Ffmpeg to duplicate an audio stream and encode this new stream

15
votes

I have some video files that I need to re-encode due to compatibility issues. They are currently mkv files with h.264 video and ac3-a52 audio. I want to keep the h.264 video, convert the container to m4v and create two audio tracks, one with the original ac3-a52 and one copied from that but in aac stereo.

I assume there has to be some sort of audio stream mapping command but I don't know how to map and re-encode at the same time. What command should I enter into ffmpeg to achieve this?

Also, what is the difference between ac3 and ac3-a52? Will an apple TV still be able to pass through ac3-a52 or does that have to be converted to ac3?

2
audioffmpegmapping

2 Answers

19
votes

this works for me:

ffmpeg -y -i Source.mkv -map 0:v -c:v copy -map 0:a -c:a copy -map 0:a -strict -2 -c:a aac out.mkv
  • -y – A global option to overwrite the output file if it already exists.
  • -map 0:v – Designate the video stream(s) from the first input as a source for the output file.
  • -c:v copy – Stream copy the video. This just muxes the input to the output. No re-encoding occurs.
  • -map 0:a – Designate the audio stream(s) from the first input as a source for the output file.
  • -c:a copy – Stream copy the audio. This just muxes the input to the output. No re-encoding occurs.
  • -strict -2 -c:a aac – Use the native FFmpeg AAC audio encoder. -strict -2 is required as a way that you acknowledge that the encoder is designated as experimental. It is not a great encoder, but it is not too bad at higher bitrates.

According to wikipedia, there is no difference between AC3 and ATSC A/52: the 1st one is the name of the codec, the 2nd is the name of the standard specifying the AC3 codec. Maybe someone have more knowledge about it?

6
votes

I'm doing the same as the OP, but with an m4v container. I'm using the MacPorts "nonfree" variant of ffmpeg so that I can use libfaac, which gives better audio quality than the built-in AAC encoder and also had the same issue as @dkam. The command line I ended using is like this:

ffmpeg -i input.m4v -map 0:v -c:v copy -map 0:a -c:a:0 copy -map 0:a -c:a:1 libfaac output.m4v

(The videos are for playback on an iPad, which doesn't seem to be able to handle ac3.)