Mute Volume with Minimal Re-encoding - audio

Question

Is it possible to mute a section of a video file (say 5 seconds) without having to re-encode the whole audio stream with ffmpeg? I know it's technically (though probably not easily) possible by reusing the majority of the existing audio stream and only re-encoding the changed section and possibly a short section before and after, but I'm not sure if ffmpeg supports this. If it doesn't, anyone know of any other library that does?

Answer 1

You can do the partial segmented encode, as you suggest, but if the source codec is DCT-based such as AAC/MP3, there will be glitches at the start and end of the re-encoded segment once you stitch it all back together.
You would use the segment muxer and concat demuxer to do this.
ffmpeg -i input -vn -c copy -f segment -segment_time 5 aud_%d.m4a
Re-encode the offending segment, say aud_2.m4a to noaud_2.m4a.
Now create a text file
file aud_0.mp4
file aud_1.mp4
file noaud_2.mp4
file aud_3.mp4
and run
ffmpeg -an -i input -f concat -safe 0 -i list.txt -c copy new.mp4

Answer 2

Download the small sample file.
Here is my plan visualized:
# original video
| video |
| audio |
# cut video into 3 parts. Mute the middle part.
| video | | video | | video |
| audio | | - | | audio |
# concatenate the 3 parts
| video | video | video |
| audio | - | audio |
# mux uncut original video with audio from concatenated video
| video |
| audio | - | audio |
Let's do this.
Store filename:
i="fridayafternext_http.mp4"
To mute the line "What the hell are you doing in my house!?", the silence should start at second 34 with a duration of 2 seconds.
Store all that for your convenience:
mute_starttime=34
mute_duration=2
bash supports simple math so we can automatically calculate the start time where the audio starts again, which is 36 of course:
rest_starttime=$(( $starttime + $duration))
Create all 3 parts. Notice that for the 2nd part we use -an to mute the audio:
ffmpeg -i "$i" -c copy -t $mute_starttime start.mp4 && \
ffmpeg -i "$i" -ss $mute_starttime -c copy -an -t ${mute_duration} muted.mp4 && \
ffmpeg -i "$i" -ss $rest_starttime -c copy rest.mp4
Create concat_videos.txt with the following text:
file 'start.mp4'
file 'muted.mp4'
file 'rest.mp4'
Concat videos with the Concat demuxer:
ffmpeg -f concat -safe 0 -i concat_videos.txt -c copy muted_audio.mp4
Mux original video with new audio
ffmpeg -i "$i" -i "muted_audio.mp4" -map 0:v -map 1:a -c copy "${i}_partly_muted.mp4"
Note:
I've learned from Gyan's answer that you do the last 2 steps in 1 take which is really cool.
ffmpeg -an -i "$i" -f concat -safe 0 -i concat_videos.txt -c copy "${i}_partly_muted.mp4"