# Audio Mixing - Podcast Production

## Overview

The blog-backup project generates podcast audio by mixing TTS (text-to-speech) with background music using ffmpeg.

## Current Working Configuration

### What It Does
- **TTS:** Uses macOS `say` command (built-in, no external API)
- **Mixing:** Music plays at 12% volume UNDER the speech (continuous bed)
- **Fades:** Music fades in at start (30%) and fades out at end (30%)
- **Format:** MP3 output

### Audio Flow
```
[Music fades in 30%] → [Speech with 12% music bed] → [Music fades out]
```

### Technical Details

**File:** `blog-backup/src/lib/tts.ts`

**Environment Variables:**
```bash
ENABLE_TTS=true
TTS_PROVIDER=macsay
ENABLE_PODCAST_MUSIC=true
INTRO_MUSIC_URL=/path/to/intro.mp3
OUTRO_MUSIC_URL=/path/to/outro.mp3
```

**ffmpeg Command (Working):**
```bash
ffmpeg -y -i "${ttsPath}" -stream_loop -1 -i "${introPath}" -i "${outroPath}" -filter_complex "
  [1:a]volume=0.3,apad=5[music];
  [2:a]volume=0.3[outro];
  [0:a][music]amix=duration=first:weights=1 0.12[speechbed];
  [speechbed]afade=t=in:st=0:d=1[in];
  [in][outro]concat=n=2:v=0:a=1[out]
" -map "[out]" -shortest "${outputPath}"
```

### Known Limitations

1. **Complex filters fail:** More elaborate ffmpeg filter chains (trimming, looping specific segments) tend to fail with "Filter has output unconnected" errors
2. **Single bed approach works:** Using the same intro as a continuous bed is reliable
3. **Pre-sliced clips would be better:** For distinct intro/speech/outro, pre-create short clips (5-10 sec) and concatenate

## Music Files

**Location:** `blog-creator/public/podcast-audio/`

| File | Duration | Use |
|------|----------|-----|
| intro.mp3 | 71 sec | Background music bed |
| outro.mp3 | 34 sec | Outro music |

### Suggested Improvements

1. **Create short intro clip:** Extract first 5-10 sec as separate file
2. **Create short outro clip:** Extract last 5-10 sec as separate file  
3. **Use simpler 2-step process:**
   - Step 1: Mix speech with looped bed
   - Step 2: Prepend intro, append outro

## Testing

```bash
# Test TTS with music
curl -X POST "http://localhost:3002/api/tts" \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -d '{
    "text": "Your text here",
    "includeMusic": true
  }'
```

## Common Errors

| Error | Cause | Fix |
|-------|-------|-----|
| "Filter has output unconnected" | Complex filter chain | Simplify to fewer inputs |
| "OPENAI_API_KEY not configured" | Wrong provider | Set TTS_PROVIDER=macsay |
| "No music files configured" | Missing env vars | Set INTRO_MUSIC_URL and OUTRO_MUSIC_URL |

## Future Enhancements

- [ ] Pre-slice intro/outro for distinct segments
- [ ] Add transition sounds between stories
- [ ] Adjust bed volume based on speech pauses
- [ ] Add compression/normalize for consistent levels