What is Muxing & Remuxing?
Muxing (multiplexing) is the process of combining separate video, audio and subtitle streams into a single container file such as MP4 or MKV without changing the streams themselves.
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Definition
Muxing & Remuxing — Muxing (multiplexing) is the process of combining separate video, audio and subtitle streams into a single container file such as MP4 or MKV without changing the streams themselves. Remuxing moves already-encoded streams from one container to another. Both are fast and lossless because nothing is re-encoded, unlike transcoding.
The vocabulary
- Muxing: putting streams together into one file.
- Demuxing: pulling streams out of a file, for example extracting the audio track.
- Remuxing: demuxing from one container and muxing into another, such as MKV to MP4.
In all three, the compressed data is copied bit for bit. Only the packaging changes. The word comes from telecommunications, where multiplexing means sending several signals over one channel and demultiplexing means separating them again.
Muxing vs transcoding
| Muxing / remuxing | Transcoding | |
|---|---|---|
| What changes | The container only | The encoded streams |
| Quality | Identical to the source | Usually a small loss |
| Speed | Seconds, even for long videos | Can take minutes or longer |
| Can change codec? | No | Yes |
Why muxing matters when downloading videos
On many big platforms, high-quality video is delivered as separate video and audio streams, a pattern used in MPEG-DASH. To give you one playable file, a downloader must fetch both and mux them. If this step is skipped, you get a silent video and a separate audio file.
SaveVidr's engine, yt-dlp with ffmpeg, performs this merge on the server. A few related points:
- Muxing only works when the target container supports the codecs. H.264 video with AAC audio fits MP4 perfectly; some combinations fit MKV more easily.
- Saving an AAC audio track as M4A can be a pure remux, so no quality is lost.
- HLS downloads involve joining many small segments, then muxing them into a single file.
Remuxing at home
Free tools such as ffmpeg, MKVToolNix and some media converters can remux files. With ffmpeg, the "copy" option for streams tells it to remux instead of re-encode. If the codecs inside are not supported by the new container, the tool will refuse or you will need to transcode.
When you just want a ready-to-play file, the video to MP4 tool takes care of the merging for you. Please only save media you have the right to keep.
FAQ
Frequently asked questions
Does remuxing lose quality?
No. Remuxing copies the existing streams into a new container without re-encoding, so quality stays exactly the same.
Why do some downloads have video but no sound?
The site delivered audio and video separately and the tool did not mux them together; a downloader that merges streams fixes this.
Glossary