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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Use FFmpeg with -c copy to put compatible MKV audio and video streams into an MP4 container without re-encoding them. But remuxing only prepares a file: it does not send that file to YouTube Live. To broadcast a video, an encoder or playout service must send YouTube an accepted live feed.
Remux an MKV file to MP4 with FFmpeg
Remuxing changes the container while copying the existing encoded streams. Because FFmpeg does not decode and re-encode those streams, the operation avoids an encoding generation, although the resulting MP4 can contain only streams its muxer supports.
1. Inspect the file and choose the tracks
Run ffprobe input.mkv to inspect stream indexes, types, codecs, and language tags. Decide which video, audio, subtitle, and other tracks you actually need before mapping them into MP4.
2. Try copying all streams
Make a separate output rather than overwriting the source, then run:
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ffmpeg -i input.mkv -map 0 -c copy output.mp4-c copy requests stream copy; -map 0 attempts to include every input stream. FFmpeg documents an MKV-to-MP4 stream-copy example and cautions that copying can fail when the output format does not support a stream type (FFmpeg documentation).
3. If a track is incompatible, map only the tracks you need
An error about an unsupported codec or stream usually means the MP4 muxer cannot accept one of the selected tracks. Try selecting the first video stream and the first audio stream if present:
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ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c copy output.mp4The question mark makes the audio selection optional if the file has no audio stream. Add other tracks only after confirming they can be muxed into MP4. You may omit an incompatible subtitle or other stream, or transcode it if it must be retained; transcoding is re-encoding, not remuxing.
4. Verify the output
Check that output.mp4 exists, plays for the expected duration, and contains the audio and tracks you intended to keep. A stream copy preserves the encoded audio and video data in principle, but track selection, subtitle support, and container-specific metadata can affect what carries over. Not every MKV can be remuxed to MP4 unchanged.
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What remuxing changes—and what it does not
The MKV and MP4 labels describe containers, not the codecs inside them. Stream copy can change the container without changing compatible encoded audio and video; it cannot change a codec that a target player or ingest path does not accept. If the stream itself is incompatible, re-encoding may be necessary.
There is no guaranteed conversion time: copying packets can be quicker than encoding, but elapsed time depends on file and storage characteristics. Do not infer that the file is ready for a particular live workflow just because the remux succeeded.
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Does an MP4 remux make the video a YouTube Live stream?
No. An MP4 is a media file; a YouTube Live stream is a feed sent by an encoder using YouTube’s live-stream setup. Remuxing may be useful preparation if your chosen encoder or playout software needs MP4 input, but it does not itself create or send a live feed. YouTube’s documentation does not say every live streamer must make an MP4 or that remuxing alone is sufficient.
Keep upload-file guidance separate from live-encoder settings. YouTube’s upload recommendations specify MP4 and include upload-oriented guidance such as H.264 video and a front-positioned moov atom (“Fast Start”); those recommendations describe uploads, not live-feed requirements (YouTube recommended upload encoding settings).
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What YouTube Live requires from an encoder
For a live feed, consult YouTube’s current encoder settings for the resolution, frame rate, and codec you plan to use. Its guidance lists RTMP/RTMPS, H.264, H.265 (HEVC), and AV1 video, frame rates up to 60 fps, AAC or MP3 audio, and constant-bitrate encoding. It recommends a two-second keyframe interval and says not to exceed four seconds. Bitrate recommendations vary by resolution, frame rate, and codec; there is no single bitrate that fits every stream (YouTube Live encoder settings).
For example, YouTube’s settings page recommends 10 Mbps for H.264 at 1080p and 30 fps. Treat that as the recommendation for that specific row, not a universal setting; check the current table for your own target format.
Connect and test the live encoder
- In YouTube Live Control Room, get the stream URL and stream key, then enter them in your encoder as directed by its interface. Treat the key like a password: anyone who obtains it may be able to send a feed to your channel.
- Configure the encoder for a protocol, codecs, bitrate mode, resolution, frame rate, and keyframe interval supported by YouTube’s current live settings.
- Start a test feed with audio and movement similar to the planned broadcast. YouTube advises, “Make sure to test before you start your live stream.”
- Watch stream health and messages in Live Control Room, and resolve warnings before relying on the feed.
YouTube’s setup guidance covers connecting an encoder and using Live Control Room (YouTube Help: live streaming with an encoder).
Copyright and channel-policy checks for a prerecorded broadcast
A file being yours to play or technically compatible does not establish that you have the rights to stream its contents. Before broadcasting a game capture, soundtrack, cutscene, or other media, check that you have the necessary rights and that the stream complies with YouTube’s policies. Repeatedly playing prerecorded material also does not by itself establish that a channel meets YouTube’s monetization requirements; review current platform rules rather than treating a successful stream as policy approval. Streamers can use this copyright safety checklist as a practical pre-stream prompt.
Troubleshooting common remux and live-feed problems
- “Could not write header” or an unsupported stream/codec error: inspect the stream list with
ffprobe, then map only compatible tracks. Omit unsupported tracks or transcode only the track that needs a different codec. - The MP4 plays, but a subtitle or audio track is missing: confirm you mapped that stream and whether MP4 supports its type. A successful video remux does not guarantee every MKV track survives.
- The file is MP4, but YouTube does not show a live broadcast: a file alone is not a live feed. Use an encoder or playout system that sends a feed to the Live Control Room URL with the stream key.
- YouTube reports poor stream health: check the encoder’s protocol, codec, constant bitrate, keyframe interval, resolution, and frame rate against the current YouTube live settings; then test and monitor again.
- The stream key is rejected or the wrong feed appears: verify that the key and URL came from the intended Live Control Room setup, and avoid sharing the key publicly.
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