Originally posted by jacob
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No, they pair it with Opus because, in their words, "it's just so good." - heck, when VP9 encodes are not (yet) available, you'll even sometimes see YouTube's h.264 encodes be paired with Opus audio!
(I do believe I'm paraphrasing that quote though - it was a presentation from years ago by a Google AV1 dev responding to the question of an AV1-like project for audio, at which they replied to just use Opus as it's already really good)
Also I realize that the majority of YouTube's formats use separate audio and video streams, therefore they can be mixed-and-matched with no regard for what video and audio formats are more normally paired together. Nevertheless, that doesn't change that it does in fact default to Opus which definitely requires more CPU grunt to decode than AAC (you can verify this yourself by digging out low-end x86 CPU hardware, possibly most easily with desktop AMD hardware made before Ryzen due to stagnant single-thread IPC and widely-supported multiplier underclocking, and then set the CPU to a 1GHz clockrate or the like and play Opus audio then AAC audio through Audacious or the like), not to mention that they'd been defaulting to VP9 for years now even before VP9 hardware decoding was a thing in shipping hardware (causing people to write extensions that basically force h.264 instead specifically to take advantage of hardware decoding).
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