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• How the ffmpeg astats crest factor of an audio track is calculated

  29 août 2017, par FranGar

  I’m scripting a ffmpeg chain process for my work. The aim is normalizing/compressing lot of audio files (mp3’s).
  It’s done in Python and the critical part is the line :

  ffmpeg -y -i "Input.mp3" -codec:a libmp3lame -b:a 96k -af acompressor=threshold=-15dB:ratio=5:attack=0.01:release=1000:knee=2,dynaudnorm=g=3:m=2:p=0.95 "Output.mp3"

  The python script it’s complete and working BUT the nature of the audios (voice recordings) are very different so I can’t use the same params for all of them.

  I make some experimenting with the values of the ffmpeg filter astats and i discovered that the crest factor (Standard ratio of peak to RMS level ) gave a good reference to programatically get the better params.

  In fact I saw that a recording with a nice dynamic range sound and smooth in shape, get crest values around 9-15 (the compress/normlz params will be somehow conservative). But audios with crest around 22-30 need more aggressive processing.
  (All empirically)

  Somebody can clarify how the crest values are really calculated ? Which are the peaks taken to account ? (Why the flat factor is always 0 ?)
  Or if somebody knows how to get a value representing the sound ’smoothness’ will be nice also.

  Thanks for the ideas.

• Why subprocess.run() have unexpected behavior in try else block ?

  25 novembre 2023, par Nikita Savenkov

  Trying to make "to mp4" converter function using ffmpeg that is going to convert a file to mp4, delete original file and return True or False for specific conditions.
But I get some unexpected behavior of a subprocess.

  


  Initially I used this construction :

  


  def to_mp4_converter(file):

    input_file = file
    output_file = f"{re.sub(r'\..+$', "", file)}.mp4"

    try:
        subprocess.run(['ffmpeg', '-i', input_file, output_file])
    except subprocess.SubprocessError as e:
        print(f"Subprocess Error: {e}")
        return False
    else:
        try:
            os.remove(path=file)
        except OSError as e:
            print(f"Can't remove {file} file: {e}")
        finally:
            return True


  


  Original file is removed, but output file is half of the expected size and quality of video is low.

  


  But if I place subprocess.run() and os.remove() into separate try else blocks like that :

  


  def to_mp4_converter(file):

    input_file = file
    output_file = f"{re.sub(r'\..+$', "", file)}.mp4"

    try:
        subprocess.run(['ffmpeg', '-i', input_file, output_file])
    except subprocess.SubprocessError as e:
        print(f"Subprocess Error: {e}")
        return False
    else:
        pass

    try:
        os.remove(path=file)
    except OSError as e:
        print(f"Can't remove {file} file: {e}")
    finally:
        return True


  


  Everything works fine.

  


  Isn't subprocess.run() should be a blocking operation, so the else statement in 1st example is unreachable until conversion is done ?

  


• avfilter/af_channelmap : Fix double-free of AVFilterChannelLayouts on error

  7 août 2020, par Andreas Rheinhardt

  avfilter/af_channelmap : Fix double-free of AVFilterChannelLayouts on error
  The query_formats function of the channelmap filter tries to allocate
  
  a list of channel layouts which on success are attached to more permanent
  
  objects (an AVFilterLink) for storage afterwards. If attaching succeeds,
  
  the link becomes one of the common owners (in this case, the only owner)
  
  of the list. Yet if the list has been successfully attached to the link
  
  and an error happens lateron, the list was manually freed, which is wrong,
  
  because it is owned by its link so that the link's pointer to the list will
  
  become dangling and there will be a double-free/use-after-free when the link
  
  is later cleaned up automatically.
  This commit fixes this by removing the custom freeing code ; this will
  
  temporarily add a leaking codepath (if attaching the list fails, the list
  
  will leak), but this will be fixed soon by making sure that an
  
  AVFilterChannelLayouts without owner will be automatically freed when
  
  attaching it to an AVFilterLink fails.
  Reviewed-by : Nicolas George <george@nsup.org>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavfilter/af_channelmap.c