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• FFmpeg what is the correct way to manually write silence through pipe:0 ?

  19 juillet 2023, par Bohdan Petrenko

  I have an ffmpeg process running with this parameters :

  


  ffmpeg -y -f s16le -ac {Channels} -ar 48000 -re -use_wallclock_as_timestamps true -i pipe:0 -f segment -segment_time {_segmentSize} -segment_list \"{_segmentListPath}\" -segment_format mp3 -segment_wrap 2 -reset_timestamps 0 -af aresample=async=1 \"{_filePath}\"


  


  I also have a DateTimeOffsetwhich represents the time when the recording was started. When an FFMpeg process is created, I need to add some some amount of silence that equals to the delay between current time and when the recording was started. This delay may be bigger than ffmpeg segments, so I calculate it relatively to the time when last ffmpeg segment should begin.
I store silence in a static byte array with length of two ffmpeg segments :

  


  _silenceBuffer ??= new byte[_segmentSize * 2 * Channels * SampleRate * 2];


  


  I tried two ways of writing silence :

  


  First code I tried is this :

  


  var delay = DateTimeOffset.UtcNow - RecordingStartDateTime;

var time = CalculateRelativeMilliseconds(delay.TotalMilliseconds); // this returns time based on current segment. It works fine.

var amount = (int)(time * 2 * Channels * SampleRate / 1000);

WriterStream.Write(_silenceBuffer, 0, amount);


  


  As the result, I have a very loud noise everywhere in output from ffmpeg. It brokes audio, so this way doesn't work for me.

  


  Second code I tried is this :

  


  var delay = DateTimeOffset.UtcNow - RecordingStartDateTime;

var time = CalculateRelativeMilliseconds(delay.TotalMilliseconds); // this returns time based on current segment. It works fine.

var amount = (int)time * 2 * Channels * SampleRate / 1000;

WriterStream.Write(_silenceBuffer, 0, amount);


  


  Difference between first and second code is that now I cast only time to int type, not the result of the whole expression. But it also doesn't work. This time at the beginning I have no silence I wrote, the recording begins with voice data I piped after writing silence. But if I use this ffmpeg command :

  


  ffmpeg -y -f s16le -ac {Channels} -ar 48000 -i pipe:0 -f segment -segment_time {_segmentSize} -segment_list \"{_segmentListPath}\" -segment_format mp3 -segment_wrap 2 -reset_timestamps 0 \"{_filePath}\"


  


  Then it works as expected. Recording begins with silence what I need, and then goes voice data I piped.

  


  So, how can I manually calculate and write silence to my ffmpeg instance ? Is there some universal way of writing and calculating silence that will work with any ffmpeg command ? I don`t want to use filters and other ffmpeg instances for offsetting piped voice data, because I do it only once per session. I think that I can write silence with byte arrays. I look forward to any suggestions.

  


• Conditional expressions not working in ffmpeg ?

  22 novembre 2019, par Don Clifton

  I’m having trouble with conditional expressions generating errors in ffmpeg. Here is an example of the problem, which is taken from the ffmpeg documentation for the blend filter :

  ffmpeg -i lyt1s.mp4 -i lyt1r.mp4 -filter_complex \
  
      'blend=all_expr=if(eq(mod(X,2),mod(Y,2)),A,B)' lytx.mp

  The following output is generated :

  ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers
  
  built with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
  
  configuration: --prefix=/Volumes/Ramdisk/sw --enable-gpl --enable-pthreads         --enable-version3 --enable-libspeex --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-avfilter --enable-libopencore_amrwb --enable-libopencore_amrnb --enable-filters --enable-libgsm --enable-libvidstab --enable-libx265 --disable-doc --arch=x86_64 --enable-runtime-cpudetect``
  
    libavutil      55. 34.101 / 55. 34.101
  
    libavcodec     57. 64.101 / 57. 64.101
  
    libavformat    57. 56.101 / 57. 56.101
  
    libavdevice    57.  1.100 / 57.  1.100
  
    libavfilter     6. 65.100 /  6. 65.100
  
    libswscale      4.  2.100 /  4.  2.100
  
    libswresample   2.  3.100 /  2.  3.100
  
    libpostproc    54.  1.100 / 54.  1.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'lyt1s.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.56.101
  
    Duration: 00:00:40.04, start: 0.000000, bitrate: 4337 kb/s
  
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 4202 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler
  
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
  
      Metadata:
  
        handler_name    : SoundHandler
  
  Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'lyt1r.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.56.101
  
    Duration: 00:00:40.04, start: 0.000000, bitrate: 2410 kb/s
  
      Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 2275 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler
  
      Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
  
      Metadata:
  
    handler_name    : SoundHandler
  
  [AVFilterGraph @ 0x7fc7f1d0a6a0] No such filter: '2)'
  
  Error initializing complex filters.
  
  Invalid argument

  If I replace the all_expr argument to something that doesn’t include a conditional expression (such as A*0.5+B*0.5) the blend filter does what it’s supposed to, without any errors.

  Can anyone tell me what’s going on here ?

• Conditional expressions not working in ffmpeg ?

  8 avril 2017, par Don Clifton

  I’m having trouble with conditional expressions generating errors in ffmpeg. Here is an example of the problem, which is taken from the ffmpeg documentation for the blend filter :

  ffmpeg -i lyt1s.mp4 -i lyt1r.mp4 -filter_complex \
  
      'blend=all_expr=if(eq(mod(X,2),mod(Y,2)),A,B)' lytx.mp

  The following output is generated :

  ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers
  
  built with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
  
  configuration: --prefix=/Volumes/Ramdisk/sw --enable-gpl --enable-pthreads         --enable-version3 --enable-libspeex --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-avfilter --enable-libopencore_amrwb --enable-libopencore_amrnb --enable-filters --enable-libgsm --enable-libvidstab --enable-libx265 --disable-doc --arch=x86_64 --enable-runtime-cpudetect``
  
    libavutil      55. 34.101 / 55. 34.101
  
    libavcodec     57. 64.101 / 57. 64.101
  
    libavformat    57. 56.101 / 57. 56.101
  
    libavdevice    57.  1.100 / 57.  1.100
  
    libavfilter     6. 65.100 /  6. 65.100
  
    libswscale      4.  2.100 /  4.  2.100
  
    libswresample   2.  3.100 /  2.  3.100
  
    libpostproc    54.  1.100 / 54.  1.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'lyt1s.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.56.101
  
    Duration: 00:00:40.04, start: 0.000000, bitrate: 4337 kb/s
  
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 4202 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler
  
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
  
      Metadata:
  
        handler_name    : SoundHandler
  
  Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'lyt1r.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.56.101
  
    Duration: 00:00:40.04, start: 0.000000, bitrate: 2410 kb/s
  
      Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 2275 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler
  
      Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
  
      Metadata:
  
    handler_name    : SoundHandler
  
  [AVFilterGraph @ 0x7fc7f1d0a6a0] No such filter: '2)'
  
  Error initializing complex filters.
  
  Invalid argument

  If I replace the all_expr argument to something that doesn’t include a conditional expression (such as A*0.5+B*0.5) the blend filter does what it’s supposed to, without any errors.

  Can anyone tell me what’s going on here ?