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Glossary · workflow

What is Mux (muxing)?

Muxing is the process of combining separate audio and video streams into a single container file (MP4, MKV, WebM). Demuxing is the reverse — splitting a combined file back into its component streams. Muxing is lossless; the codec data is unchanged, only the wrapper.

Also called:muxer · multiplexing · remux

YouTube serves video and audio as separate streams for any quality above 720p. The video file has no audio; the audio file has no video. To produce the single file you actually want to save, the two need to be combined — that's muxing.

Muxing happens at the byte level. The encoded H.264 frames and AAC audio frames are dropped into an MP4 container without being decoded or re-encoded. This is why a clean mux of a 1080p video takes seconds, not minutes — there is no "processing", just file assembly.

VidPickr does muxing in your browser. The video and audio streams come from YouTube's CDN, get fed into a JavaScript MP4 muxer (mp4-muxer), and the combined file streams to your disk via the File System Access API. No server side mux, no full-file buffer in RAM — bytes flow through.

Common questions

Is muxing the same as transcoding?
No. Muxing is repackaging existing codec data into a container. Transcoding decodes the data and re-encodes it — usually with quality loss. Muxing is fast and lossless; transcoding is slow and lossy.
Why does YouTube split video and audio?
It allows the player to choose video quality independently of audio. Switching from 720p to 1080p mid-playback only requires swapping the video stream — the same audio stream keeps playing. Storing combined files would multiply the storage cost.

Related terms

Container (file format)

A container is the file format that wraps one or more audio and video streams into a single file.

Codec

A codec is the algorithm that encodes (compresses) and decodes raw audio or video into a smaller stream.

Fragmented MP4

Fragmented MP4 (fMP4) is an MP4 variant where the file is split into many short chunks ("fragments"), each containing its own header.

M4A

M4A is an audio-only file format that wraps AAC-encoded audio in an MP4 container.

VidPickr is a free, browser-based YouTube downloader. Every term in this glossary either describes how YouTube delivers video or why your downloads behave the way they do. Try the downloader →