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• How do I merge few audios with ffmpeg without volume renormalization ?

  20 septembre 2018, par mr_blond

  There is the amix command for mixdown few audio files into one output.

  ffmpeg -i INPUT1 -i INPUT2 -filter_complex amix=inputs=2:duration=first:dropout_transition=0 OUTPUT

  When two files are being merged with amix and dropout_transition = 0 there is the volume renormalization works for both files. ffmpge reduces volume of each file if they sounds in same time. Then when one file is longer then another, it starts to sound louder when shortest file ends sound. So longest file have not constant volume in output mix.

  How do I merge few audios with ffmpeg without volume renormalization with constant volume of each input file in mix ?

• FFmpeg delay and mix audio streams while keeping overall volume constant

  5 octobre 2020, par unstuck

  I have about 100 audio streams, all with the same intro music/sound, and in some of them the intro is delayed by a few seconds. I want to align and mix all the audio streams such that all the intros play at the same time and the output remains pretty much the same volume throughout. I know in advance how much each stream needs to be delayed by.

  


  Like this in Audacity. Each audio stream is aligned to the intro, and the duration before the intro is arbitrary. (This doesn't solve the volume problem though.)

  


  What I have so far uses adelay and amix. It looks something like this but with more audio streams.

  


  ffmpeg -i 00.oga \
       -i 01.oga \
       -i 02.oga \
       -i 03.oga -filter_complex \
"[0]adelay=delays=     123S:all=1[a0]; \
 [1]adelay=delays=    2718S:all=1[a1]; \
 [2]adelay=delays= 6283185S:all=1[a2]; \
 [3]adelay=delays=11235813S:all=1[a3]; \
 [a0][a1][a2][a3]amix=inputs=4" output.oga


  


  In this example the first stream is delayed by 123 samples, the second by 2 718, the third by 6 283 185, and the by fourth 11 235 813.

  


  This works, except at the beginning of the output it's very quiet. When fed n streams, amix makes each stream 1/nth its original volume, which is a good thing in principle. In this case it's not an entirely good thing, because at the beginning of the output 3 of the 4 audio streams are silent (adelay fills delayed streams with silence), meaning the only audible stream is 1/4 = 25% of its original volume. When the second stream becomes audible, the overall volume is 2/4, with three audible streams 3/4, and with all four streams audible it's 4/4 = 100%.

  


  Instead, I want the the first stream to be at 100% volume when it's the only audible one, 50% volume each when there are two audible streams, etc.

  


  Is there a way to make it so when there are n audio streams but m non-silent audio streams, the volume for each of the audio streams is 1/m not 1/n ? amix does this when streams end ; if one stream ends it changes the volume of the others from 1/n to 1/n-1 over a period of time (dropout_transition : https://ffmpeg.org/ffmpeg-filters.html#amix).

  


  I found a similar question where someone wanted to do something like this but only with 2 audio streams. The answer was to split, trim, and change the volume manually. This would be incredibly complicated with 100 audio streams or more, like in my situation.

  


  Is there any easy way to achieve this, even without FFmpeg ?

  


• ffmpeg : Select frames at % position of video

  28 mars 2017, par Matt Joiner

  I’m trying to create a 2x2 tile thumbnail of a video, that contains frames from 20%, 40%, 60%, and 80% through a video. I see that I need to use a video filter, with select, but can’t work out what options will do this. I want to do something like this :

  ffmpeg -i video.avi -frames 1 -vf "select=not(mod(pos\,0.2)),tile=2x2" tile.png

  Where position would be the position in the video from 0.0 to 1.0. How do I do this, or what are my options ?