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2026-07-14 00:26:01 +02:00
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+# Data Transfer Fix - AudioCoder
+
+## Problem Identified
+The GUI would show "CLOCK LOCK pattern=AA" repeatedly but never receive any data. The decoder got stuck in the RX_SEARCH state.
+
+## Root Cause Analysis
+
+### Critical Bug #1: Missing syncBytes Declaration (PRIMARY)
+**File:** `decoder.c` / `decoder.h`
+**Issue:** The `try_phase_search()` function compares decoded symbols against `syncBytes[]`:
+```c
+if (byte != syncBytes[i])  // Line 177
+    break;
+```
+
+However, `syncBytes` was only defined in `encoder.c` and **NOT** declared as `extern` in `decoder.h`. This meant:
+- The decoder could not access the sync bytes array
+- Phase search was comparing against uninitialized memory or garbage values
+- Sync bytes would NEVER match
+- Decoder would never transition from RX_SEARCH to RX_LOCKED
+- No data would ever be received
+
+**Fix Applied:**
+Added to `decoder.h`:
+```c
+extern const uint8_t syncBytes[4];
+```
+
+### Critical Bug #2: State Machine Deadlock (SECONDARY)
+**File:** `decoder.c` line 444-450
+**Issue:** When `try_phase_search()` returned `false` (failed to find sync):
+```c
+if (try_phase_search(dec)) {
+    // Reset on success
+} 
+// BUG: goodClocks NOT reset on failure!
+```
+
+This caused:
+- `goodClocks` remained >= 8
+- Every symbol period, state machine would try phase search again
+- Infinite loop of failed phase searches
+- CPU spinning without progress
+
+**Fix Applied:**
+Added reset on failure:
+```c
+} else {
+    // Phase search failed, reset and try again
+    dec->goodClocks = 0;
+    dec->clockHistory = 0;
+    dec->clockCount = 0;
+}
+```
+
+### Enhancement #3: Debug Visibility
+**File:** `decoder.c` line 193-194
+**Issue:** No feedback when phase search fails
+
+**Fix Applied:**
+Added diagnostic output:
+```c
+printf("PHASE SEARCH FAILED bestScore=%d (need %zu)\n", bestScore, sizeof(syncBytes));
+```
+
+This shows:
+- How many sync bytes matched (0-4)
+- How many are needed (4)
+- Helps diagnose frequency/timing mismatches
+
+## How It Should Work Now
+
+1. **Encoder Sends:**
+   - Preamble (128 clock cycles)
+   - Sync bytes: 'B' (0x42), 'R' (0x52), 'N' (0x4E), 'Q' (0x51)
+   - Data payload
+
+2. **Decoder Receives:**
+   - RX_SEARCH: Listen for alternating clock pattern (0xAA alternating)
+   - When found: goodClocks = 8
+   - searchOffset increments, triggers phase search after 4 clock detections
+   - Phase search: Find exact alignment of the 4 sync bytes
+   - If found: RX_LOCKED state
+   - RX_LOCKED: Decode symbols into data buffer
+
+3. **Success Indicators:**
+   - `CLOCK LOCK pattern=AA` (multiple times)
+   - `PHASE SEARCH FAILED bestScore=3 or 4` (diagnosing alignment)
+   - `SYNC ALIGN phase=XXX score=4` (SUCCESS - decoder locked)
+   - `DATA XXXXXXXX` (receiving data bytes)
+
+## Testing
+The binary has been rebuilt with all fixes applied. Expected output should progress from CLOCK LOCK → PHASE SEARCH FAILED (or SUCCEED) → SYNC ALIGN → DATA.