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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.

• Anomalie #4402 (Fermé) : Liste de notices

  12 février 2021, par cedric -

  je ferme ce ticket, car un ticket qui référence un wiki qui liste des patches, ça commence à ressembler à un jeu de pistes
  + depuis novembre 2019 ces notices ont été corrigée
  + maintenant proposer une PR pour chaque cas quand on tombe dessus ça sera beaucoup plus efficace

• The proper way to limit framerate when recording screen on macOS

  5 juillet 2024, par jsx

  macOS 14.5, FFmpeg 7.0.1. To record the screen, if I use the code from Wiki, that is,

  


  ffmpeg -f avfoundation -i 1 output.mp4


  


  the frame rate seems to be so high that when I press q to stop recording, the following message persists until my MacBook Pro starts to noise by its cooler : "[q] command received. Exiting."

  


  My current solution is to add -r after -i :

  


  ffmpeg -f avfoundation -i 1 -r 24 output.mp4


  


  But I heard that a more correct solution is to replace -r with -framerate and to put it before instead of after -i :

  


  


  ffmpeg -f avfoundation -framerate 24 -i 1 output.mp4


  


  Note that I used -framerate as an input option instead of -r 24 as an output option, so I'm telling avfoundation to record at 24fps instead of recording at the default fps and then forcing FFmpeg to drop or duplicate frames to give you the desired 24.

  


  


  This doesn't work for me currently, and appears to be the same as I don't use -r at all.

  


  And so, what is the proper way to fix the "too high framerate" issue when recording the screen on macOS ? Do we need -r or instead -framerate, and where to put it, before or after -i ?

  


  Just in case, here is the log of ffmpeg -f avfoundation -i 1 output.mp4 -v verbose :

  


  https://github.com/jsx97/test/blob/main/ffmpeg.log