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FFMPEG Audio/video out of sync after cutting and concatonating even after transcoding
4 mai 2020, par Ham789I am attempting to take cuts from a set of videos and concatonate them together with the concat demuxer.



However, the audio is out of sync of the video in the output. The audio seems to drift further out of sync as the video progresses. Interestingly, if I click to seek another time in the video with the progress bar on the player, the audio becomes synced up with the video but then gradually drifts out of sync again. Seeking to a new time in the player seems to reset the audio/video. It is like they are being played back at different rates or something. I get this behaviour in both Quicktime and VLC players.



For each video, I decode it, trim a clip from it and then encode it to 4k resolution at 25 fps with its audio :



ffmpeg -ss 0.5 -t 0.5 -i input_video1.mp4 -r 25 -vf scale=3840:2160 output_video1.mp4



I then take each of these videos and concatonate them together with the concat demuxer :



ffmpeg -f concat -safe 0 -i cut_videos.txt -c copy -y output.mp4



I am taking short cuts of each video (approximately 0.5s)



I am using Python's subprocess to automate the cutting and concatonating of the videos.



I am not sure if this happens because of the trimming or concatenation steps but when I play back the intermediate cut video files (
output_video1.mp4
in the above command), there seems to be some silence before the audio comes in at the start of the video.


When I concatonate the videos, I sometimes get a lot of these warnings however the audio still becomes out of sync even when I do not get them :



[mp4 @ 0000021a252ce080] Non-monotonous DTS in output stream 0:1; previous: 51792, current: 50009; changing to 51793. This may result in incorrect timestamps in the output file.



From this post, it seems to be a problem with cutting the videos and their timestamps. The solution proposed in the post is to decode, cut and then encode the video however I am already doing that.



How can I ensure the audio and video are in sync ? Am I transcoding incorrectly ? This seems to be the only solution I can find online however it does not seem to work.



UPDATE :



I took inspiration from this post and seperated the audio and video from
output_video1.mp4
using :


ffmpeg -i output_video1.mp4 -acodec copy -vn video.mp4



and



ffmpeg -i output_video1.mp4 -vcodec copy -an audio.mp4



I then compared the durations of
video.mp4
andaudio.mp4
and got 0.57s and 0.52s respectively. Since the video is longer, this explains why there is a period of silence in the videos. The post then suggests transcoding is the solution however as you can see from the code above that does not work for me.


Sample Output Log for the Trim Command



built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
 configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.2 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.0.1.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.0.1.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
 libavutil 56. 31.100 / 56. 31.100
 libavcodec 58. 54.100 / 58. 54.100
 libavformat 58. 29.100 / 58. 29.100
 libavdevice 58. 8.100 / 58. 8.100
 libavfilter 7. 57.100 / 7. 57.100
 libavresample 4. 0. 0 / 4. 0. 0
 libswscale 5. 5.100 / 5. 5.100
 libswresample 3. 5.100 / 3. 5.100
 libpostproc 55. 5.100 / 55. 5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input_video1.mp4':
 Metadata:
 major_brand : isom
 minor_version : 512
 compatible_brands: isomiso2avc1mp41
 encoder : Lavf58.29.100
 Duration: 00:00:04.06, start: 0.000000, bitrate: 14266 kb/s
 Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 3840x2160, 14268 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
 Metadata:
 handler_name : Core Media Video
 Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 94 kb/s (default)
 Metadata:
 handler_name : Core Media Audio
File 'output_video1.mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
 Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
 Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x7fcae4001e00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x7fcae4001e00] profile High, level 5.1
[libx264 @ 0x7fcae4001e00] 264 - core 155 r2917 0a84d98 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output_video1.mp4':
 Metadata:
 major_brand : isom
 minor_version : 512
 compatible_brands: isomiso2avc1mp41
 encoder : Lavf58.29.100
 Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 3840x2160, q=-1--1, 25 fps, 12800 tbn, 25 tbc (default)
 Metadata:
 handler_name : Core Media Video
 encoder : Lavc58.54.100 libx264
 Side data:
 cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
 Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)
 Metadata:
 handler_name : Core Media Audio
 encoder : Lavc58.54.100 aac
frame= 14 fps=7.0 q=-1.0 Lsize= 928kB time=00:00:00.51 bitrate=14884.2kbits/s dup=0 drop=1 speed=0.255x 
video:922kB audio:5kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.194501%
[libx264 @ 0x7fcae4001e00] frame I:1 Avg QP:21.06 size:228519
[libx264 @ 0x7fcae4001e00] frame P:4 Avg QP:22.03 size: 85228
[libx264 @ 0x7fcae4001e00] frame B:9 Avg QP:22.88 size: 41537
[libx264 @ 0x7fcae4001e00] consecutive B-frames: 14.3% 0.0% 0.0% 85.7%
[libx264 @ 0x7fcae4001e00] mb I I16..4: 27.6% 64.3% 8.1%
[libx264 @ 0x7fcae4001e00] mb P I16..4: 9.1% 10.7% 0.2% P16..4: 48.5% 7.3% 3.9% 0.0% 0.0% skip:20.2%
[libx264 @ 0x7fcae4001e00] mb B I16..4: 1.1% 1.0% 0.0% B16..8: 44.5% 2.9% 0.2% direct: 8.3% skip:42.0% L0:45.6% L1:53.2% BI: 1.2%
[libx264 @ 0x7fcae4001e00] 8x8 transform intra:58.2% inter:93.4%
[libx264 @ 0x7fcae4001e00] coded y,uvDC,uvAC intra: 31.4% 62.2% 5.2% inter: 11.4% 30.9% 0.0%
[libx264 @ 0x7fcae4001e00] i16 v,h,dc,p: 15% 52% 12% 21%
[libx264 @ 0x7fcae4001e00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 19% 33% 32% 2% 2% 2% 4% 2% 4%
[libx264 @ 0x7fcae4001e00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 20% 39% 9% 3% 4% 4% 12% 3% 4%
[libx264 @ 0x7fcae4001e00] i8c dc,h,v,p: 43% 36% 18% 3%
[libx264 @ 0x7fcae4001e00] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7fcae4001e00] ref P L0: 69.3% 8.0% 14.8% 7.9%
[libx264 @ 0x7fcae4001e00] ref B L0: 88.1% 9.2% 2.6%
[libx264 @ 0x7fcae4001e00] ref B L1: 90.2% 9.8%
[libx264 @ 0x7fcae4001e00] kb/s:13475.29
[aac @ 0x7fcae4012400] Qavg: 125.000```