is YouTube’s standard high-quality audio format for premium streaming.
As AV1 hardware penetration exceeds 70% in new devices (2026 estimates), expect “399” to become the new gold standard for adaptive streaming.
The middle identifier, 399 , is likely the internal ID for a specific video stream quality. In the internal dictionaries of platforms like YouTube, these Itag numbers are legendary.
In practice, AV01008M08 targets —aggressive by 2025 standards, aiming to match H.265 quality at nearly half the bitrate. codecs av01008m08 399 opus 251 new
: Specifies the AV1 video codec. It succeeds VP9 and competes directly with the proprietary HEVC (H.265) format.
: Refers to Profile 0 (Main), which supports 8-bit or 10-bit color.
On YouTube, is an internal format code for an AV1-encoded video stream. It is frequently used for high-definition, such as 1080p, often at lower framerates (like 24 or 30 fps). Advantages of AV1/399 In the internal dictionaries of platforms like YouTube,
To provide a clearer comparison, here are the audio streams you might encounter alongside opus (251) :
This pairing is in YouTube’s DASH (Dynamic Adaptive Streaming over HTTP) manifests.
<Codec>av01.0.08M.08</Codec> <!-- Bitrate 399000 bps, audio opus itag 251 --> It succeeds VP9 and competes directly with the
AV1 itself is an advanced, royalty-free codec that can achieve up to than its predecessor, VP9, while maintaining the same visual quality. This makes the av01.0.01M.08 combination — with its moderate level (2.1) — ideal for 720p and 1080p video streaming .
The landscape of online video streaming is constantly shifting, driven by the need for higher resolutions, faster loading times, and lower bandwidth consumption. As of 2026, YouTube has largely solidified its adoption of modern, open-source codecs to meet these demands. When you open "Stats for Nerds" on a high-definition video, you might frequently see the combo: .
Users save on mobile data and bandwidth while enjoying the same or better quality.
: Higher visual quality than the older H.264 (AVC) version. Audio Codec: Opus 251