Why is the output audio out of sync after transcoding?

Audio can end up out of sync after transcoding for a few common reasons:

1. Different timestamps / timebases

   • The source file may have irregular or non-standard timestamps.
   • During transcoding, the encoder/container may rewrite timestamps slightly differently, causing drift.

2. Variable frame rate video

   • If the input video is VFR and the output is forced to CFR (or vice versa), audio/video timing can shift.

3. Audio resampling or codec delay

   • Some audio codecs add encoder delay or padding.
   • If that delay isn’t handled correctly, audio can start too early or too late.

4. Dropped or duplicated frames

   • If video frames are dropped, duplicated, or reordered during conversion, the video duration changes relative to audio.

5. Container mismatch

   • Sometimes the audio stream is fine, but the output container has incorrect metadata or timestamps, making playback appear out of sync.

6. Bad source media

   • The original file may already have sync issues, and transcoding just makes them more obvious.

Typical fixes

• Preserve timestamps carefully during transcoding.
• Use proper audio delay compensation if needed.
• If working with VFR input, try to keep it VFR or convert both streams consistently.
• Remux instead of re-encoding if possible.
• Use tools like ffmpeg options such as:
  • -vsync
  • -af aresample=async=1
  • -fflags +genpts
  • -avoid_negative_ts make_zero

If you want, I can help diagnose it with your exact ffmpeg command and media info.