Inconsistent quality behavior?

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Hello, I have a little question about what seems to be inconsistent behavior regarding format/quality selection. Or perhaps my expectations about said options are inaccurate? I hope someone can help clarify if that is the case.

I’ll explain/show my actions and results below.

Video used as example: https://www.youtube.com/watch?v=f25R9Gl6FWc

Download without any parameters(for reference): http://imgur.com/GS4fSku

Media info: http://imgur.com/kT0uW9R

Now for the interesting part, download with parameters taken from the “Format selection examples” section. (https://github.com/rg3/youtube-dl#format-selection-examples)

Download best mp4 format available or any other best if no mp4 available

$ youtube-dl -f ‘bestvideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best’

http://imgur.com/VOrVs9N

Media info: http://imgur.com/LnjUqHl

Now for the final result using -f best http://imgur.com/LVQP4QI

Media info: http://imgur.com/BDuaDNx

Note the significantly larger file size and higher bit rates for both audio and video.

Why does “# Download best mp4 format available or any other best if no mp4 available” not actually return the best quality mp4 available but -f best does? Am I misunderstanding things? Am I doing something wrong? Are my expectations off?

Please explain if you can, thanks in advance.

[yan12125]

It’s a limitation of youtube-dl: sometimes the actual quality is unknown until the file is downloaded. In this example, bestvideo is:

136          mp4        1280x720   720p  278k , avc1.4d401f, 30fps, video only, 48.46MiB

And best is:

22           mp4        1280x720   hd720 , avc1.64001F,  mp4a.40.2@192k (best)

As both formats have the same dimension, the only factor is their bitrates. Unfortunately, YouTube does not provide the bitrate information for format 22 of this video. In most cases, bestvideo is better than best, so we make it the default. This video is a good counterexample, but we can do nothing without sufficient information. If you know how to determine video qualities without actual downloading, please propose it. There are already lots of related feature requests. For example, a not-so-good idea was proposed at #6055. (It downloads some portion of the video, which slows down the extraction step.)

[yan12125]

bigger is usually better

Not always true. Comparing bitrates among different codecs is meaningless. See https://github.com/rg3/youtube-dl/issues/6018#issuecomment-114008839 and https://en.wikipedia.org/wiki/Video_quality. You need some mathematical metrics, such as SSIM, PSNR, etc.