title: FFMPEG: How to avoid audio/video desync in output of crossfaded clips when input is variable frame rate video
body: <p>I'm doing screen recordings of gameplay (Dota2) using my NVIDIA graphics card GeForce experience hardware recording (NVEC Encoder). This creates a variable frame rate output video. My NVIDIA settings are 60 fps 15000 kbps. I have paid a guy to make a program that generates scripts that given start/stop timepoints can extract clips from the video and merge them with crossfade. See example code below. The script works for many input recordings but fails often: The audio and video are desynchronized (usually audio delay) in many of the clips, ca 0.5 seconds. I think it fails more when frame rate dropped more during recording. He does not know how to fix the problem, and I wonder if anyone could point out if anything could be fixed in the script (example below)?</p>
  
  <p>Processing speed is quite important (now making a 10 min 'highlight' video takes ca 7-10 min). Solutions increasing that amount very much more is not of too big interest, unfortunately. His approach has been to work separately with audio and video and merge in the end. He already has a program to make ffmpeg code for working with different scenarios (also adding overlays, adding music, intro/outro) so it would be preferable with some easy fixes to his code and not dramatic redesigning of the logic. But if nothing else can fix the problem, a redesign in logic is ok. Using other tools than ffmpeg is also ok, but should be automatable (scripts/cli) and not increase processing times too much.</p>
  
  <p>Running the program "mediainfo" on the input video shows that framerate dropped quite low for this input video: </p>
  
  <p>Frame rate mode: Variable</p>
  
  <p>Frame rate : 60.000 FPS</p>
  
  <p>Minimum frame rate: 3.059 FPS</p>
  
  <p>Maximum frame rate: 63.739 FPS</p>
  
  <p>Full report here: <a href="https://pastebin.com/TX061Wih" rel="nofollow noreferrer">https://pastebin.com/TX061Wih</a></p>
  
  <p>The input video can be downloaded from dropbox here (6 GB): 
  <a href="https://www.dropbox.com/s/ftwdgapazbi62pr/fullgame.mp4?dl=0" rel="nofollow noreferrer">https://www.dropbox.com/s/ftwdgapazbi62pr/fullgame.mp4?dl=0</a></p>
  
  <p>Here the example of a script when asked to extract two clips from input video at 9:57 (41 sec length) and 15:45 (28 sec length) and crossfade merge them with a 0.5 crossfade time. There might be some code-remnants from options that are not used in this example (overlays, music, intro/outro). Using the input video above, this creates audio/video desync. </p>
  
  <p>6 commands excecuted in sequence:</p>
  
  <pre><code>ffmpeg.exe -loglevel warning -ss 00:09:57 -i fullgame.mp4 -t 00:00:41 -filter_complex "[0:a]afade=t=out:st=40.5:d=0.5[a1]" -map "[a1]" -y out_temp_00.mp4.wav
  
  ffmpeg.exe -loglevel warning -i fullgame.mp4 -ss 00:09:57 -t 00:00:41 -an -vcodec copy -f mpegts -avoid_negative_ts make_zero -y out_temp_00.mp4.ts
  
  ffmpeg.exe -loglevel warning -ss 00:15:45 -i fullgame.mp4 -t 00:00:28 -filter_complex "[0:a]afade=t=in:st=0:d=0.5[a1]" -map "[a1]" -y out_temp_01.mp4.wav
  
  ffmpeg.exe -loglevel warning -i fullgame.mp4 -ss 00:15:45 -t 00:00:28 -an -vcodec copy -f mpegts -avoid_negative_ts make_zero -y out_temp_01.mp4.ts
  
  ffmpeg.exe -loglevel warning -i out_temp_00.mp4.wav -i out_temp_01.mp4.wav -y -filter_complex "[0:a]adelay=0|0[a0];[1:a]adelay=40500|40500[a1];[a0][a1]amix=inputs=2:dropout_transition=68.5,atrim=duration=68.5[outa0];[outa0]loudnorm[outa]" -map "[outa]" -ar 48000 -acodec aac -strict -2 fullgame_Output.mp4.aac
  
  ffmpeg.exe -loglevel warning -i out_temp_00.mp4.ts -i out_temp_01.mp4.ts -y -i fullgame_Output.mp4.aac  -filter_complex "[0:v]trim=start=0.5,setpts=PTS-STARTPTS[0c];[1:v]trim=start=0.5,setpts=PTS-STARTPTS[1c];[0:v]trim=40.5:41,setpts=PTS-STARTPTS[fo];[1:v]trim=0:0.5[fi];[fi]format=pix_fmts=yuva420p,fade=t=in:st=0:d=0.5:alpha=1[z];[fo]format=pix_fmts=yuva420p,fade=t=out:st=0:d=0.5:alpha=1[x];[z]fifo[w];[x]fifo[q];[q][w]overlay[r];[0c][r][1c]concat=n=3[outv]" -map "[outv]" -map 2:a -shortest -acodec copy -vcodec libx264 -preset ultrafast -b 15000k -aspect 1920:1080 fullgame_Output.mp4
  </code></pre>
  
  <p>P.S.</p>
  
  <p>I already asked for help at an ffmpeg chat room. One guy said he knew what the problem was, but didnt know how to fix it(?):</p>
  
  <pre><code>[00:10] &lt;kepstin&gt; oh, wait, you're using -vcodec copy
  [00:10] &lt;kepstin&gt; that explains everything.
  [00:10] &lt;kepstin&gt; when you're using -vcodec copy, the start time (set with -ss) is rounded to the nearest keyframe
  [00:10] &lt;kepstin&gt; it's not exact
  [00:11] &lt;kepstin&gt; depending on the keyframe interval, this will result in possibly quite large shifts
  [00:11] &lt;kepstin&gt; (also, your commands are applying audio filters on commands with -an, which is confusing/contradictory)
  [00:12] &lt;birdboy88&gt; so the problem is that the audio temporary clips are not being extracted from the same excat timepoints?
  [00:13] &lt;kepstin&gt; birdboy88: yeah, your audio is being re-encoded to wav so it's being cut sample-accurate, but the video's not being precisely cut.
  [00:16] &lt;birdboy88&gt; kepstin: so I need to use slow seek (?) to extract video accurately? Or somehow extract audio only where there are video keyframes?
  [00:17] &lt;kepstin&gt; birdboy88: i don't know how to extract audio starting at video keyframes with ffmpeg cli. You're already doing slow seek, which doesn't help (you should move the -ss option to before the -i option to speed it up)
  [00:17] &lt;kepstin&gt; if you want accurate video cutting when saving to a file, you have to re-encode the video
  [00:18] &lt;kepstin&gt; (doing this in a single ffmpeg command means you don't have to save to a file, so you can avoid the issue)
  [00:18] * kepstin is off for a bit now
  </code></pre>
  
  <p>EDIT:
  Everything is done with the latest ffmpeg version.</p>
  
  <p>I was unable to get Gyan's code to work.  It always loses some audio (audio is either 40.5 or 27.5, so only one audio is used). This is the only one working for me (changes were adelay=40500|40500 and amix=inputs=2[a0];[a0]loudnorm):</p>
  
  <pre><code>ffmpeg -i fullgame.mp4 -filter_complex "[0]split=2[vpre][vpost];
  [0]asplit=2[apre][apost];
  [vpre]trim=start='00:09:57':duration='00:00:41',setpts=PTS-STARTPTS[vpre-t];
  [apre]atrim=start='00:09:57':duration='00:00:41',asetpts=PTS-STARTPTS,afade=t=out:st=40.5:d=0.5[apre-t];
  [vpost]trim=start='00:15:45':duration='00:00:28',setpts=PTS-STARTPTS,format=yuva420p,fade=t=in:st=0:d=0.5:alpha=1,setpts=PTS+40.5/TB[vpost-t];
  [apost]atrim=start='00:15:45':duration='00:00:28',asetpts=PTS-STARTPTS,afade=t=in:st=0:d=0.5,adelay=40500|40500[apost-t];
  [vpre-t][vpost-t]overlay[v];
  [apre-t][apost-t]amix=inputs=2[a0];[a0]loudnorm[a]" -map "[v]" -map "[a]" -y -c:v libx264 -preset ultrafast -b:v 15000k -aspect 1920:1080 -c:a aac fullgame_Output.mp4
  </code></pre>
  
  <p>Then I tried using a similar setup but with 3 clips, but on one machine I got error: "Error while filtering: Cannot allocate memory". And my 16 GB memory machine the processing speed is 0.02x! Any way to avoid this? This is the code I tried:</p>
  
  <pre><code>ffmpeg -i fullgame.mp4 -filter_complex "[0]split=3[vpre][vpost][v3];
  [0]asplit=3[apre][apost][a3];
  [vpre]trim=start=357:duration=41,setpts=PTS-STARTPTS[vpre-t];
  [apre]atrim=start=357:duration=41,asetpts=PTS-STARTPTS,afade=t=out:st=40.5:d=0.5[apre-t];
  [vpost]trim=start=795:duration=28,setpts=PTS-STARTPTS,format=yuva420p,fade=t=in:st=0:d=0.5:alpha=1,fade=t=out:st=40.5:d=0.5:alpha=1,setpts=PTS+40.5/TB[vpost-t];
  [apost]atrim=start=795:duration=28,asetpts=PTS-STARTPTS,afade=t=in:st=0:d=0.5,afade=t=out:st=27.5:d=0.5,adelay=40500|40500[apost-t];
  [v3]trim=start=95:duration=30,setpts=PTS-STARTPTS,format=yuva420p,fade=t=in:st=0:d=0.5,setpts=PTS+41+28-0.5/TB[v3-t];
  [a3]atrim=start=95:duration=30,asetpts=PTS-STARTPTS,afade=t=in:st=0:d=0.5,adelay=68500|68500[a3-t];
  [vpre-t][vpost-t]overlay[v1];
  [v1][v3-t]overlay[v];
  [apre-t][apost-t][a3-t]amix=inputs=3[a0];
  [a0]loudnorm[a]" -map "[v]" -map "[a]" -y -c:v libx264 -preset ultrafast -b:v 15000k -aspect 1920:1080 -c:a aac fullgame_Output.mp4
  </code></pre>