Convert specific video and audio track only with ffmpeg

ffmpegvideo conversion

I have an MKV file (vidui.mkv) with 6 tracks in it.

Track 1 - video - xvid - 1920x1080
Track 2 - video - xvid - 720x576
Track 3 - audio -  AAC - 1240kbps - English
Track 4 - audio -  AAC - 648kbps - Spanish
Track 5 - audio -  AAC - 648kbps - Commentary 1
Track 6 - audio -  AAC - 648kbps - Commentary 2

I want to convert the above file to a mp4 format with one h264 video and one AC3 audio. Also I want to convert track 1 (video) and track 5(audio).

If I use

ffmpeg.exe -i vidui.mkv -f mp4 -vcodec libx264 -acodec ac3 -crf 20 -sn -n vidui.mp4

it converts the first video track and first audio track, but what I would like it to do is convert track 1 and track 5.

Best Answer

You can use the -map option (full documentation) to select specific input streams and map them to your output.

The most simple map syntax you can use is -map i:s, where i is the input file ID and s is the stream ID, both starting with 0. In your case, that means we select track 0 and 4:

ffmpeg -i vidui.mkv -c:v libx264 -c:a ac3 -crf 20 -map 0:0 -map 0:4 vidui.mp4

If you want to choose video, audio or subtitle tracks specifically, you can also use stream specifiers:

ffmpeg -i vidui.mkv -c:v libx264 -c:a ac3 -crf 20 -map 0:v:0 -map 0:a:1 vidui.mp4

Here, 0:v:0 is the first video stream and 0:a:1 is the second audio stream.

Setting video quality

For controlling video quality, set the crf parameter, which defaults to 23. Lower means better quality, but higher file size. Try values between 19 and 26 to see what fits best. You can also set a certain bit rate, depending on which file size you want. Here, for example, 500 kBit/s:

ffmpeg -i input.mkv -c:v libx264 -crf 23 …
ffmpeg -i input.mkv -c:v libx264 -b:v 500k …

For audio, you can set the bit rate too, with -b:a.

Multiple channel audio

If your audio stream is using multiple channels (e.g. 5.1 sound), you need to use another AAC encoder (libfdk_aac). This encoder is not available in the static builds, but can be obtained with the pre-packaged / Homebrew versions of ffmpeg.

ffmpeg -i input.mkv -c:v libx264 -crf 23 -c:a libfdk_aac -b:a 384k out.mp4

Copying all streams

In case your input file has more than one video, audio and subtitle stream, ffmpeg by default does not convert all of them.

Use -map 0 to instruct ffmpeg to take all streams from the input file (see the FFmpeg Wiki for more info). This is useful for retaining different languages and subtitles that might be in the original.

ffmpeg -i input.mkv -c:v libx264 -c:a aac -map 0 out.mp4

FFmpeg Audio – Convert and Keep Audio Track with FFmpeg

You can use the -map and -codec options with stream specifiers to choose what streams you want and if you would like to stream copy or encode the streams:

ffmpeg -i input.mkv \
-map 0 -c copy \
-c:s mov_text \
-map 0:a:0 -c:a:0 libfdk_aac -b:a:0 128k \
-map 0:a:1 -c:a:1 libfdk_aac -b:a:1 128k \
output.mp4

The mapping in this example goes from general to specific. At first all inputs are stream copied, but the later options refine the behavior so the subtitles are converted instead of copied and more maps are added to include the audio to be re-encoded.

Best Answer

Related Solutions

Convert MKV with DTS sound to MP4 video with AAC or AC3 audio

Setting video quality

Multiple channel audio

Copying all streams

FFmpeg Audio – Convert and Keep Audio Track with FFmpeg

Related Question