If you have ever enabled "Stats for Nerds" on a YouTube video, you may have noticed a cryptic string like av01.0.08M.08 / opus (251). This sequence represents the cutting-edge technology YouTube uses to deliver high-quality video and audio while consuming as little data as possible.

  • Opus : open, low-latency audio codec, superior to AAC at low bitrates.
  • 251 : In YouTube’s internal itag system, audio format 251 is Opus at 160 kbps, 48 kHz, stereo.

8) Debugging & interoperability

  • If playback fails:

    So av01008m08 should be read as: av01.0.08M.08

    Modern YouTube Encoding: A Technical Overview of AV1 and Opus