Film-Tech Forum ARCHIVE: IMAX 3D Grey Patches

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Posted by Andy Linde (Member # 5664) on 04-11-2010, 07:53 PM:

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??

Posted by Darryl Spicer (Member # 711) on 04-11-2010, 09:27 PM:

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.

Posted by Andy Linde (Member # 5664) on 04-11-2010, 10:17 PM:

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???

Posted by Kurt Zupin (Member # 2751) on 04-12-2010, 12:08 AM:

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.

Posted by Jack Ondracek (Member # 1466) on 04-17-2010, 09:24 PM:

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?

Posted by Gordon McLeod (Member # 33) on 04-17-2010, 11:06 PM:

Shaft encoder on the projector provides bi-phase to either a dubber or a timecode generator

Posted by Sean McKinnon (Member # 612) on 04-18-2010, 12:18 PM:

I always thought they added slugs to non 3d imax films to keep the sound in sync if anyone film needed to be removed?

Posted by Paul Mayer (Member # 355) on 04-18-2010, 12:37 PM:

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.

Posted by Jack Ondracek (Member # 1466) on 04-22-2010, 06:08 PM:

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.

Posted by Thomas Pitt (Member # 4322) on 04-28-2010, 12:41 PM:

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.

Posted by Paul Mayer (Member # 355) on 04-28-2010, 04:04 PM:

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.

Posted by John Wilson (Member # 269) on 04-28-2010, 04:15 PM:

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.

Posted by Mark J. Marshall (Member # 1409) on 04-28-2010, 08:35 PM:

quote: John Wilson
Well, that is obviously no longer an issue with current IMAX decision makers

Posted by Gordon McLeod (Member # 33) on 04-28-2010, 08:35 PM:

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

Posted by Christopher Seo (Member # 7) on 05-04-2010, 10:55 PM:

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?

Posted by Richard Hamilton (Member # 321) on 05-05-2010, 09:47 AM:

Anyone have any sources to buy some 8/70 black film, and what the average cost should be?

As far as DTS, I think I installed 1 or 2 systems that the customer insisted on it. If I remember correctly, the DTS units chased our generated timecode.

Rick

Posted by Cameron Glendinning (Member # 3516) on 05-07-2010, 07:03 AM:

I'm not sure if IMAX in NZ uses / has ever or still has IMAX shutter glasses but they too sometime caused similar issues if the sync transmitter plays up.

Were the glasses like Real D sunglasses or space age looking thingies?

Posted by Andy Linde (Member # 5664) on 05-24-2010, 07:12 AM:

The space age ones.

Posted by Dave Macaulay (Member # 813) on 12-31-2010, 08:33 AM:

The problem isn't film area, there's a lot of room outside the sprocket holes in 70mm. 70mm is the release film, the camera negative film is 65mm which has edges outside the sprockets similar to 35mm. The extra 5mm on the release film was for the magnetic sound tracks.
The Imax 15/70 projector uses the rolling loop transport system through the gate. This system does not contact the image area of the film so a film can be shown thousands of times without image damage. In practice damage occurs in the storage reels and the picture dyes bleach out but one can run a film loop all day and not be able to see any scratches or marks in the image area.
The edges take a beating though. There's a lot of surface wear on the "sound track" area both inside and outside the image area. Emulsion wears off fairly quickly (several hundreds of passes) so any data printed there photographically has a limited life. The Imax frame size was defined before any thought of digital sound so the frame lines are very narrow, and some Imax installations use so much film image area that the narrow ultrasonic splices are visible on screen. There was some thought given to having a narrow "bar code" printed in the frame line to identify the title and frame number allowing sound sync among other benefits but having this visible on screen would be nasty and the code had to be wider than was thought acceptable with the reader technology available.
Experiments I saw with DTS code stretched it much like with 5/70-24 or 5/70-30 just more stretched. I don't know what print life they get, I wasn't aware there was any Imax 15/70 using DTS timecode printed on. Print life in Iwerks 5/70 systems using loop cabinets and Ballantyne projectors is limited by DTS data degradation from the emulsion wearing off.
In my experience venues using DTS sound (this was unusual but they existed) the shaft encoder was used for DTS synchronizing. They used a converter box that generated SMPTE time code from the biphase shaft encoder to sync the DTS unit, no timecode data from the film. You had to set a sync frame in the projector and reset the timecode converter every show. Slugs still had to be spliced in to replace damaged film with this sound system.