The string "av01.0.08M.08.399 / opus 251" represents a specific combination of high-efficiency video and audio codecs typically found in YouTube's "Stats for Nerds". It signifies that you are watching a video using the video format paired with high-quality Video Codec: av01.0.08M.08 (399) This technical identifier breaks down the AV1 (AOMedia Video 1) stream parameters: : The four-character code for the , a royalty-free, open-source format developed by the Alliance for Open Media : Refers to (Main), which supports 8-bit or 10-bit YUV 4:2:0. : Indicates Level 3.0 (Main Tier) . On YouTube, this specific level is often used for resolution videos at standard frame rates. : Specifies an 8-bit depth for color and luma data. : This is an internal YouTube itag (Format ID)
: This likely refers to the recent widespread rollout where YouTube has begun forcing these modern, royalty-free codecs across more devices to save bandwidth while maintaining (or slightly improving) visual and audio fidelity. Key Benefits of This Setup Efficiency : AV1 is roughly 30–50% more efficient codecs av01008m08 399 opus 251 new
Here’s a developer-focused guide to interpreting and implementing the string "codecs av01008m08 399 opus 251 new"—likely related to DASH/HLS manifest parsing, codec selection, or stream validation. The string "av01
Some potential areas of research and development include: 251 is the WebM-in-DASH codec identifier for Opus
opus + 251251 is the WebM-in-DASH codec identifier for Opus (see WebM DASH spec).av010m10 (10‑bit) for better gradients and compression efficiency, even for SDR content..mkv (not .mp4 for Opus+AV1) – make sure new doesn’t use experimental AV1 features that break muxing.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