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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.

  


• Revision 9922e4344a : Enable Hadamard transform based cost estimate for all block sizes This commit t

  3 avril 2015, par Jingning Han

  Changed Paths :
   Modify /vp9/encoder/vp9_pickmode.c

  
  
  Enable Hadamard transform based cost estimate for all block sizes

  This commit turns on the Hadamard transform based rate distortion
  estimate for all block sizes in RTC coding mode. It conditionally
  skips the rate distortion estimation if all zero block flag is set
  on. No significant encoding speed change is observed. The
  compression performance of speed -6 is improved by 1.7% over using
  it only for block sizes of 32x32 and below.

  Change-Id : I768145e6f05c737b05b5b5f1ee674e929532cafb

• Revision e39ecfaa98 : Preliminary code for variance based paritioning Brings back most of Jim's previ

  4 mars 2014, par Deb Mukherjee

  Changed Paths :
   Modify /vp9/encoder/vp9_encodeframe.c

  
   Modify /vp9/encoder/vp9_mcomp.c

  
   Modify /vp9/encoder/vp9_onyx_if.c

  
   Modify /vp9/encoder/vp9_pickmode.c

  
  
  Preliminary code for variance based paritioning

  Brings back most of Jim's previous patch for choosing
  partitioning based on variance while making it compatible
  with the current state of the code. Also adds a
  nonrd_use_partition() function to recursively encode for any
  arbitrary sb_type decisions within a 64x64 block ; and
  includes some refactoring.

  Currently, when the VAR_BASED_PARTITIONING mode is turned on
  for speed 7, there is a 10+% speed-up observed.

  Experiments/improvements with this new partitioning method
  will be conducted subsequently.

  Change-Id : Ie6f43bfbde30583e941f450bf07c3b48828c9571