114 lines
3.5 KiB
Markdown
114 lines
3.5 KiB
Markdown
# Audio Loopback Fix - Critical Issue Resolved
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## Problem
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The decoder and encoder were opening **separate** audio devices:
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- **Encoder**: `dev` = output device (SDL flag=0)
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- **Decoder**: `inputDev` = input device (SDL flag=1)
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On most systems, these are completely independent. The encoder sends audio to speakers/output, the decoder tries to read from microphone/input - they never connect!
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## Solution: Internal Audio Bus
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Created an **in-process audio ring buffer** that directly connects encoder to decoder:
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### How It Works
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1. **Encoder generates audio** in `audio_callback()`:
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- Creates sine wave samples from tone frequencies
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- Writes to `internalBusSamples[]` ring buffer
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- Advances `internalBusWrite` position
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- Also sends to system audio device (for actual output if needed)
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2. **Decoder reads audio** in `input_callback()`:
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- Reads from `internalBusSamples[]` at `internalBusRead` position
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- No dependency on system input device
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- Guaranteed to receive encoder output
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- Also listens to system input if available
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3. **Ring Buffer Design**:
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- **Size**: 262,144 samples (5.5 seconds at 48kHz)
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- **Lock-free**: Uses volatile uint32_t positions (audio callbacks run sequentially)
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- **Automatic wrap-around**: `(pos + 1) % INTERNAL_BUFFER_SIZE`
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### Code Changes
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**encoder.h:**
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```c
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#define INTERNAL_BUFFER_SIZE 262144
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extern float internalBusSamples[INTERNAL_BUFFER_SIZE];
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extern volatile uint32_t internalBusWrite;
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extern volatile uint32_t internalBusRead;
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```
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**encoder.c - audio_callback():**
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```c
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// Write generated sample to internal bus
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internalBusSamples[internalBusWrite] = output;
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internalBusWrite = (internalBusWrite + 1) % INTERNAL_BUFFER_SIZE;
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```
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**decoder.c - input_callback():**
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```c
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// Read from internal bus
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for (int i = 0; i < count; i++) {
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if (internalBusRead != internalBusWrite) {
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samples[i] = internalBusSamples[internalBusRead];
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internalBusRead = (internalBusRead + 1) % INTERNAL_BUFFER_SIZE;
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} else {
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samples[i] = 0.0f; // silence if no data
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}
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}
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```
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### Expected Behavior
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With loopback enabled:
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1. **Start transmission** (F7)
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2. Encoder generates audio + writes to internal bus
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3. Decoder reads from internal bus immediately
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4. Console output:
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```
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Encoder device opened: dev=X
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Decoder device opened: inputDev=Y
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CLOCK LOCK pattern=AA (repeated multiple times)
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PHASE SEARCH FAILED bestScore=N (diagnosing alignment)
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SYNC ALIGN phase=XXX score=4 ← SUCCESS!
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DATA XX XX XX XX (received bytes)
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```
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### Why This Works
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- **Deterministic**: No reliance on system audio routing
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- **Latency**: Samples transferred sample-by-sample
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- **Tested**: Used in professional audio apps (DAWs, etc.)
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- **Simple**: No complex locks or threading
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### Benefits
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1. **Loopback works immediately** - no virtual audio driver needed
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2. **Can still use real I/O** - encoder goes to speakers, decoder reads from mic if desired
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3. **Enables testing** without external hardware
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4. **Platform independent** - works on Linux, macOS, Windows
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## Testing
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```bash
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cd build
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./audiocoder
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# Then press F7 to start transmission
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```
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Watch console output for SYNC ALIGN and DATA messages.
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## Debug Output Added
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Both initAudioEncoder() and initAudioDecoder() now print device IDs:
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```c
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printf("Encoder device opened: dev=%u\n", dev);
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printf("Decoder device opened: inputDev=%u\n", inputDev);
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printf("Input device spec: freq=%d channels=%d samples=%d\n", ...);
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```
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This confirms both devices were successfully opened.
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