bestaudio not so "best" after all?

[CatPlanet]

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I'm not entirely sure if I should classity thiss issue as a bug so I decided to mark it as 'other'.
https://i.imgur.com/fRNJnz4.png
When using -f bestaudio parameter youtube-dl will use 171 format (it's new). However, as it shows on Spek's output, it's just 128k with nice and hard cut after 15kHz, compared to juicier 251 format.

You can clasify this a bug from user's perspective, but it's not a bug when you look at format output: 151k is more than 141k. On the other hand format description says completely different story.

[CatPlanet]

So, as I understand with just closing issue and leaving no comment, this is indended behavior?

[alexvong1995]

@CatPlanet

Firstly, next time please do not post images if you can post text. It is
impossible to copy and paste commands posted in an image.

Secondly, I agree with your assertion: 251 is of higher quality than 171
because the format of 251 is opus @160k, while the format of 171 is
vorbis@128k. Note that 251 has higher rated bitrate (160k) than that of
171 (128k). The fact that 251 has lower actual bitrate is precisely
because opus is meant to be a replacement for vorbis.
It provides better compression with no apparent loss in quality.

As we can see, the youtube-dl way of detecting best formats using the actual
bitrate is only a general heuristic. It does not always yield the best format.

However, I don't think changing the behaviour of youtube-dl is a good idea
because different users have different requirements. There isn't a simple way
to satisfy all users, e.g. I prefer free formats over patent-encumbered
formats (so vp8, vp9 > h264), as long as they have no worse resolution and
rated bitrate.

Therefore, I think we should follow the unix philosophy,
keeping youtube-dl minimal, and instead modularly create a script which takes a
list of formats and print the best format based on user configuration.

This can be done because youtube-dl can dump the list of formats as json.
Actually, this is something I want to do for a long time.