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• How to re-encode an audio to match another one, to avoid re-encoding the whole audio

  21 mars 2024, par Bernard Wiesner

  I have an audio editor in the browser using ffmpeg (WebAssembly), and I want to insert new audio into the existing audio without having to re-encode everything. Re-encoding everything takes a long time, especially in the browser, so I would like to only re-encode the inserted file, match it to the original one and concatenate them using the copy command.

  


  On ffmpeg concatenate docs it says :

  


  


  All files must have the same streams (same codecs, same time base, etc.)

  


  


  But it is not clear what is meant by time base. So far I have observed I need to match :

  


  

  • codec

  


  • bit rate

  


  • sample rate

  


  • channels (mono, stereo)

  


  


  Is there anything else I need to match so that the resulting audio is not corrupt/broken when concatenating ?

  


  I have observed with mp3 for example it has VBR, CBR, and ABR. If the original audio has a bit rate of 128 kb/s, I am assuming it is a CBR, so I match it with :

  


  ffmpeg -i original.mp3
# > Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 128 kb/s

ffmpeg -i input.mp3 -b:a 128k -ar 44100 -ac 2 re_encoded.mp3

# then merge
# concat_list.txt contains the original audio and the re_encoded.mp3

ffmpeg -f concat -i concat_list.txt -safe 0 -c copy merged.mp3


  


  And that works fine for CBR such as 8, 16, 24, 32, 40, 48, 64, 80, 96, 112, 128, 160, 192, 224, 256, or 320 (docs), as far as I have tested.

  


  The issue is when the original.mp3 has a VBR (variable bit rate) or ABR, such as 150 kb/s.

  


  If I try to match it like below :

  


  ffmpeg -i input.mp3 -b:a 150k -ar 44100 -ac 2 re_encoded.mp3
ffmpeg -i re_encoded.mp3
# Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 160 kb/s


  


  The resulting bitrate is rounded to the nearest CBR which is 160.

  


  I can solve this with mp3 by using -abr 1 :

  


  ffmpeg -i input.mp3 -abr 1 -b:a 150k -ar 44100 -ac 2 re_encoded.mp3
ffmpeg -i re_encoded.mp3
# Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 150 kb/s


  


  Now the bitrate matches the original audio, however I am not sure this is correct since I am modifying the new audio to an ABR and concatenating it with a VBR ? I am not even sure how to check with ffmpeg if the audio is VBR, CBR or ABR, or if that even matters when concatenating.

  


  Another issue also happens with aac files. When I try to match the original audio bitrate I can't.

  


  ffmpeg -i input.mp3 -b:a 128k -ar 44100 -ac 2 re_encoded.aac
ffmpeg -i re_encoded.aac
# Stream #0:0: Audio: aac (LC), 44100 Hz, stereo, fltp, 135 kb/s


  


  The resulting bitrate always seems to be variable (135 in this case), and hence I can't match it to the original one.

  


  So my question is, what conditions need to be met when concatenating audios with different streams, and how can I achieve re-encoding only one audio to match the other one. Or if there is some package that can do this, it would be of great help.

  


• Create MPEG-DASH Initialization segment

  5 janvier 2016, par Mahout

  I am looking to convert between HLS and MPEG Dash. I do not access to the original fully concatenated video file, only the individual HLS segments.

  In doing this transformation to MPEG Dash I need to supply an initialziation segment for the Dash manifest .mpd file.

  My questions are :

  1. What is the structure of a Dash video initialization segment ?
  2. How can I generate/create one without the need for the original full file ?

  Perhaps a solution would involve getting MP4Box to convert the ’.ts’ HLS segments to Dash ’.m4s’ segments which are self initializing, but I am unsure how to go about this this ?

  Any ideas are much appreciated.

  Many thanks.

  UPDATE :
  Snippet to stream using original hls segments. Video plays all the way through but is just black.

    <representation width="426" height="238" framerate="25" bandwidth="400000">
  
      <segmentlist timescale="25000" duration="112500">
  
             <segmenturl media="video_0_400000/hls/segment_0.ts"></segmenturl>
  
             <segmenturl media="video_0_400000/hls/segment_1.ts"></segmenturl>
  
            <segmenturl media="video_0_400000/hls/segment_2.ts"></segmenturl>
  
      </segmentlist>
  
     </representation>

• Windows Batch - Change the beginning of a path but keep the rest

  21 juin 2014, par o_ren

  I’m running FFMPEG for video encoding.
  I have a batch file which will encode files you drop on it, keeping the file’s original name and copy the encoded files to a specific directory.
  I would like that script to "know" the original’s file path and copy the encoded file to a path relative to it, meaning :
  original file dropped on batch file is in C :\some\folder\show\season\episode\file.mov
  encoded file should be encoded to D :\different\directory\show\season\episode\file.mp4
  The part of the path up until \show is fixed.

  This is what I have so far :
  

  @echo on
  
  
  
  set count=0
  
  for %%a in (%*) do (<br />
  
    if exist %%a (
  
  
  
          md \\specific\path\"%%~na"
  
  
  
          %MYFILES%\ffmpeg.exe -i "%%~a" -vcodec libx264 -preset medium -vprofile baseline -level 3.0 -b 500k -vf scale=640:-1 -acodec aac -strict -2 -ac 2 -ab 64k -ar 48000 "%%~na_500.mp4"
  
          copy "%%~na_500.mp4" "\\specific\path\"%%~na"\%%~na_500.mp4"
  
          copy "%%~na_500.mp4" "\\specific\path\"%%~na"\%%~na_500.mp4"
  
          del "%%~na_500.mp4"

  set /a count+=1

  ) else (
  echo Skipping non-existent %% a
  
  

  Thank you,
  Oren