ffmpeg, stereo + 5.1 surround, and YouTube's "Recommended upload encoding settings"

Question

I'm trying to make sense of something YouTube has listed on their recommended upload encoding settings for the best possible quality. Here's the link: https://support.google.com/youtube/answer/1722171#zippy=%2Caudio-codec-aac-lc

Specifically this line:

• Channels: Stereo or Stereo + 5.1

I'm Googling for "Stereo + 5.1" and anything to do with ffmpeg but I can't seem to find much of anything.

I have trailers with surround audio I need to export for a client for YouTube.

Answer 1

You need to create a stereo downmix. (5.1 to stereo).

See https://trac.ffmpeg.org/wiki/AudioChannelManipulation#a5.1stereo

Then you combine your

• video
• audio (stereo) and
• audio (5.1)

into one file with a total of 3 tracks.