ffmpeg used to extract audio track, video track and then merge them, produces smaller mp4

Question

Here is what I did with "original.mp4" (00:15:22 duration, ~80MB filesize):

1. Extracted video stream from mp4

   ffmpeg -i original.mp4 -c copy -an vid_only.mp4

2. Extracted audio stream from mp4

   ffmpeg -i original.mp4 -vn -acodec copy aud_only.aac

3. Merged the extracted audio and video streams to form new mp4

   ffmpeg -i aud_only.aac -i vid_only.mp4 new_mix.mp4

And found that "new_mix.mp4" was mixed accurately, duration was same as before, but the file-size had dropped to 36MB (from ~80MB). Using ffprobe on both "original.mp4" and "new_mix.mp4", I found that the difference is in bitrate of the audio and video streams.

Step #1 and #2, extracted the audio and video streams without any reencoding. Step #3, supposedly merged audio and video streams, but took quite a long time, so probably performed reencoding. Is that right ? And if so, is there a way to tell ffmpeg to not reencode while merging, to do it faster and retain original bitrate ?

Answer 1

-c copy tells ffmpeg to streamcopy all streams being processed. That's missing.

ffmpeg -i aud_only.aac -i vid_only.mp4 -c copy new_mix.mp4

Also, for step 2, it's better to extract to a container like M4A

ffmpeg -i original.mp4 -vn -acodec copy aud_only.m4a

If your audio has timestamp gaps, .aac will lose them.

Answer 2

Okay, my bad. Looks like the answer was only a search away. Mods may mark the question as duplicate. Here is what I needed to do to achieve the original file-size, and retain the original (higher) bitrates, i.e. merge audio + video streams without reencoding (thus much faster):

`ffmpeg -i aud_only.aac -i vid_only.mp4 -acodec copy -vcodec copy mixed_copy.mp4`

Here's the original Q&A here on this site: How to merge audio and video file in ffmpeg