: This is an internal YouTube ID (itag) representing high-quality Opus audio.
As soon as I removed an album from my downloads, it went to 141 mp4a. I've heard that Opus at 251 is actually pretty good, but it' Reddit·r/YoutubeMusic codecs av01008m08 399 opus 251 new
From 2025 onward, expect to see av01.0.12M.10 opus 251 (10-bit, level 12) as the new high-end standard. The string 399 will disappear, replaced by av1 + average bitrate in separate JSON metadata. : This is an internal YouTube ID (itag)
The term is emerging in technical forums as a placeholder or internal specification for a hybrid A/V sync profile . While not an official codec name (like H.264 or AAC), it appears to describe a constraint set: The string 399 will disappear, replaced by av1
📍 : You need a modern browser or device (Chrome, Firefox, or a 2020+ Smart TV) to hardware-decode AV1 efficiently.
The string refers to the AOMedia Video 1 (AV1) codec, a royalty-free, open-source video format developed by the Alliance for Open Media .