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[Help] 5.1 to 4.1
#1
Hi guys, so I had this idea, just wanted to have more experienced opinions. So if a movie had a 4.1 theatrical mix with mono surrounds, but they were split and stereo-ized for the home video release to make it an upmixed 5.1 layout, if I combine the surrounds back into mono, will I restore the original 4.1 mix? Logically it should be doable, but technically? Case in point - Close Encounters Of The Third Kind (4.1 mix has been enhanced to 5.1).
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#2
Technically doable? Sure.
Philologically correct? Dunno...

If the process used made something like, sound A should go to surround left (and surround right is silent), then sound B should go to surround right (and surround left is silent) it should be correct.

But, as I suspect, they split the mono channel with phase/time "tricks", mixing the result surround back to mono will not lead to original surround channel.
Sadly my projects are lost due to an HDD crash... Sad
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#3
(2021-07-29, 08:14 PM)allldu Wrote: So if a movie had a 4.1 theatrical mix with mono surrounds, but they were split and stereo-ized for the home video release to make it an upmixed 5.1 layout, if I combine the surrounds back into mono, will I restore the original 4.1 mix? Logically it should be doable, but technically?

You can't restore the mono surround exactly because both the stereoizing of the surrounds and the conversion back to mono are lossy processes; however, sometimes it's the best you can do – for example, if there is no 4.1 source, or you want to splice 5.1 content into an existing 4.1 track without altering the latter (when combining different cuts of a film, or whatever).  You can perform the conversion in AviSynth:

Code:
LoadPlugin("AudioLimiter.dll")#http://avisynth.nl/index.php/AudioLimiter
FL=WavSource("L.wav")
FR=WavSource("R.wav")
C=WavSource("C.wav")
LFE=WavSource("LFE.wav")
SL=WavSource("SL.wav").ConvertAudioToFloat()
SR=WavSource("SR.wav").ConvertAudioToFloat()
SUR=MixAudio(SL, SR, 1.0, 1.0).SoftClipperFromAudX(0.0).ConvertAudioTo24bit()# or ConvertAudioTo16bit() if the source is 16-bit

Merge = MergeChannels(FL,FR,C,LFE,SUR)

return Merge# or to dub the audio to a clip:
#AudioDub(
#\BlankClip(Merge, height=720, width=480, fps=24000, fps_denominator=1001),
#\Merge
#\)

Set the channel-mask to 271 (for example, in BeHappy); otherwise, the output will probably read as 5.0 by default.

EDIT: LD 12646 of CETK has mono surrounds.
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#4
Question is if that's all that was done. If the process consisted merely of duplicating the back channel and maybe phase flipping one of the two, then yeah, you could do some form of summing to restore the original. More likely more mixing differences exist in the mix in most cases. (I don't know about this specific case, you might very well be correct if you researched it)
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Thanks given by: allldu
#5
Thanks guys for your response - I thought so much that this might not be a simple trick of just combining the two surrounds together, that is, one needs to investigate how the stereoizing was done... However, thanks to @Chewtobacca for reminding me that the Director's Cut has mono surrounds on LD, and I happen to have it. If I want to restore the theatrical cut, I guess my best bet would be to swap its surronds to the ones from the Director's cut and patch the missing scenes with the combined ones.
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#6
@allldu
Even if CETK's 5.1 is a remix (that is, with stereo surrounds that are qualitatively different from the original mono surround), stereoizing them is probably the best that you can do if your intent is to base a restoration of the theatrical audio on the LD track and splice in the alternate material, which I guessed it might be when you mentioned CETK.  Assuming that you do this, you might present your 4.1 as 5.1 with mono surrounds, to match how the track is presented on LD, and because it's often a good idea anyway.  (In other words, 5.1→4.1→5.1-with-mono-surrounds.)  If so, the script would look like this:

Code:
LoadPlugin("AudioLimiter.dll")#http://avisynth.nl/index.php/AudioLimiter
FL=WavSource("L.wav")
FR=WavSource("R.wav")
C=WavSource("C.wav")
LFE=WavSource("LFE.wav")
SL=WavSource("SL.wav").ConvertAudioToFloat()
SR=WavSource("SR.wav").ConvertAudioToFloat()
SUR=MixAudio(SL, SR, 1.0, 1.0).SoftClipperFromAudX(0.0).Amplify(0.707).ConvertAudioTo24bit()# or ConvertAudioTo16bit() if the source is 16-bit

Merge = MergeChannels(FL,FR,C,LFE,SUR,SUR)

return Merge# or to dub the audio to a clip:
#AudioDub(
#\BlankClip(Merge, height=720, width=480, fps=24000, fps_denominator=1001),
#\Merge
#\)

The call to Amplify() is to preserve the global volume level.  After editing, encode as 5.1. In this case, there should be no need to set a channel-mask.
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#7
I wonder if there is a DAW tool that could take phase-shifted or micro-delayed 2 channel material and sum / restore to a relatively respectable monaural signal with as much of the shared L-R signal as possible? Finding out when the 5.1 was made could also narrow down the tools they used to make it...some of the early tools they used to spread mono with were not great...
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#8
Basically they can do whatever tomfoolery they want to once they upmix it. Theoretically if they didn't you could sum the rears to one channel and add the filtered out bass back into the main L and R channels. However they've already done all that in reverse and left it in a 5.1 container.
It also doesn't help most receivers and home matrix formats try to stereoize rear mono surrounds anyway.
Damn Fool Idealistic Crusader
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Thanks given by: allldu
#9
Also be aware that there needs to be a 3dB adjustment for the rear(s) when converting btw 4x & 51 to maintain equal power.

I admit I upscale 4x LCRS to 51 for compatibility reasons, and it's extremely unlikely that any home layout now has an exact 4x cine layout.
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