In some encoding pipelines (especially SVT-AV1 or libaom with custom params), the encoder adds a program ID or bitrate hint. 399 almost certainly means 399 kbps – a plausible average bitrate for AV1 1080p video.
Example yt-dlp -F output snippet:
codecs av01008m08 399 opus 251 new is not a valid codec – it’s a . codecs av01008m08 399 opus 251 new
: Despite the seemingly lower bitrate compared to older formats, 160 kbps Opus is often cited by audiophiles on Reddit In some encoding pipelines (especially SVT-AV1 or libaom
than VP9, meaning you get the same video quality while using significantly less data. Superior Audio codecs av01008m08 399 opus 251 new