How to identify 5.1 Dolby Stereo unfolds/70mm Six-Track Mixes

[izzybell]

While it's a common practice for most major studios to do brand-new 5.1 Surround mixes for the classic catalog titles, sometimes they just repurpose old stems.

If you use this forum Dolby Stereo doesn't need an introduction but if you somehow don't know the TLDR is essentially Dolby Stereo was a technique of getting 4 channels of audio (LCRS) to fit into the usual optical stereo track on a 35mm reel via matrixing (if you want to a longer explanation, check here: https://fanrestore.com/thread-3467.html). Sometimes these mixes would play in discrete "Baby Boom" format in 70mm, hence the 70mm Six-Track part of the thread name. Now in the early days of surround sound on Home Media, many releases would choose to just use their existing Dolby Stereo track and process them back to surround (Much like a theater would've done during the original run). Having an interest in these mixes, I have found a way to identify if a release is using an unfolded Dolby Stereo track. 

Software Needed -
Audacity + FFMPEG Plugin
gmkvextractGUI (optional, if extracting from a remuxed source)

For my main example, I'll be using a personal favorite of mine that has two 5.1 tracks on different home media release. Dune (1984).

Below is a screenshot of the surround channels on the 1998 Universal DVD



Now you might already be able to tell that the surround channels look to be the same in both channels. This is the first step in identifying as Dolby Stereo only ever used a single mono surround channel. However the second step is the real smoking gun to prove that this is a Dolby Stereo unfold/Six-Track Mix
By right click on the the leftmost side of the track in Audacity, you can change the track view to Spectrogram which reveals...



Matrixing in the surrounds! The spectrogram can easily show the results of the matrix decoding process which makes it HIGHLY likely a track is an unfolded Dolby Stereo track. Now this isn't 100% definitive for one of those 2 reasons.
1. This is age/budget dependant, Some films had discrete mono surrounds while other relied on matrixing, There's also rare case of a film's original mix having stereo surrounds but being released in Dolby Stereo (i.e Indiana Jones, Popeyes).
2. There are rare cases of remixes using matrixed surrounds (again, Indiana Jones) and/or stereoizing the mono surrounds (Blade Runner). So beware of those.

With that being said I hope this is a useful guide for those looking to find 70mm Six-Tracks or Dolby Stereo unfolds on older releases, Cheers!

[stwd4nder2]

What about the spectrogram shows the matrix decoding? Are you just confirming the surrounds are mono or is there something else I'm missing?

[izzybell]

Discrete surround channels have much cleaner looking spectrograms than matrixed surround channels.

Using the same Dune example, here's the spectrogram for the remixed 5.1 surround channels.

[Stamper]

I'm a bit new to this, how do you deduce the matrixing from the spectrogram? All I see is both channels are similar.

[izzybell]

Take a closer look at them, you will noticed matrixed surround channels have harder cutoffs and generally "stranger" looking forms compared to a discrete track.

[stwd4nder2]

Take a closer look at them, you will noticed matrixed surround channels have harder cutoffs and generally "stranger" looking forms compared to a discrete track.

Are you sure that isn't just compression related?

[izzybell]

Take a closer look at them, you will noticed matrixed surround channels have harder cutoffs and generally "stranger" looking forms compared to a discrete track.

Are you sure that isn't just compression related?

I'm sure it's not. Here an example using the recently leaked Japanese DCP PCM 5.1 for My Neighbor Totoro, a film that has never had a 5.1 JP track on any home release and was mixed in Dolby Stereo. Note how it has the same characteristics of the Dune DVD example.

It's also important to note that quality of the decoding will depending on what the studio used. It might not be super obvious so you have to have a keen eye for it

[Turisu]

I accept that some 5.1 tracks repurpose the original LRCS channels of a Dolby Stereo or 70mm mix. But these would still be entirely discrete channels with no matrixing. I've never heard of a studio 'unfolding' a matrixed 2-channel track into a 5.1 mix. Are you suggesting that the original LRCS channels for Dune were unavailable for the DVD and they had to resort to decoding the matrixed track? I find that rather difficult to believe. In fact, it has been postulated that the 5.1 track on this DVD is derived from the 70mm 6-track mix, which would not have been matrixed in the first place.

[izzybell]

It's entirely possible they decoded it, Listening to the surround channels on their own they sound rather low quality and a little "hazy" which matches other decoded tracks I've heard but at the same time the track overall sounds higher quality than the Arrow 2.0 so perhaps they decoded it from a higher quality mag source, I have no clue to be entirely honest. I found this method from a friend who was checking the Indiana Jones DVD 5.1s (the ones that are in fact the 70mm Six-Tracks) and they have the same attributes (both in listening and spectrogram). Below is an example from Temple of Doom.

I've already said this method isn't 100% but I believe that this could still be useful

[Turisu]

But 70mm 6-track audio is not matrixed. These tracks are entirely discrete to start with. So what you're seeing on the Indiana Jones surround channels has nothing to do with matrixing or decoding.