FFmpeg amix + volume filters create saturated output?

user_6857671 0
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First of all, this is a question asked more out of curiosity than desperate need of fixing. I seem to have "fixed" it by upgrading from FFmpeg 3.4.6 to FFmpeg 4.3.1. Or so I think, at least the result is much better.

I'm trying to mix 2 audio files (of the same length, so no scale variation here) using FFmpeg amix filter:

  • A music which starts pretty loud in the beginning and then is almost quiet until the end,
  • An audio that starts with silence (basically as long as the music is loud), then continues with a person speaking at an "average" volume

Now, I learnt that the filter will divide each input volume by 1/nb_active_inputs (by 2 in my case then), so I added a volume filter on the output to multiply its volume by 2. My command basically looks like this:

ffmpeg -i music.mp3 -i voice.mp3 -filter_complex amix=inputs=2:duration=first[mix];[mix]volume=2[out] -map [out] output.mp3

After that complex filter, my final output is completely distorted. The voice sounds overall very loud and saturated.

I'm not an expert with audio nor with FFmpeg so I assume I'm missing something. I'm trying to understand what is going on and why I am having this result since I use the same inputs, filters and filter options in both cases. Only the FFmpeg version differs.

I've tried to read the code of af_amix.c for both versions but can't seem to find any explanation for that difference. I've seen that the 4.3.1 has a weights option for inputs but from what I understood, they default to 1.0 if not specified. Any clue would be much appreciated.

ffmpeg
2 Answers

You have to set number of input and output method of sound reproduction(stereo,mono) and set volume one by one like this:

ffmpeg -i $firstAudio -i $secondAudio -filter_complex [0:a]volume=$firstAudioVolume[A];[1:a]volume=$secondAudioVolume[B];[A][B]amerge=inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3 $outputAudio

and this sample for using:

ffmpeg -i music.mp3 -i voice.mp3  -filter_complex '[0:a]volume=3.0[A];[1:a]volume=1.0[B];[A][B]amerge=inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3'   output.mp3
user_117475550

You can set the volume of both mp3 files accordingly and generate new mp3 file(output.mp3) like this:

<?php
   $path = dirname(__FILE__);        // Absolute directory path
   $music = $path.'/background.mp3'; // Absolute mp3 input file path
   $voice = $path.'/voice.mp3';      // Absolute mp3 input file path
   $output = $path.'/output.mp3';    // Absolute mp3 output file path

   $voice_volume = '1bB';
   $bg_music_volume = '-10bB'; // -(minus) denotes decrease volume.

   echo shell_exec('ffmpeg -i '.$music.' -i '.$voice.' -filter_complex "[0:0][1:0]concat=n=2:v=0:a=1,volume='.$bg_music_volume.',aformat=fltp,pan=stereo|c0=c0|c1=c1[a0];[1]volume='.$voice_volume.',aformat=fltp,pan=stereo|c0=c0|c1=c1[a1];[a0][a1]amerge,aformat=fltp[a]" -map "[a]" -strict -2 -y '.$output.'');
?>

Default Volume:

  1. $music(background.mp3) value '-10dB'
  2. $voice(voice.mp3) value '1dB'

You can adjust the volume accordingly

user_165605480
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