AV1 ( av01 ), which is a modern, royalty-free codec designed for better compression and quality compared to older formats.
According to technical discussions on Reddit and GitHub , 399 is often preferred because: 399 mp4
(In this example, 399 is the 1080p AV1 video and 140 is the standard M4A audio stream.) Why Choose 399? AV1 ( av01 ), which is a modern,
Supports up to 60fps (High Frame Rate/HFR), offering smoother playback for action-heavy content. AV1 ( av01 )
If you want to manually select this format to ensure you get the AV1 1080p version, you would use a command similar to this in your terminal: yt-dlp -f 399+140 [VIDEO_URL] Use code with caution. Copied to clipboard