# config.xml - Audio Bucket Configuration Guide

## Audio Bucket Structure

The audio buckets in `config.xml` define the bitrate tiers used for audio encoding based on channel count.

### Current Configuration

```xml
<audio>
  <stereo>
    <low>128000</low>
    <medium>160000</medium>
    <high>192000</high>
  </stereo>
  <multi_channel>
    <low>384000</low>
    <medium>448000</medium>
    <high>640000</high>
  </multi_channel>
</audio>
```

## Bitrate Usage by Resolution and Channels

### Stereo (2 Channels) - All Resolutions
- **720p**: Up to `high` (192 kbps) if needed
- **1080p**: Up to `high` (192 kbps) if needed  
- **4K (2160p)**: Up to `high` (192 kbps) if needed

Stereo bitrates are the same across all resolutions.

### Multi-Channel (6+ Channels)

#### 1080p (Max 6 channels)
- **Minimum**: `low` (384 kbps)
- **Default**: `medium` (448 kbps)
- **Maximum**: `medium` (448 kbps) ⚠️ **High tier NOT used**

Reason: Balanced compression for 1080p file size

#### 4K / 2160p (Max 8 channels)
- **Minimum**: `low` (384 kbps)
- **Default**: `medium` (448 kbps)
- **Maximum**: `high` (640 kbps) ✅ **High tier NOW available**

Reason: 4K containers larger; can better accommodate higher quality audio

#### 720p (Max 2 channels)
- Uses stereo bitrate tier (2 channels)
- Never uses multi-channel tier

## Audio Encoder Selection

### Stereo (2 channels)
- **Codec**: AAC
- **Use Case**: Commentary tracks, all 720p sources
- **Quality**: ~128-192 kbps depending on source

### Multi-Channel (6+ channels)
- **Codec**: EAC3 (Enhanced AC-3)
- **Use Case**: Main audio tracks with surround sound
- **Quality Tiers**:
  - Low: Aggressive compression, smaller files
  - Medium: Balanced quality/size (default)
  - High: High fidelity (4K only)

## Configuration Recommendations

### For 4K HDR Content
```xml
<multi_channel>
  <low>384000</low>      <!-- Option for aggressive compression -->
  <medium>448000</medium> <!-- Standard quality -->
  <high>640000</high>    <!-- High-quality surround (NEW in 4K mode) -->
</multi_channel>
```

### For Bandwidth-Conscious Setup
Reduce high tier if needed:
```xml
<multi_channel>
  <low>384000</low>
  <medium>384000</medium>  <!-- Same as low to reduce file size -->
  <high>448000</high>      <!-- Moderate high tier for 4K -->
</multi_channel>
```

### For Maximum Quality
Increase all tiers:
```xml
<multi_channel>
  <low>448000</low>
  <medium>512000</medium>
  <high>768000</high>      <!-- Higher quality for 4K -->
</multi_channel>
```

## How the Selection Works

### Example 1: 6-Channel Audio on 1080p
1. Detected: 6 channels, 500 kbps bitrate, 1080p resolution
2. Channel clamping: 6 ≤ 6 (max for 1080p) → stays 6ch
3. Bitrate selection: 500 kbps, 1080p max is `medium` (448 kbps) → uses 448 kbps
4. Output: EAC3 codec, 6 channels, 448 kbps

### Example 2: 8-Channel Audio on 4K
1. Detected: 8 channels, 640 kbps bitrate, 4K resolution
2. Channel clamping: 8 ≤ 8 (max for 4K) → stays 8ch
3. Bitrate selection: 640 kbps is exactly `high` → uses 640 kbps
4. Output: EAC3 codec, 8 channels, 640 kbps

### Example 3: 8-Channel Audio on 1080p
1. Detected: 8 channels, 640 kbps bitrate, 1080p resolution
2. Channel clamping: 8 > 6 (max for 1080p) → **downmix to 6ch**
3. Bitrate selection: 640 kbps → clamped to `medium` (448 kbps) for 1080p
4. Output: EAC3 codec, 6 channels, 448 kbps (downmixed)

## Key Takeaways

- ✅ Stereo bitrates remain unchanged across all resolutions
- ✅ Multi-channel at 1080p: Max `medium` (448 kbps), no downmixing below 6ch
- ✅ Multi-channel at 4K: Can use `high` (640 kbps) for 6-8 channels
- ✅ Audio channels automatically clamped: 720p→2ch, 1080p→6ch, 4K→8ch
- ✅ Bitrate selection respects both detected quality and resolution ceiling