H264 Audio Sync


Hi. I have a number of trailers that I’ve downloaded from the internet. All of them are MPEG-4 AVC, MPEG AAC Audio, with a frame rate of 23.976215, and all have a noticeable audio lag of around 200ms when played on my Vero 4k (adjust display refresh rate is set to ‘on start/stop’ and sync playback to display is off). If I disable accelerate h264, the lag goes away - audio and video are then perfectly in sync. Pass through is disabled as I’m currently running the vero via hdmi directly into a reasonably old HD tv. These files play without problem via VLC on my laptop and also on a raspberry pi3 running LibreElec. Also worth noting I don’t have any av sync issues with my local media. Wondering if this is a known issue? I can upload logs but not for a few days as I’m away with work. Thanks.


The frame rate shows that either there is something wrong with either the encoding or the container. 24000/1001p is 23.976024, which is enough different from the rate you show that it’s not just a rounding error, so a re-mux with the frame rate forced to correct wouldn’t hurt. The difference isn’t really that much, and it won’t cause an audio delay as large as you say, but it is a sign that at least one thing was done wrong when creating the files.

Note, too, that even the slight difference between frame rates would be enough to cause a noticeable audio offset by the end of a 2-hour movie.


The next update will improve lip-sync issues.


Appreciate the response - very useful information - and will do some research into remuxing with adjusted frame rate. Handbrake is the only software I’m currently aware of that could do the job. I’d be cautious about doing this though, as the rpi3 and vlc on my laptop handle these files without problem, so my feeling is these trailers, although not perfect encodes, aren’t as bad as they appear to be on the vero. Thanks again.


Pleased to hear this! Are these improvements directly related to the type of file I’m having issues with? I only ask as I’ve never had an issue with lip-sync on the vero with my locally ripped media. Thanks.


It should improve any hardware accelerated content


Thanks Sam, and apologies, I’ve just realised there’s a rather long thread that already exists about this subject. I’ll make sure I check the forums properly before starting a new topic in future. Thanks.


Use MKVToolNix GUI:

  1. Drag the file into the “source files” box on the multiplexor tab
  2. Click on the video track in the “tracks, chapters and tags” box
  3. Change the FPS in the “Default duration/FPS” dropdown to 24000/1001p
  4. Click on the “Fix bitstream timing info” checkbox to enable it
  5. Click “Start Multiplexing”

This will fix the frame rate without re-encoding the video.


Ah, awesome thanks. Didn’t think of using mkvtoolnix. Interestingly, all my local Blu-ray and UHD Blu-ray mkvs, created via makemkv, are read by VLC on a windows laptop as 23.976215. This has me confused given your initial comment regarding what the correct frame rate should be, as these are all proper rips directly from disc.


I assumed you were referring to the MediaInfo report as showing that frame rate. I don’t know what VLC is doing to get that completely random number (it’s not rounding, and it’s not any other commonly used frame rate), but it means you can’t trust anything it reports.

Mediainfo will give you the true, correct framerate.


Good to know, thanks. Will check Mediainfo.