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Background

I need to convert AVI files to H264 (MP4). I am successfully doing so with the following:

-i mymovie.avi -c:v libx264 -crf 19 -preset slow -c:a libfaac -pix_fmt yuv420p -f mp4 -b:a 192k -y -an output.mp4

I am calling the command via a C# .Net Winforms applications, although I suspect this will have little impact on the answer.

The Question

How do I add an audio track? I need to add one, as I need to be able to upload the videos to Instagram, which it seems rejects files without an audio track (to my annoyance). In reality I do not need an audio track that is very long. My thoughts were a couple of seconds of "silence" in MP3 format.

I think I will need AAC audio as the output.

For the record I also realise the -an command will provide no audio and I have taken this away in my later attempts.

Attempts so far

I have gathered I will need to use the -shortest command as the audio will be shorter than the video. In reality I will be adding a blank MP3 (or similar) just so while encoding I get an audio stream embedded in the video.

I also have tried using the -map command but without success.

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2 Answers 2

Ok here is the command I have working. I discovered I did not have libfaac in my build of ffmpeg so i switched. I did not need the -map options either as FFMPEG can deal with 2 inputs fine without extra config.

-i test.avi -i music.mp3 -c:v libx264 -crf 19 -preset slow -c:a libvo_aacenc -pix_fmt yuv420p -f mp4 -b:a 192k -y -shortest output.mp4

The differences between my original and what works are:

  • libvo_aacenc replaces libfaac
  • -shortest is added to deal with shorter audio than video
  • -i is used to provide the audio input

Update

The command after some great input in the comments is now

-i test.avi -i music.mp3 -c:v libx264 -crf 19 -preset slow -c:a aac -strict experimental -pix_fmt yuv420p -f mp4 -b:a 192k -y -shortest output.mp4
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Note that of all the AAC encoders you can use with ffmpeg, libvo_aacenc offers the worst quality. Better to use -c:a aac -strict experimental and a bitrate of over 96k. This is the ffmpeg-internal encoder. –  slhck Dec 3 '13 at 11:36
    
How do I add the bitrate or will it take it from the source file? Thanks for improving my answer! Have I done anything else wrong? –  Graham Smith Dec 3 '13 at 11:40
1  
-b:a sets the audio bitrate, so in your example you'd only need to change the encoder part. Some minor things: the -y option should go before the inputs, and -f mp4 is not needed. Apart from that it looks good to me! –  slhck Dec 3 '13 at 11:43
    
Also see the FFmpeg and AAC Encoding Guide. –  LordNeckbeard Dec 3 '13 at 18:47

You can generate a silent audio stream with the aevalsrc audio source:

ffmpeg -y -i test.avi -f lavfi -i aevalsrc=0 -c:v libx264 -crf 19 -preset slow \
-pix_fmt yuv420p -shortest output.mp4

This is easier than using a blank audio file as an input.

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How does the -shortest command work with this setup? –  Graham Smith Dec 3 '13 at 20:31
1  
@GrahamSmith Apparently it does nothing in this case. I assumed your video has a finite duration and that aevalsrc will generate silence indefinitely, so I added -shortest to make sure the output ends at the end of the video, but it appears that the output will end with the video anyway. –  LordNeckbeard Dec 3 '13 at 20:49
    
Awesome input and great follow up. Thought my comment maybe of use to others reading this. Thanks again. –  Graham Smith Dec 3 '13 at 23:32

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