This string contains a typo. The official standard is av01.0.08M.08 , which the YouTube ecosystem and tools like yt-dlp use to label an .
: Refers to AV1 (AOMedia Video 1) , the state-of-the-art video compression format developed by the Alliance for Open Media.
: Represents the decoder capability "Level" and "Tier." The 08 refers to Level 4.0, which easily handles resolutions up to 1080p at 60 frames per second or standard 2K video. The M designates the Main Tier (as opposed to High Tier used for extreme high-bitrate applications).
: YouTube separates the high-quality AV1 video tracks from the Opus audio tracks on its servers.
This string explicitly tells the browser's media player engine how the video bitstream was constructed so the device can safely decode it. Low framerate for some YouTube videos · Issue #79 - GitHub codecs av01008m08 399 opus 251 new
The "251 New" part of the designation suggests that this codec is a newer iteration, possibly with enhanced features or improvements over its predecessors. With a bit rate of 251 kbps, this codec is suitable for applications where high-quality audio is required, such as music streaming, voice over IP (VoIP), and video conferencing.
This specific combination of terms refers to the high-efficiency playback formats currently used by YouTube, which you can often see when enabling "Stats for Nerds" on a video. Decoding the Terms av01.0.08M.08 : This represents the AV1 (AOMedia Video 1) video codec. : The specific identifier for AV1. : Profile 0 (Main profile).
If you have a file that requires this codec string (e.g., for remuxing, streaming, or media info validation), correct it as follows:
A power user’s dream , but a normie’s nightmare . If you’re building a personal media archive or streaming to AV1‑capable devices, this config is top‑tier. For sharing or compatibility, transcode to H.265 + AAC instead. This string contains a typo
Decoding YouTube's Future: A Deep Dive into av01.0.08M.08 (399) and Opus (251)
"The file uses AV1 (profile 0, level 8, main tier, 8-bit) at 399 kbps video, and Opus audio track 251. It’s newly encoded."
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The string refers to a high-efficiency combination of video and audio streams commonly found in YouTube's "Stats for Nerds" . This specific pairing indicates the use of the AV1 video codec alongside the Opus audio codec, both of which are modern standards designed to provide high quality at lower bitrates . Video Stream: av01.0.08M.08 (399) : Represents the decoder capability "Level" and "Tier
Modern streaming platforms rarely send a video as a single, combined file containing both video and audio. Instead, they use a technology called .
Video Codec: AV01.0.08M.08 (AV1) This string refers to a specific profile of the codec, often seen in YouTube's internal metadata. AV01 : Identifies the AV1 codec. 0 : Base profile. 08M : Level 4.1 (Main tier). 08 : 8-bit color depth. Performance : Offers 30-40% better compression than VP9.
as superior to 256 kbps AAC (format 141) due to its superior compression algorithms. Audio Characteristics : Detailed waveform analysis shows that format 251 typically removes frequencies above
: The internal YouTube Format ID (itag) assigned to a specific AV1 video stream (typically high-definition 1080p or higher).
Because AV1 ( av01 ) is a newer, highly complex compression algorithm, it requires more computational power to decode than older formats.