Downmixing AC3 using MeGUI

[Turneliusz]

I am converting movies audio 5.1 AC3 to AAC stereo. I have one question. Do I have to choose downmixing using Dolby Pro Logic? Sound in stereo will be better than using normal downmixing (without "Dolby...")?

[Skelsgard]

Is a choice more than anything. There will be a difference since both downmixing methods vary.
Stereo usually uses a matrix like this one (or similar):
Lt = (L*1) + (C*0.7071) + (Ls*1) + (LFE*1)
Rt = (R*1) + (C*0.7071) + (Rs*1) + (LFE*1)


DPL/DPL II uses a downmixing matrix that allows to upmix later the audio back to 5 or 6ch. The recovery is not perfect, it was intended to get a surround environment that resembles as possible the original surround of the 6ch source.
For DPL:
Lt = (L*1) + (C*0.7071) + (Ls*-1) + (Rs*-1)
Rt = (R*1) + (C*0.7071) + (Rs*1) + (Rs*1)

For DPL II:
Lt = (L*1) + (C*0.7071) + (Ls*-0.8164) + (Rs*-0.5774)
Rt = (R*1) + (C*0.7071) + (Ls*0.5774) + (Rs*0.8164)
Dolby doesn´t recommend to mix the LFE into the Lt/Rt.
* This matrices are not perfect. They don´t achieve TRUE DPL/DPL II downmixing, wich includes phase shifting and some else.

Anyway, the sound should "sound" god to u, and that´s all. U can even try your own matrices to get a narrower or wider stereo sound if u want :).
If u want to use your audio with a DPL/DPL II decoder to get surround, then u should downmix with a DPL/DPL II matrix.
If it´s just to be heard thru a 2ch speaker system than a simple stereo matrix is a valid choice too.

Cheers.

[tebasuna51]

Good answer Skelsgard.

Is a user choice, but remember: the recovery of 4 channels (DPL I) or 5 channels (DPL II) is more perfect with more quality (bitrate)

Then if you want very low bitrates, with aac PS feature (aac+v2), forget DPL I/DPL II.

[Skelsgard]

True, I forgot to mention te need for bitrates that can mantain as good as possible the higher frequencies :D.