Converting a screen recording to MP4? Check the codecs before re-encoding

An MKV file and an MP4 file can contain the same H.264 video and AAC audio. If yours does, you can change the container without encoding the pixels again. For a screen recording with small text, that saves time and avoids a second generation of compression.

1. Check the streams

ffprobe -v error -show_entries stream=index,codec_type,codec_name \
  -of default=noprint_wrappers=1 recording.mkv

Look for the first video stream and, if the recording has sound, the first audio stream. For a broadly compatible MP4, H.264 video and AAC audio are a practical starting point. The extension alone doesn't tell you which codecs the file contains.

2. Remux H.264 and AAC

ffmpeg -i recording.mkv -map 0:v:0 -map '0:a:0?' \
  -c copy -movflags +faststart recording.mp4

-c copy carries the encoded streams over as-is. The question mark makes audio optional, so a silent clip works too. The command keeps the first video and at most the first audio track. If you need a second track or subtitles, inspect and map them explicitly. +faststart moves MP4 metadata to the front, which lets compatible web players start sooner when the file is served over HTTP.

3. Re-encode when compatibility requires it

If the capture is VP9 in WebM and the receiving editor specifically needs H.264, convert the video stream:

ffmpeg -i recording.webm -map 0:v:0 -map '0:a:0?' \
  -c:v libx264 -crf 20 -preset medium -pix_fmt yuv420p \
  -c:a aac -b:a 128k -movflags +faststart recording.mp4

CRF 20 is a starting point, and the resulting size depends on the recording. Preview small text and cursor motion in the receiving app. If only the audio codec needs changing, you can keep the video with -c:v copy and encode just the audio to AAC.

The commands were checked on two synthetic clips. The H.264/AAC MKV kept both codecs after remuxing. The silent VP9 WebM produced an H.264 MP4 with no audio track. Run ffprobe on your own output and test playback before discarding the original.

If a teammate prefers a browser UI for a shareable MOV or MKV, VideoBee's online video converter offers MP4 output. It processes uploads in the cloud, so use the local FFmpeg commands for recordings that should stay on your machine.

Reference: FFmpeg stream copy and stream selection.

Disclosure: This article was drafted by an AI agent for a VideoBee submission. It includes a link to VideoBee, and the command examples were verified on generated test clips.