399 Mp4 -

According to technical discussions on Reddit and GitHub , 399 is often preferred because:

It provides better visual quality at a lower bitrate than the older H.264 (AVC) codec.

Supports up to 60fps (High Frame Rate/HFR), offering smoother playback for action-heavy content. 399 mp4

Format is a video-only stream used by YouTube for high-quality content. Key characteristics include: Resolution: 1080p (Full HD).

(In this example, 399 is the 1080p AV1 video and 140 is the standard M4A audio stream.) Why Choose 399? According to technical discussions on Reddit and GitHub

AV1 ( av01 ), which is a modern, royalty-free codec designed for better compression and quality compared to older formats.

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 Key characteristics include: Resolution: 1080p (Full HD)

Usually wrapped in an MP4 container, though it is technically "video-only" and must be merged with an audio stream (like itag 140) to create a playable video file. How to use it in yt-dlp