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» Film-Tech Forum   » Operations   » Large Format Forum   » IMAX 3D Grey Patches (Page 1)

 
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Author Topic: IMAX 3D Grey Patches
Andy Linde
Film Handler

Posts: 19
From: Nelson, New Zealand
Registered: Mar 2010


 - posted 04-11-2010 07:53 PM      Profile for Andy Linde   Author's Homepage   Email Andy Linde   Send New Private Message       Edit/Delete Post 
A while back I went to Auckland, NZ to see Avatar 3D on the country's only IMAX screen. (And what a horrible screen it is!) Throughout the feature, i think about 5 times, one eye would completely grey out for anywhere between 1 and 5 seconds at a time. Looked as if something was blocking one lens. I thought it was just me, but realised the whole auditorium started wiping and playing with their glasses [Smile] I haven't much knowledge about IMAX 3D, but is this normal during a 3D feature??

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Darryl Spicer
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From: Lexington, KY, USA
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 - posted 04-11-2010 09:27 PM      Profile for Darryl Spicer     Send New Private Message       Edit/Delete Post 
Well this question should be in the large format section but here is the most likely answer. If this was a film based Imax presentation then there was for some reason or another some black slug inserted for 1 to 5 seconds to keep the left and right eye in sync with each other. If you loose footage on one eye the missing footage must be replaced with the black slug. you can't just cut the same amount out of the other eye because the sound would end up out of sync with the picture.

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Andy Linde
Film Handler

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From: Nelson, New Zealand
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 - posted 04-11-2010 10:17 PM      Profile for Andy Linde   Author's Homepage   Email Andy Linde   Send New Private Message       Edit/Delete Post 
Ah of course that would explain it. Thanks! I'm wondering now how they managed to lose so much film then? There were about 5 different spots. Possibly at reel changes???

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Kurt Zupin
Jedi Master Film Handler

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From: Maricopa, Arizona
Registered: Oct 2004


 - posted 04-12-2010 12:08 AM      Profile for Kurt Zupin   Email Kurt Zupin   Send New Private Message       Edit/Delete Post 
Andy, Darryl is correct on the insertion of slug film to keep the eyes in sync and running together. 5 diffrent spots on the one eye and none on the other is troubling. Reel changes in IMAX film based presentations happen about every 4-6 min. Avatar alone had 47 in each eye if I remember correctly.

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Jack Ondracek
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From: Port Orchard, WA, USA
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 - posted 04-17-2010 09:24 PM      Profile for Jack Ondracek   Author's Homepage   Email Jack Ondracek   Send New Private Message       Edit/Delete Post 
quote: Darryl Spicer
you can't just cut the same amount out of the other eye because the sound would end up out of sync with the picture.

Hmmm... ok. Then how does the sound stay synched under normal conditions?

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Gordon McLeod
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From: Toronto Ontario Canada
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 - posted 04-17-2010 11:06 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Shaft encoder on the projector provides bi-phase to either a dubber or a timecode generator

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Sean McKinnon
Phenomenal Film Handler

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From: Peabody Massachusetts
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 - posted 04-18-2010 12:18 PM      Profile for Sean McKinnon   Email Sean McKinnon   Send New Private Message       Edit/Delete Post 
I always thought they added slugs to non 3d imax films to keep the sound in sync if anyone film needed to be removed?

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

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From: Albuquerque, NM
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 - posted 04-18-2010 12:37 PM      Profile for Paul Mayer   Author's Homepage   Email Paul Mayer   Send New Private Message       Edit/Delete Post 
Yes, slug is added to IMAX prints 2D and 3D to keep the picture synched with the sound. You never want to cut the sound to match the picture (though I have seen this done). Doing so creates a one-off soundtrack that can only be played with that particular print. You always add slug to match the sound. That way any print of that title can play with any soundtrack for that title.

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Jack Ondracek
Film God

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From: Port Orchard, WA, USA
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 - posted 04-22-2010 06:08 PM      Profile for Jack Ondracek   Author's Homepage   Email Jack Ondracek   Send New Private Message       Edit/Delete Post 
Is there a reason why they wouldn't use a timecode, like DTS? Then, if they lost a frame or two from one strip they could cut the same off the other. That might glitch the picture, but it might not be so noticeable, and the sound would continue to track.

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Thomas Pitt
Master Film Handler

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From: Leeds, West Yorkshire, UK
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 - posted 04-28-2010 12:41 PM      Profile for Thomas Pitt   Email Thomas Pitt   Send New Private Message       Edit/Delete Post 
There's no timecode on IMAX so that the entire film width (apart from sprockets) can be used for image. Putting a timecode or soundtrack along the edge of the film would shrink the image and result in lower resolution.

How is it kept in sync? The film is loaded up with the projector on a specially marked START frame for each eye. Provided the right amount of black slug is put on each eye when the film is damaged, they should both stay in sync with the soundtrack.

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

Posts: 3835
From: Albuquerque, NM
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 - posted 04-28-2010 04:04 PM      Profile for Paul Mayer   Author's Homepage   Email Paul Mayer   Send New Private Message       Edit/Delete Post 
At the time IMAX got started, timecode would have been a relatively expensive and complex solution for keeping sync. Bi-phase has been around forever and is simple and commonplace (at least for double-system operations, like studio screening and mixdown rooms). I imagine cost and complexity issues at the time (1970) greatly favored the bi-phase solution for IMAX sound sync.

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John Wilson
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From: Sydney, Australia.
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 - posted 04-28-2010 04:15 PM      Profile for John Wilson   Email John Wilson   Send New Private Message       Edit/Delete Post 
quote: Thomas Pitt
Putting a timecode or soundtrack along the edge of the film would shrink the image and result in lower resolution.
Well, that is obviously no longer an issue with current IMAX decision makers, so why not add one? DTS wasn't around when they went this route but it is now.

We always ran a dubber back-up whenever we could for our digital sound at IMAX. Quite a few times it came in very handy when one of the three decks didn't sync up properly. Without it, those sessions would have been cancelled.

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Mark J. Marshall
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From: New Castle, DE, USA
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 - posted 04-28-2010 08:35 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
quote: John Wilson
Well, that is obviously no longer an issue with current IMAX decision makers
[thumbsup] [thumbsup] [thumbsup]

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Gordon McLeod
Film God

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From: Toronto Ontario Canada
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 - posted 04-28-2010 08:35 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Thomas said "There's no timecode on IMAX so that the entire film width (apart from sprockets) can be used for image. Putting a timecode or soundtrack along the edge of the film would shrink the image and result in lower resolution."
Not true the negative uses 65mm film so the strip area is actually free and dts time code has been used on imax films on special order for a couple of venues with non imax produced product
bi phase has been around and as paul said works reliably

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Christopher Seo
Jedi Master Film Handler

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From: Los Angeles, CA
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 - posted 05-04-2010 10:55 PM      Profile for Christopher Seo   Email Christopher Seo   Send New Private Message       Edit/Delete Post 
Yes, image size is not an issue since DTS-70 timecode rides outside the perfs. Gordon, how was the track done for those DTS 15/70 prints? Was it "stretched" 3X from normal DTS-70 timecode?

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