Dubbing, Subtitle & Video Sync
This article explains why subtitles, speech, and video may go out of sync after video translation, and how to fix it.
Why Does Sync Drift Occur?
Different languages express the same meaning with different sentence lengths and syllable counts. For example:
| Original | Duration | Translation | Duration |
|---|---|---|---|
| Hello (Chinese) | 0.5 sec | Hello | 0.5 sec |
| I am very happy today (Chinese) | 1.5 sec | I am very happy today | 2.5 sec |
| Germany's capital is Berlin (Chinese) | 2.0 sec | Die Hauptstadt Deutschlands ist Berlin | 3.0 sec |
The dubbed audio duration inevitably differs from the original video duration, causing audio-visual misalignment.
Sync Strategies
The software provides the following strategies to address sync issues:
Strategy 1: Dubbing Speedup+ (Enabled by Default)
When dubbing duration > original video duration, the dubbing is automatically sped up to match the original length.
Best for: Dubbing 1–2x longer than the original video Pros: Fast processing; no impact on video Cons: Excessive speedup makes the voice sound higher-pitched
Strategy 2: Slow Video
When dubbing duration > original video duration, the video playback speed is reduced to match the dubbing duration.
Best for: Dubbing more than 2x longer than the original video Pros: Preserves original dubbing voice quality Cons: Video may appear slightly choppy; file size increases
Strategy 3: Audio + Video Coordination (Recommended)
When dubbing speedup exceeds 1.2x, audio speedup and video slowdown each handle half of the time difference.
Best for: Dubbing significantly longer than the original video Pros: Side effects cancel each other out for the best result
Strategy 4: Condensed Translation
Use an LLM or manual editing to reduce the word count in the target language, shortening the dubbing duration from the source.
Strategy 5: Remove Silence Around Dubbing
Eliminates the silent buffer that TTS engines automatically add at the beginning and end of sentences.
Corresponding Settings in the Software
Main Interface
| Setting | Location | Description |
|---|---|---|
| Dubbing Speedup+ | Main interface Row 5 | Speeds up overly long dubbing |
| Slow Video | Main interface Row 5 | Slows down video to match dubbing |
| Dubbing Rate | Main interface Row 5 | Globally adjusts dubbing speed |
Advanced Options
| Setting | Location | Description |
|---|---|---|
| Max Audio Speedup | Subtitle-Audio-Video Alignment | Default 100 |
| Max Video Slowdown | Subtitle-Audio-Video Alignment | Default 10 |
| Remove Silence Before/After Dubbing | Dubbing Settings | Optional |
Best Practices
- Enable Dubbing Speedup+ by default — handles most scenarios
- When dubbing is much longer than the video, also enable Slow Video
- When translating Chinese to English, set Dubbing Rate to
+10or+15to speed up the overall pace - When using the clone voice, keep subtitle duration between 3–10 seconds
- Use STT again for more precise timecodes after dubbing is complete
Technical Details
The software uses a SpeedRate alignment engine at its core. The key logic:
- Preprocessing: Extends each subtitle's end time to the next subtitle's start time (absorbing silent gaps)
- Calculation: Based on the dubbing/subtitle duration ratio, determines which alignment strategy to use
- Execution: Audio speed change (Rubber Band or FFmpeg atempo) + Video speed change (PTS setpts)
- Stitching: Merges all processed audio segments in order, filling gaps with silence
For detailed technical implementation, see Audio-Visual Sync Technical Explanation