how mpv does handle 5.1 downmix to stereo?

[geextahslex]

Hi, I would like to know how mpv does handle 5.1 downmix to stereo? for example ffmpeg leaves LFE out in the default -ac 2 option.

Thanks for any help :)

[Astralyfuse]

I don't really know how mpv do it but if anybody want mpv to include lfe channel in the downmix, they could use audio-swresample-o=lfe_mix_level=1. There are other better ways if you want to control how it's mixed but I just use this because it's simple.

[spinktvis]

You can also use the following by copying it into your mpv config to enable 7.1/5.1 channel downmixing to stereo with the LFE channel enabled. The surround channel values below are according to the Dolby downmix to stereo specification AFAIK, and this is also how ffmpeg functions with -ac 2 (which excludes the LFE channel as you discovered but which is manually added here). Edit: Forgot to mention that this is also how mpv seems to handle it, see https://github.com/mpv-player/mpv/blob/master/audio/chmap.c#L69

In addition there is also a lowpass on the LFE channel to exclude non bass sound on this channel. For more information on pan and lowpass functionality and how to customize it further you can read these sections in the ffmpeg manual here:
https://ffmpeg.org/ffmpeg-filters.html#pan
https://ffmpeg.org/ffmpeg-filters.html#lowpass

af=lavfi="lowpass=c=LFE,pan=stereo|FL < 1.0*FL + 0.7071*FC + 0.5*FLC + 0.7071*BL + 0.7071*SL + 1.0*LFE|FR < 1.0*FR + 0.7071*FC + 0.5*FRC + 0.7071*BR + 0.7071*SR + 1.0*LFE"