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What YouTube does with the videos


Uploading videos to YouTube in a given format doesn't mean that's the format that will play for all users.

Most of the time we don't notice but YouTube will re-encode the video we upload into multiple videos using different codecs at different bitrate/resolution combinations.

According to the first article, Which Codecs Does YouTube Use?, the version of your video that you will get is based on the number of views the video has accumulated. As far as I understand it, the list goes like this

The default conversion is to MP4 with a resolution of up to 1080p.

When a video gets 3500 views it will be encoded using the VP9 codec.

A video with several million views (couldn't find the exact count) will be additionally encoded with the AV1 codec.

So, no matter what codec or resolution you use, different users will get different versions of the video based on their device's characteristics (not all devices support all codecs, particularly at the lower end of the spectrum), and available bandwidth.

This also means that, even if you upload a 4k video, it will only be encoded and shown using the MP4 codec at 1080p resolution

So what formats are available for my video? #

The first thing to look at is what format the current browser is playing.

When you right-click on a Youtube video you will get a list of options. The one we're interested in is the one at the bottom of the menu, stuff for nerds

YouTube contextual menu, showing stuff for nerds at the bottom
YouTube contextual menu, showing stuff for nerds at the bottom

When you right-click on a YouTube video currently playing you will get a window similar to the one in image 2. The information includes both video and audio codecs.

Stuff for nerds menu. Codec highlighted
Stuff for nerds menu. Codec highlighted

stuff for nerds will only tell you what audio and video codecs the current browser is playing, in this case using the AV1 video and the Opus audio codecs. However, it won't tell you all the formats that are available for the video.

Youtube-dl is a download manager that allows command line-based interaction with YouTube videos.

We will not get into the legality of the tool, we'll use it to check the streams available for a given video.

The command to run is:

youtube-dl -F <url for the YouTube video>

Using The Living Years by Mike + The Mechanics as an example, the command looks like this

youtube-dl -F

You will get the following results:

[info] Available formats for 5hr64MxYpgk:
format code  extension  resolution note
249          webm       audio only tiny   47k , webm_dash container, opus @ 47k (48000Hz), 1.82MiB
250          webm       audio only tiny   62k , webm_dash container, opus @ 62k (48000Hz), 2.42MiB
251          webm       audio only tiny  125k , webm_dash container, opus @125k (48000Hz), 4.84MiB
140          m4a        audio only tiny  129k , m4a_dash container, mp4a.40.2@129k (44100Hz), 4.99MiB
160          mp4        196x144    144p   36k , mp4_dash container, avc1.4d400b@  36k, 25fps, video only, 1.40MiB
394          mp4        196x144    144p   50k , mp4_dash container, av01.0.00M.  50k, 25fps, video only, 1.95MiB
278          webm       196x144    144p   75k , webm_dash container, vp9@  75k, 25fps, video only, 2.90MiB
133          mp4        328x240    240p   69k , mp4_dash container, avc1.4d400d@  69k, 25fps, video only, 2.69MiB
395          mp4        328x240    240p   72k , mp4_dash container, av01.0.00M.  72k, 25fps, video only, 2.78MiB
242          webm       328x240    240p   85k , webm_dash container, vp9@  85k, 25fps, video only, 3.30MiB
396          mp4        492x360    360p  121k , mp4_dash container, av01.0.01M. 121k, 25fps, video only, 4.68MiB
134          mp4        492x360    360p  144k , mp4_dash container, avc1.4d4015@ 144k, 25fps, video only, 5.57MiB
243          webm       492x360    360p  165k , webm_dash container, vp9@ 165k, 25fps, video only, 6.36MiB
397          mp4        656x480    480p  210k , mp4_dash container, av01.0.04M. 210k, 25fps, video only, 8.09MiB
244          webm       656x480    480p  276k , webm_dash container, vp9@ 276k, 25fps, video only, 10.64MiB
135          mp4        656x480    480p  287k , mp4_dash container, avc1.4d401e@ 287k, 25fps, video only, 11.06MiB
18           mp4        492x360    360p  273k , avc1.42001E, 25fps, mp4a.40.2 (44100Hz) (best)

The video has 21 million views so, as expected, the video has MP4, VP9, and AV1 streams available.

The most exciting thing is that several streams are DASH. It appears that YouTube assembles videos from different audio and video streams at runtime.

So, to fully answer the question in the title of this post: YouTube will re-encode the video based on the number of views and serve a DASH muxed video with the best audio and video streams based on device capability and network conditions

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