127 lines
3.7 KiB
Markdown
127 lines
3.7 KiB
Markdown
# config.xml - Audio Bucket Configuration Guide
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## Audio Bucket Structure
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The audio buckets in `config.xml` define the bitrate tiers used for audio encoding based on channel count.
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### Current Configuration
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```xml
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<audio>
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<stereo>
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<low>128000</low>
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<medium>160000</medium>
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<high>192000</high>
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</stereo>
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<multi_channel>
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<low>384000</low>
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<medium>448000</medium>
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<high>640000</high>
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</multi_channel>
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</audio>
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```
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## Bitrate Usage by Resolution and Channels
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### Stereo (2 Channels) - All Resolutions
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- **720p**: Up to `high` (192 kbps) if needed
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- **1080p**: Up to `high` (192 kbps) if needed
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- **4K (2160p)**: Up to `high` (192 kbps) if needed
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Stereo bitrates are the same across all resolutions.
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### Multi-Channel (6+ Channels)
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#### 1080p (Max 6 channels)
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- **Minimum**: `low` (384 kbps)
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- **Default**: `medium` (448 kbps)
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- **Maximum**: `medium` (448 kbps) ⚠️ **High tier NOT used**
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Reason: Balanced compression for 1080p file size
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#### 4K / 2160p (Max 8 channels)
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- **Minimum**: `low` (384 kbps)
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- **Default**: `medium` (448 kbps)
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- **Maximum**: `high` (640 kbps) ✅ **High tier NOW available**
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Reason: 4K containers larger; can better accommodate higher quality audio
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#### 720p (Max 2 channels)
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- Uses stereo bitrate tier (2 channels)
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- Never uses multi-channel tier
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## Audio Encoder Selection
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### Stereo (2 channels)
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- **Codec**: AAC
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- **Use Case**: Commentary tracks, all 720p sources
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- **Quality**: ~128-192 kbps depending on source
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### Multi-Channel (6+ channels)
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- **Codec**: EAC3 (Enhanced AC-3)
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- **Use Case**: Main audio tracks with surround sound
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- **Quality Tiers**:
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- Low: Aggressive compression, smaller files
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- Medium: Balanced quality/size (default)
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- High: High fidelity (4K only)
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## Configuration Recommendations
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### For 4K HDR Content
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```xml
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<multi_channel>
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<low>384000</low> <!-- Option for aggressive compression -->
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<medium>448000</medium> <!-- Standard quality -->
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<high>640000</high> <!-- High-quality surround (NEW in 4K mode) -->
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</multi_channel>
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```
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### For Bandwidth-Conscious Setup
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Reduce high tier if needed:
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```xml
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<multi_channel>
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<low>384000</low>
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<medium>384000</medium> <!-- Same as low to reduce file size -->
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<high>448000</high> <!-- Moderate high tier for 4K -->
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</multi_channel>
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```
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### For Maximum Quality
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Increase all tiers:
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```xml
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<multi_channel>
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<low>448000</low>
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<medium>512000</medium>
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<high>768000</high> <!-- Higher quality for 4K -->
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</multi_channel>
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```
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## How the Selection Works
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### Example 1: 6-Channel Audio on 1080p
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1. Detected: 6 channels, 500 kbps bitrate, 1080p resolution
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2. Channel clamping: 6 ≤ 6 (max for 1080p) → stays 6ch
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3. Bitrate selection: 500 kbps, 1080p max is `medium` (448 kbps) → uses 448 kbps
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4. Output: EAC3 codec, 6 channels, 448 kbps
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### Example 2: 8-Channel Audio on 4K
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1. Detected: 8 channels, 640 kbps bitrate, 4K resolution
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2. Channel clamping: 8 ≤ 8 (max for 4K) → stays 8ch
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3. Bitrate selection: 640 kbps is exactly `high` → uses 640 kbps
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4. Output: EAC3 codec, 8 channels, 640 kbps
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### Example 3: 8-Channel Audio on 1080p
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1. Detected: 8 channels, 640 kbps bitrate, 1080p resolution
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2. Channel clamping: 8 > 6 (max for 1080p) → **downmix to 6ch**
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3. Bitrate selection: 640 kbps → clamped to `medium` (448 kbps) for 1080p
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4. Output: EAC3 codec, 6 channels, 448 kbps (downmixed)
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## Key Takeaways
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- ✅ Stereo bitrates remain unchanged across all resolutions
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- ✅ Multi-channel at 1080p: Max `medium` (448 kbps), no downmixing below 6ch
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- ✅ Multi-channel at 4K: Can use `high` (640 kbps) for 6-8 channels
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- ✅ Audio channels automatically clamped: 720p→2ch, 1080p→6ch, 4K→8ch
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- ✅ Bitrate selection respects both detected quality and resolution ceiling
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