 0.5x frequency (octave down)
-
-#### Option B: Resample Spectrograms (Complex)
-- Generate spectrograms at "reference tempo" (e.g., 1.0)
-- At playback time, resample to match current tempo
-- **Pros:** Preserves pitch
-- **Cons:** CPU-intensive, requires interpolation
-
-#### Option C: Time-Stretching (Best Quality)
-- Use phase vocoder or similar algorithm
-- Stretches/compresses time without changing pitch
-- **Pros:** High quality
-- **Cons:** Very complex, may exceed 64k budget
-
-#### Option D: Pre-render at Reference Tempo (Recommended for 64k)
-**Key Insight:** For a 64k demo, pre-render everything at a fixed internal tempo.
-
-```cpp
-// At pattern trigger time:
-// 1. Composite pattern at "reference tempo" (e.g., 120 BPM baseline)
-int frame_offset = (int)(event.beat * REFERENCE_BEAT_DURATION * 32000.0f / DCT_SIZE);
-
-// 2. Store tempo multiplier with voice
-struct Voice {
-  float playback_speed;  // NEW: 1.0 = normal, 2.0 = double speed, etc.
-  // ... existing fields
-};
-
-// 3. In synth_render(), advance at variable rate:
-void synth_render(float* output_buffer, int num_frames) {
-  for each voice {
-    // Advance through spectral frames at tempo-adjusted rate
-    float frame_advance = playback_speed / DCT_SIZE;
-
-    // Use fractional frame index
-    float spectral_frame_pos;  // NEW: float instead of int
-
-    // Interpolate between frames
-    int frame0 = (int)spectral_frame_pos;
-    int frame1 = frame0 + 1;
-    float frac = spectral_frame_pos - frame0;
-
-    // Blend spectral data from frame0 and frame1
-    for (int j = 0; j < DCT_SIZE; ++j) {
-      windowed_frame[j] = lerp(spectral[frame0][j], spectral[frame1][j], frac);
-    }
-  }
-}
-```
-
-**Trade-off:** This changes pitch! But for a demo, this might be acceptable.
-
----
-
-## Sync with Physical Audio Device
-
-### Current System
-```
-Audio Callback (32kHz fixed)
-    ↓
-synth_render(buffer, num_frames) ← pulls samples
-    ↓
-Voices advance: ++buffer_pos, ++current_spectral_frame
-    ↓
-Fixed playback rate (one spectral frame = DCT_SIZE samples)
-```
-
-### With Variable Tempo
-```
-Audio Callback (32kHz fixed - CANNOT CHANGE)
-    ↓
-synth_render(buffer, num_frames, current_playback_speed)
-    ↓
-Voices advance: buffer_pos += playback_speed
-    ↓
-Variable playback rate (playback_speed can be > 1.0 or < 1.0)
-```
-
-**Key Point:** Hardware audio rate (32kHz) is FIXED. We cannot change it.
-
-**Solution:** Adjust how fast we consume spectrogram data:
-- `playback_speed = 2.0` → consume frames 2x faster → music plays 2x speed
-- `playback_speed = 0.5` → consume frames 0.5x slower → music plays 0.5x speed
-
----
-
-## Implementation Roadmap
-
-### Step 1: Remove BPM from TrackerScore ✅ Ready
-- Remove `float bpm` field
-- Update tracker_compiler to not generate BPM
-- Update tests
-
-### Step 2: Add Global Tempo State
-```cpp
-// In audio/tracker.h or main.cc
-float g_current_tempo = 1.0f;  // Multiplier: 1.0 = normal, 2.0 = double speed
-
-void tracker_set_tempo(float tempo_multiplier);
-float tracker_get_tempo();
-```
-
-### Step 3: Update tracker_update()
-```cpp
-void tracker_update(float music_time) {
-  // music_time is now abstract, not physical seconds
-  // Patterns trigger when music_time >= trigger.music_time
-}
-```
-
-### Step 4: Update Pattern Compositing (Keep Reference Tempo)
-```cpp
-// Use fixed reference tempo for compositing (e.g., 120 BPM)
-const float REFERENCE_BPM = 120.0f;
-const float reference_beat_to_sec = 60.0f / REFERENCE_BPM;
-
-int frame_offset = (int)(event.beat * reference_beat_to_sec * 32000.0f / DCT_SIZE);
-```
-
-### Step 5: Variable Playback in Synth (Advanced)
-```cpp
-// Store playback speed with each voice
-struct Voice {
-  float playback_speed;  // Set when voice is triggered
-  float spectral_frame_pos;  // Change to float for interpolation
-  // ...
-};
-
-// In synth_trigger_voice():
-v.playback_speed = g_current_tempo;
-
-// In synth_render():
-// Advance with fractional frame increment
-v.spectral_frame_pos += v.playback_speed / DCT_SIZE;
-
-// Interpolate spectral data (required for smooth tempo changes)
-```
-
-### Step 6: Update Main Loop
-```cpp
-// In main.cc
-static float g_music_time = 0.0f;
-static float g_tempo_scale = 1.0f;  // Can be animated!
-
-void update_game_logic(double physical_time) {
-  float dt = get_delta_time();
-
-  // Music time advances at variable rate
-  g_music_time += dt * g_tempo_scale;
-
-  // Animate tempo (example)
-  g_tempo_scale = 1.0f + 0.5f * sinf(physical_time * 0.1f);  // Oscillate 0.5x - 1.5x
-
-  tracker_set_tempo(g_tempo_scale);
-  tracker_update(g_music_time);
-}
-```
-
----
-
-## Open Questions
-
-1. **Pitch Shifting Acceptable?**
-   - If tempo changes, should pitch change too?
-   - For demoscene effect: probably YES (classic effect)
-   - For "realistic" music: NO (need time-stretching)
-
-2. **Tempo Curve Continuity**
-   - Should tempo changes be smoothed (acceleration curves)?
-   - Or instant jumps (may cause audio glitches)?
-
-3. **Spectral Frame Interpolation**
-   - Linear interpolation sufficient?
-   - Or need cubic/sinc for quality?
-
-4. **Tracker Compilation**
-   - Should tracker_compiler still know about tempo?
-   - Or output pure "beat" units?
-
----
-
-## Recommendation
-
-**For a 64k demo, I recommend:**
-
-### Minimal Change Approach (Easiest)
-1. Keep BPM for compositing (as "reference tempo")
-2. Add `g_current_tempo` multiplier
-3. Make synth advance through frames at `tempo * playback_speed`
-4. Accept pitch shifting as intentional effect
-
-**Changes needed:**
-- Add `float g_current_tempo` global
-- Update `synth_render()` to use `playback_speed`
-- Main loop: `music_time += dt * g_current_tempo`
-- Tracker: rename `time_sec` → `music_time` (conceptual only)
-
-**Size impact:** ~100 bytes
-**Complexity:** Low
-**Enables:** Full variable tempo with pitch shift effect
-
-### Advanced Approach (Better Quality)
-- Implement spectral frame interpolation
-- Add time-stretching (phase vocoder lite)
-- Size impact: +2-3 KB
-
----
-
-## Conclusion
-
-**Is the code ready?** ❌ NO
-
-**What needs to change?**
-1. BPM must become a "reference tempo" for compositing only
-2. Add global `tempo_scale` variable
-3. Synth must support variable playback speed
-4. Main loop must track `music_time` separately from physical time
-
-**Sync with audio device:**
-- Hardware rate (32kHz) is FIXED
-- We control playback speed by advancing through spectral frames faster/slower
-- This inherently changes pitch (unless we add time-stretching)
-
-**Recommended next step:** Implement minimal change approach first, then iterate.