[FFmpeg-user] audio artefacts after segment and transcode

Tue May 7 09:06:21 EEST 2019

Hi everyone

I am transcoding larger videos on a set of computers in parallel. I do this by segmenting an input file at key-frames (ffmpeg -i ... -f segment), then transcode parts using GNU parallel, then recombine parts into one output file using ffmpeg -f concat -i ...). This works well, but I had issues with audio being not in sync with videos or having audio "artefacts". I solved that by transcoding audio separately, but I would prefer the more direct solution to transcode both audio and video in one step.

If segmented video parts are concat together I sometimes experience audio "artefacts" (audio discontinues and then starts again) and I can't find a way to avoid it. I would very much like some insights from your side why this happens or how this artefacts can be avoided. I could not find any solution using aresample filter, async, vsync, forcing constant frame rate.

I prepared a simple working example (all files stored in /tmp; I increased I-Frames in the input file so that audio artefacts are getting more pronounced). I also included ffprobe of the input and output-file after the example, maybe you already see a problem there without executing this example (I might just don't see the trees in the forest anymore...).

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# generated Output file is at http://www.pentachoron.net/output.mov, if you only want to hear/see the result

# step 1: download example file, input.avi

cd /tmp && wget http://www.pentachoron.net/input.avi

# step 2: create segments

ffmpeg -y -hide_banner -i /tmp/input.avi -f segment -segment_time 0.5 -reset_timestamps 1 -segment_list /tmp/input_part.list -segment_list_type ffconcat -r 25 -c:v copy -c:a copy -strict experimental -c:s copy -map v? -map a? -map s? /tmp/input_part_%06d.mp4

# step 3: process each "segment" (parts created in step 2)

for f in `seq -f %06g 1 59`; do ffmpeg -y -hide_banner -i input_part_$f.mp4 -c:v libx265 -map v? -map a? -map s? /tmp/output_part_$f.mp4; done

# step 4: create a ffconcat file for the output file

for f in /tmp/output_part_*.mp4; do echo "file '$f'" >>/tmp/output_part.list; done

# step 5: create output file

ffmpeg -y -hide_banner -safe 0 -f concat -i /tmp/output_part.list -c:v copy -c:a copy -c:s copy -map v? -map a? -map s? /tmp/output.mov

############

ffprobe of the input file:

user at sonne:/tmp$ ffprobe input.avi
ffprobe version 4.1.3 Copyright (c) 2007-2019 the FFmpeg developers
  built with gcc 8 (Debian 8.3.0-4)
  configuration: --disable-decoder=amrnb --disable-decoder=libopenjpeg --disable-libopencv --disable-outdev=sdl2 --disable-podpages --disable-sndio --disable-stripping --enable-libaom --enable-avfilter --enable-avresample --enable-gcrypt --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libfdk-aac --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libkvazaar --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx265 --enable-libxvid --enable-libzvbi --enable-nonfree --enable-opencl --enable-opengl --enable-postproc --enable-pthreads --enable-shared --enable-version3 --enable-libwebp --incdir=/usr/include/x86_64-linux-gnu --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --toolchain=hardened --enable-frei0r --enable-chromaprint --enable-libx264 --enable-libiec61883 --enable-libdc1394 --enable-vaapi --enable-libmfx --disable-altivec --shlibdir=/usr/lib/x86_64-linux-gnu
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, avi, from 'input.avi':
  Metadata:
    IAS1            : Deutsch
    IAS2            : English
    encoder         : Lavf58.20.100
  Duration: 00:00:30.04, start: 0.000000, bitrate: 861 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile) (xvid / 0x64697678), yuv420p, 576x432 [SAR 1:1 DAR 4:3], 719 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
    Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 128 kb/s

ffprobe of the output file:

user at sonne:/tmp$ ffprobe output.mov
ffprobe version 4.1.3 Copyright (c) 2007-2019 the FFmpeg developers
  built with gcc 8 (Debian 8.3.0-4)
  configuration: --disable-decoder=amrnb --disable-decoder=libopenjpeg --disable-libopencv --disable-outdev=sdl2 --disable-podpages --disable-sndio --disable-stripping --enable-libaom --enable-avfilter --enable-avresample --enable-gcrypt --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libfdk-aac --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libkvazaar --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx265 --enable-libxvid --enable-libzvbi --enable-nonfree --enable-opencl --enable-opengl --enable-postproc --enable-pthreads --enable-shared --enable-version3 --enable-libwebp --incdir=/usr/include/x86_64-linux-gnu --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --toolchain=hardened --enable-frei0r --enable-chromaprint --enable-libx264 --enable-libiec61883 --enable-libdc1394 --enable-vaapi --enable-libmfx --disable-altivec --shlibdir=/usr/lib/x86_64-linux-gnu
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'output.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    encoder         : Lavf58.20.100
  Duration: 00:01:01.25, start: 0.000000, bitrate: 342 kb/s
    Stream #0:0(eng): Video: hevc (Main) (hev1 / 0x31766568), yuv420p(tv, progressive), 576x432 [SAR 1:1 DAR 4:3], 172 kb/s, 23.94 fps, 50 tbr, 12800 tbn, 25 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 162 kb/s (default)
    Metadata:
      handler_name    : SoundHandler

if you would execute the steps 1-5 above and watch /tmp/output.mov you'll notice that the sound "clacks" (stops and then starts again). Do you have an explanation or do you know how this audio artefacts can be solved? Can it be that it's just an issue with codec timebases or because libx265 is using a variable frame rate (ffprobe of output.mov has an effective fps of 23.94 while input.avi has a constant frame rate of 25 fps)? I would very much appreciate some help.

Thank you,

Philipp