youtube-dl. Is 'best' better?

[pooksahib]

Hi. I have a query about the size/quality of youtube videos. Take a look at the screenshot and you'll see that the 'best' version comprises video at 508k plus audio at 96k, total size 1.17Gb.
However, the better video would appear to be 134 as it's 634k. Similarly, 140 looks like the better audio – 128k.
The 'best' video totals 1.17Gb whereas 134+140 = 676.73Mb+304.8Mb i.e. less than 1Gb.
Seems odd to me. Can anyone explain why 134+140 isn't bigger than 'best'? Thank you.

[poisondeathray]

I downloaded them; they use different settings for video. The reported audio bitrate matches what mediainfo says

-f 18 video is 410kb/s 965Mib, uses worse specs (1 ref, no CABAC, no bframes)
-f 134 video is 285kb/s 671Mib, uses better specs (3 ref, CABAC, 2 b frames)

1.43x more bitrate, but worse encoding settings.

Which do you think is "better". A quick look and, visually they are both "bad", but some frames are better in one, some better in the other. One isn't clearly better overall when you compare a few frames or watch a few seconds. You'd have spend more to examine it more closely. Usually higher bitrate trumps everything, but at low bitrates ranges, better encoding settings can make a larger proportionate difference to visual quality.

[ponens]

Can anyone explain why 134+140 isn't bigger than 'best'? Thank you.

Probably because of profile differences.

134 is avc1.4d401e, which is main profile
18 is avc1.42001e, which is baseline.

Baseline is typically considered the least efficient (in terms of size) of the H.264 profiles, while main is considered to be more efficient but also more demanding of hardware.

Baseline is often used for the quality on youtube where the video/audio are already together, because this quality is often served to mobile devices, many of which might be old or have limited hardware capability.

[pooksahib]

Thanks, guys, for going to all that trouble. I'll have to look into CABAC, ref and b frames as I don't currently know much about these. From what you say, though, I think I'll continue selecting individual video and audio if they appear to be better than 'best'. But with such old recordings, it probably won't make any difference. Thanks again.

[pedrothelion]

I look at the sound quality of this

-f 251 audio only 156k opus @160k (48000Hz)

isn't it better than this one?

-f 140 audio only m4a, mp4a.40.2@128k (44100Hz)

[pooksahib]

Yes, but I recall having trouble with opus in the past so I just ignored it.

[sneaker]

Note that in youtube-dl nomenclature "best" has a very specific meaning:

You can also use special names to select particular edge case formats:

• best: Select the best quality format represented by a single file with video and audio.
• worst: Select the worst quality format represented by a single file with video and audio.
• bestvideo: Select the best quality video-only format (e.g. DASH video). May not be available.
• worstvideo: Select the worst quality video-only format. May not be available.
• bestaudio: Select the best quality audio only-format. May not be available.
• worstaudio: Select the worst quality audio only-format. May not be available.

So separate streams may be - and usually are - of better quality than "best". For example higher resolutions and fps are only offered as separate streams and "-f best" will actually NOT give you the best quality.

Either way youtube-dl can only see resolution, fps, codec and bitrate. Since codec, codec settings and subjective preferences may differ the selection process is not perfect. If in doubt you need to download multiple versions and compare with your own eyes and ears.

https://github.com/ytdl-org/youtube-dl/blob/master/README.md#format-selection

[pooksahib]

Thanks for that, sneaker. It would be that much easier if youtube simply hosted the uploader's actual file...