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  1. 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?
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    Join Date
    Jun 2009
    Search Comp PM
    Use eac3to with the option "-down6" instead of ffmpeg.
    "Like this facility, I don't exist."
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  3. 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).
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  4. Thank you for the information!
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