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I want the loudest peak sound in a movie clip to be as loud as the codec allows, then have every other sound amplified accordingly.

What's a practical example in order to accomplish this using ffmpeg?

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You're looking to have the audio 'normalized.' I found this thread and there's lots of good information in there. Hope it helps! –  bobsbarricades Aug 14 '11 at 20:02

1 Answer 1

up vote 36 down vote accepted

For the lazy ones:

I created a Python script to normalize media files – you simply download ffmpeg, then run the script. For example, to normalize the audio of every MP4 file to 0 dB, run this:

./normalize.py -i *.mp4 -l 0

It'd output WAV files with the normalized audio. You have to merge the audio back into the original files. See this answer on how to do it.

Using ffmpeg to normalize audio

In ffmpeg you can use the volume filter to change the volume of a track. Make sure you download a recent version of the program.

This guide is for peak normalization, meaning that it will make the loudest part in the file sit at 0 dB instead of something lower. There is also RMS-based normalization which tries to make the average loudness the same across multiple files. To do that, do not try to push the maximum volume to 0 dB, but the mean volume to the dB level of choice (e.g. -26 dB).

Find out the gain to apply

First you need to analyze the audio stream for the maximum volume to see if normalizing would even pay off:

ffmpeg -i video.avi -af "volumedetect" -f null /dev/null

Replace /dev/null with NUL on Windows. This will output something like the following:

[Parsed_volumedetect_0 @ 0x7f8ba1c121a0] mean_volume: -16.0 dB
[Parsed_volumedetect_0 @ 0x7f8ba1c121a0] max_volume: -5.0 dB
[Parsed_volumedetect_0 @ 0x7f8ba1c121a0] histogram_0db: 87861

As you can see, our maximum volume is -5.0 dB, so we can apply 5 dB gain. If you get a value of 0 dB, then you don't need to normalize the audio.

Apply the volume filter:

Now we apply the volume filter to an audio file. Note that applying the filter means we will have to re-encode the audio stream. What codec you want for audio depends on the original format, of course. Here are some examples:

  • Plain audio file: Just encode the file with whatever encoder you need:

    ffmpeg -i input.wav -af "volume=5dB" output.mp3

    Your options are very broad, of course.

  • AVI format: Usually there's MP3 audio with video that comes in an AVI container:

    ffmpeg -i video.avi -af "volume=5dB" -c:v copy -c:a libmp3lame -q:a 2 output.avi

    Here we chose quality level 2. Values range from 0–9 and lower means better. Check the MP3 VBR guide for more info on setting the quality. You can also set a fixed bitrate with -b:a 192k, for example.

  • MP4 format: With an MP4 container, you will typically find AAC audio. We can use ffmpeg's build-in AAC encoder.

    ffmpeg -i video.mp4 -af "volume=5dB" -c:v copy -c:a aac -strict experimental -b:a 192k output.mp4

    Here you can also use other AAC encoders. Some of them support VBR, too. See this answer and the AAC encoding guide for some tips.

In the above examples, the video stream will be copied over using -c:v copy. If there are subtitles in your input file, or multiple video streams, use the option -map 0 before the output filename.

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I'm using your answer, thanks. Usually in Step 1 I only output 20min of audio (-t 00:20:00), in Step 2 I run normalize-audio with -n (--no-adjust) parameter (just output info) and look on the gain. If it's less then 1dB I just don't bother with audio level otherwise I run ffmpeg when transcoding with volume=5dB. I reckon 20min is pretty representative. –  Art Shayderov Jul 16 '12 at 12:09
There is also tools/normalize.py in the ffmpeg source, but I've never really looked into it. Might be worth a try. –  LordNeckbeard Nov 26 '14 at 23:14
@LordNeckbeard Thanks. I've never seen that either. It calculates the loudness according to EBU R128, so it might give different results. –  slhck Nov 27 '14 at 6:11
@slhck Maybe I'll attempt to document the tools...or at least add it to my todo list, but I'll probably have no clue about a good number of them. Good way to learn I guess. –  LordNeckbeard Nov 27 '14 at 7:01

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