Request feature of merging video and audio separately served at YouTube

[m13253]

From the screenshot we can know YouTube now serves video and audio separately.

I think it will be great if youtube-dl can automatically merge them.

I give three ways. Either of them can be implemented into youtube-dl:

ffmpeg -i video.webm -i audio.m4a -c copy output.mkv

mkvmerge -o output.mkv video.webm audio.m4a

MP4Box -add video.mp4 -add audio.m4a output.new.mp4 && mv output.new.mp4 output.mp4

Thank you.

BTW: The worst/best decision is not accurate now, since the 'dash' version is not always the worst and the 'non-dash' version serves only up to 720p.

[AGSPhoenix]

We already have support for this, though it's undocumented until we work out a few problems. Just specify two format codes with a + between them. For example, -f 137+140, which will get you a 1080p MP4 file. You can also combine this with specifying multiple formats by preference; for example -f 137+140/136+140/22 would try to get 1080p DASH, then 720p DASH, and finally normal 720p.

This feature isn't enabled by default, because it requires either ffmpeg or avconv to be available and in the $PATH to mux the video, and we need to be able to automatically detect their availability, and change what format is considered the 'best' if they are available. It's certainly something that could be done, but no one has taken the time to do so.

[jaimeMF]

As @AGSPhoenix said, this is already implemented, but it needs some improvements. If you just want the best quality you may prefer to use -f bestvideo+bestaudio.

[PacoH]

@AGSPhoenix: Why isn't this in the man page?

BTW didn't work. After downloading webm video and m4a audio:

user$ youtube-dl -f 272+141 https://www.youtube.com/watch?v=rkby8QrWMhs
[youtube] Confirming age
[youtube] rkby8QrWMhs: Downloading webpage
[youtube] rkby8QrWMhs: Downloading video info webpage
[youtube] rkby8QrWMhs: Extracting video information
[youtube] rkby8QrWMhs: Downloading DASH manifest
[download] Destination: 4K-Italy-Long-rkby8QrWMhs.f272.webm
[download] 100% of 4.09GiB in 12:43
[download] Destination: 4K-Italy-Long-rkby8QrWMhs.f141.m4a
[download] 100% of 40.12MiB in 00:05
[ffmpeg] Merging formats into "4K-Italy-Long-rkby8QrWMhs.webm"
ERROR: Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument

Yet I can merge the original downloaded files no problem using:

user$ ffmpeg -i 4K-Italy-Long-rkby8QrWMhs.webm -i 4K-Italy-Long-rkby8QrWMhs.m4a -vcodec libvpx -acodec libvorbis 4K-Italy-Long-rkby8QrWMhs-aduio-added.webm

This is a 4k video.

[phihag]

@PacoH It is documented now. We're still waiting for someone to contribute code that automatically detects whether merging is possible so that we could even default to this.

The webm container does not support m4a content, and that's why youtube-dl fails (If you think it should detect this situation and maybe choose a better container, please file a bug saying that you'd be interested.

In your command-line, you are converting the audio to vorbis, whereas youtube-dl tries to do it losslessly. By the way, why are you recoding the video? That will lead to a loss of quality.

[PacoH]

I tried -c:v copy and it failed before. This time I tried -c:v copy again and it worked.

 user$ fmpeg -i 4K-Italy-Long-rkby8QrWMhs.webm -i 4K-Italy-Long-rkby8QrWMhs.m4a -c:v copy -acodec libvorbis 4K-Italy-Long-rkby8QrWMhs-aduio-added.webm

Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #1:0 -> #0:1 (aac (native) -> vorbis (libvorbis))

[m13253]

The webm container does not support m4a content, and that's why youtube-dl fails (If you think it should detect this situation and maybe choose a better container, please file a bug saying that you'd be interested.

My suggestion is that, according to the codecs that YouTube are currently using, most of them can be fitted into an MP4 container or a WebM one. General MKV can be used as the last resort:

Use MP4 for H.264/FLV1/H.263/MPEG4 + AAC/MP3/AMR (coming from .mp4 .flv .3gp(only 144p) .m4a .mp3)

Use WebM for VP9/VP8 + Opus/Vorbis (coming from .webm)

Use MKV for the remaining combinations.=

[PacoH]

OK, it works perfectly if you don't mix formats. I successfully downloaded both wbem-video+webm-audio and mp4-video+m4a-audio formats and they were properly muxed into webm and mp4 containers respectively. Both were 4k video quality but the webm is ~1.5x the size of the mp4 and ~1.5x the bitrate.