Fixing Audio Sync Issues

Audio sync problems usually fall into two categories: a constant offset (audio is always early/late) or drift (the gap grows over time). This guide shows how to diagnose the cause and fix it reliably.

Identify the type of sync issue

Constant offset

• Audio is always early or late by the same amount.
• The gap does not grow over time.

Drift

• The first seconds are in sync, then the audio slowly slips.
• The longer the video, the worse the gap becomes.

Knowing which type you have is the fastest way to pick the right fix.

Common causes

• Mismatched frame rates (29.97 vs 30, or VFR sources)
• Different sample rates (44.1 kHz vs 48 kHz audio)
• Mixed sources (video exported from editor + audio recorded separately)
• Player limitations (some players handle timestamps poorly)

Fix constant offset

1. Play the output and note whether audio is early or late.
2. Use Audio Offset in small steps:
   • Positive values delay audio.
   • Negative values start audio earlier.
3. Re‑merge and test again.

Start with ±0.1s to ±0.5s, then fine‑tune.

Fix drift over time

Drift is usually caused by frame‑rate or timestamp differences.

Recommended steps:

1. Re‑encode video to a constant frame rate (CFR), like 30.
2. Keep audio in Copy mode if possible.
3. Re‑merge and test a 2–3 minute segment before the full file.

If you have only one source, exporting again from your editor with CFR often fixes drift.

Audio sample rate mismatch

If your audio is 44.1 kHz and the video expects 48 kHz, drift can appear.

Fix:

• Re‑encode audio to 48 kHz AAC before merging, or
• Use the audio codec re‑encode option if available.

When to re‑encode

Use re‑encoding when:

• The player cannot read timestamps correctly.
• The source uses variable frame rate.
• Audio sample rate does not match the video workflow.

Recommended:

• Video: H.264
• Audio: AAC (48 kHz)

Using -shortest

Enable -shortest when your audio or subtitles are shorter than the video. It prevents extra silence or blank space at the end, but it does not fix drift.

Testing checklist

• Test on two players (browser + desktop player)
• Compare the start, midpoint, and end of the file
• Use a short clip to validate settings before full merges

Summary

• Constant offset → adjust Audio Offset.
• Drift → re‑encode to constant frame rate and consistent sample rate.
• Always test with a short segment first.