This is another example of using FFMPEG to manipulate video and audio files. In this case, one would like to replace the second audio stream in a video file by another audio file.
Here is the command you may want to try:
ffmpeg -i video.mkv -i audio.mp3 -map 0:v -map 0:a:0 -map 1:a -codec copy output.mkv
There is yet another common use of FFMPEG, i.e. adding a new audio track to an existing file which can be a video-only file or a video file already with an audio track. A use case will be adding a second audio track to a file already with video and sound. The following is an example of the command to add audio.dts audio track to an existing mkv file:
ffmpeg -i input.mkv -i audio.dts -map 0 -map 1 -c copy output.mkv
Due to various reasons, the audio and video tracks sometimes may become out-of-sync in a video clip, i.e. there are some delay between the tracks affecting the overall viewing experience. Use the following as examples to fix the issue.
CASE 1: Audio happens before video (aka “need to delay audio stream 1”):
ffmpeg -i clip.mp4 -itsoffset 0.150 -i clip.mp4 -vcodec copy -acodec copy -map 0:0 -map 1:1 output.mp4
The “itsoffset” in the above example is placed before file 1 (remember that linux counts from 0, so 0 is the first and 1 is the second), so when the mapping happens, it says “Take the video of file 0 and the audio of file 1, leave the video of file 0 alone and apply the offset to the audio of file 1 and merge them into a new output file”. The delay is only .15 seconds.
CASE 2: Video happens before audio (aka “need to delay video stream 0”):
ffmpeg -i clip.mp4 -itsoffset 0.150 -i clip.mp4 -vcodec copy -acodec copy -map 0:1 -map 1:0 output.mp4
The “itsoffset” in the above example is placed before file 1. When the mapping happens, it says “Take the audio of file 0 and the video of file 1, leave the audio of file 0 alone and apply the offset to the video of file 1 and merge them into a new output file”. The delay is only .15 seconds.
Very often videos shot by smart phones do not carry a proper orientation indicator as intended due to various reasons. In order to rotate these videos without re-encoding, try the following command.
ffmpeg -i input.mp4 -c copy -metadata:s:v:0 rotate=90 output.mp4
Many video file formats allow you to have multiple sound tracks (e.g. karaoke videos) where the first audio track is treated as the default by many audio-visual editing applications. Many of those can only handle one audio track which is the first one. If you want to switch the audio tracks, you can use FFMPEG without re-encoding the media. The following example is the command to switch the two audio tracks in a video file:
ffmpeg -i input -map 0:v:0 -map 0:a:1 -map 0:a:0 -c copy output
I needed for a reliable, fast and non-destructive way to increase gain on a video’s audio track. I found it in using the ffmpeg tool, which is a swiss army knife for dealing with video and audio files. It’s conveniently cross-platform and works wonders.
To increase the volume of the first audio track for 10dB use:
ffmpeg -i inputfile -vcodec copy -af “volume=10dB” outputfile
To decrease the volume of the first audio track for 3dB use:
ffmpeg -i inputfile -vcodec copy -af “volume=-3dB” outputfile