The number in this context likely denotes a combined encoding efficiency score —a metric where lower is better. Traditional H.264 + AAC-LC scores around 580 for 1080p/60. The AV01008M08 + Opus 251 pairing reportedly achieves 399 on the same perceptual quality index (PQI), representing a 31% improvement in bandwidth-to-quality ratio.
: An internal identifier mapping specific color space matrix coefficients, transfer characteristics, and color primaries used during the encoding pass. 2. Audio Component: opus (251)
: Like 399 , 251 is a YouTube itag , but this time for an audio-only stream. Specifically, itag 251 denotes an Opus audio stream at a target bitrate of 160 kbps , sampled at 48 kHz, within a WebM container. It's widely considered the best quality audio stream provided by YouTube for most videos.
This is the identifier for the AV1 codec. It is designed to be up to 50% more efficient than older standards like H.264. codecs av01008m08 399 opus 251 new
This essay explores the technical synergy and practical implications of combining the AV1 video codec with the Opus audio format in modern digital streaming. The Evolution of Modern Media Compression
If av01008m08 ≈ AV1:
When a YouTube video switches to itag 251, the system delivers an Opus audio stream variable-encoded at approximately . Because Opus is highly efficient, a 160 kbps Opus stream provides perceived audio quality that matches or exceeds a traditional 256 kbps MP3 or AAC stream. It represents the premium audio tier for standard web playback. Why "New" Streams Are Separated (DASH) The number in this context likely denotes a
: Higher visual quality than the older H.264 (AVC) version. Audio Codec: Opus 251
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Youtube premium opus 251kbps vs Youtube music mp4a/aac 141kbps : An internal identifier mapping specific color space
On the audio side, is not a new codec but a specific bitrate and sampling mode within the Opus format (IETF RFC 6716). Opus uses numeric identifiers for its internal configurations:
Check with ffprobe on a sample file: