DTS-HD MA 7.1 to 5.1 AC3 ffmpeg - is downmixing automatic?

[Msuix]

Hi all,

I've been scouring the net for an answer to this. Given a 7.1 DTS-HD MA source and using ffmpeg to convert to AC3 (5.1 is maximum supported by ac3 codec), if I just simply use

ffmpeg -c:a ac3 -b:a 640k

will it downmix the 7.1 to 5.1 properly? I've seen on the net that people generally specify the channels to get 5.1 like:

ffmpeg -c:a ac3 -b:a 640k -ac 6

I ran both on the same source file and the resulting ac3 files actually had different hashes (but same filesize), which makes me think there is a difference. I wonder if i've been sub-optimally converting the DTS audio in the content i've been working with lately (though the resulting audio files sound the same to me).

Anybody know the answer?

[Msuix]

[El Heggunte]

Use eac3to with the option "-down6" instead of ffmpeg.

[pandy]

Not likely but check ffmpeg logs to be sure - most probably core part (5.1) is selected and used directly by ffmpeg - downmixing from 7.1 to 5.1 can be performed explicitly but probably you will need to specify donwmixing matrix manually (and perhaps apply additional processing like delay/phase shift).

[Msuix]

Thank you for the information!