Film-Tech Forum ARCHIVE: Dolby 3D dual projector setups?

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Posted by Carsten Kurz (Member # 5396) on 01-08-2010, 04:43 AM:

I heard that recently two large screens in germany have been equipped with dual-projector Dolby 3D systems (Kinoton installations).

http://www.dcinematoday.com/dc/PR.aspx?newsID=1652

Someone mentioned to me that these projectors still contain Dolby wheels.
Is anyone familiar with this kind of installation, is it 'officially' Dolby approved? Why would they still use filter wheels inside instead of static 'Infitec/Dolby' filters in front of the lens? The only reason I could imagine would be that with filter wheels, both projectors could still be used as single 3D machines temporarily on smaller screens without reinstalls.
However my source mentioned that these would not contain standard Dolby wheels but special versions. Still I would think that static filters offer slightly more light - at least if these wheels still have segments and need blanking times.

Has anyone heard of Dolby providing static filters for projectors at all? Are they that much different from Infitec Filters?

- Carsten

Posted by Demetris Thoupis (Member # 833) on 01-08-2010, 05:12 AM:

There are two ways to implement a two projector setup. In both cases the desired result is to increase the light output on big screens.

One case is to use both projectors showing both eyes. In this case you are mentioning, most probably both projectors are flashing both eye images inside them and thus using the dolby filter wheel prior the light engine. The other case is where each projector shows one eye only thus having projector 1 showing left eye, and projector 2 showing right eye. That is where you use a static filter but I am not sure in Dolbys' case if its inserted prior the light engine or in front as I have never done such a setup myself. Most probably Mark or Gordon or Julio or Steve can inform us better.

Demetris

Posted by Mark Lensenmayer (Member # 134) on 01-08-2010, 07:32 AM:

They ran 4-Barco projectors (2 for each eye) using Dolby 3D at the Empire Leicester Square theatre for the premiere in London. Don't know if they are still using them or not.

Posted by Carsten Kurz (Member # 5396) on 01-08-2010, 08:11 AM:

Yes, 4 DLP+Dolby 3D for the AVATAR premiere there.
But only for the premiere I heard, afterwards they stayed with a 2 projector setup.

- Carsten

Posted by Chase Taylor (Member # 778) on 01-08-2010, 10:06 AM:

I believe but could be wrong but when using a BARCO DP-3000 in a dual projector set up the color wheel that goes into each projector is one filter that spins so basically the wheel is not split into two filters just one and each projector has the filter for one eye and the other projector has the filter for the other eye.

Someone please correct me if I'm wrong but I think this what I remember how it's done with those.

Posted by Pete Naples (Member # 761) on 01-08-2010, 12:35 PM:

That is my understanding too Chase, a static filter wouldn't stand up to the heat.

Posted by Demetris Thoupis (Member # 833) on 01-08-2010, 12:40 PM:

rotating filter with single eye filter

Posted by Mark Gulbrandsen (Member # 72) on 01-08-2010, 03:14 PM:

I am pretty sure dual projector uses the exact same filters. You have two controllers and one just has to phase them both correctly.
Mark

Posted by Kevin Fairchild (Member # 4985) on 01-08-2010, 06:39 PM:

Dolby has the Large Screen 3D installation instructions on dolbesecure.com

It says that there are two seperate filters.
Cat # 932 is the left eye filter.
Cat # 933 is the right eye filter.

Posted by Sam D. Chavez (Member # 1841) on 01-08-2010, 08:33 PM:

Kevin is correct.

Posted by Demetris Thoupis (Member # 833) on 01-09-2010, 01:40 AM:

After reading thoroughly via the Dolby Extranet, it says that they are static filters although they implement colour wheel like the standard solution so they are placed again prior the light engine. The BIG difference is that the images are not flashed at all as they are shown without any gaps as each projector shows one eye only so the brightness is much higher.
Demetris

Posted by Kris Verhanneman (Member # 5562) on 01-09-2010, 06:33 AM:

Much higher? How long do we project 'black' between a flash of image? Anybody who can say how much (%) that is?

Posted by Julio Roberto (Member # 4976) on 01-09-2010, 07:14 AM:

Aprox. 7%

Per projector.

So in this case, they get a 14% increase in light, compared to fitting both projectors with traditional filters.

Of course, having two projectors already "doubles" the light output to begin with.

Posted by Mark Gulbrandsen (Member # 72) on 01-10-2010, 03:22 PM:

I never thought of that aspect. Either would work but one is going to give lots more light.

Posted by Mark Uhde (Member # 5357) on 01-19-2010, 12:53 AM:

Sorry Julio, that math is wrong. You can't add the light loss from each projector to get the total. You have to average it. Since each one is, by your numbers, losing about 7% the average is also about 7% of the total.

To demonstrate why you can't add it, imagine a hypothetical 15-projector setup. If you added up the 7% loss in each projector to get total loss, you would be losing 105% of your light [Smile]

Posted by Tristan Lane (Member # 1169) on 01-19-2010, 01:15 AM:

Julio is referring only to the light gains garnered from eliminating the dark time between alternating frames required on a traditional single projector configuration. It makes sense to me.

Can anyone explain why the filters can't be placed in front of the lenses in a two projector config? Is efficiency increased by placing it in the light path prior to the engine, or is it required for functionality.

It would seem much more economical to use passive filters in front of the lens.

Posted by Julio Roberto (Member # 4976) on 01-19-2010, 03:11 AM:

Different ways to look at the same thing.

What I meant is that each time you put a projector into a time-sequential mode, like you do with Dolby (or RealD or Master Image, for that matter), the projector stops sending aprox. 7% of the light in "black" for the timing interval to the screen. Say this is equivalent to the light of 100Watts.

If you double-up two projectors and put them both in time-sequential mode, each one would lose 7% of "potential" light output. Say this is equivalent to 200Watts. 100Watts for each projector.

If instead you fit them with passive or none-time sequential filters, like we are talking here, each projector will put through the lens 7% more light.

If you wanted to "compensate" for this by adding a "third projector", this projector would have to be able to output "200Watts" worth of light and perfectly halve-it to each eye without blanking.

All this is aproximate as, i.e., the blanking interval for a Zscreen is closer to 10% while it can be closer to 5% for some active shutter glasses. I don't know what the mechanical Master Image is able to obtain, but I can see it's not a very good design by switching the filter horizontally instead of vertically and having such a wide gap between the filters in wheel.

No idea why Dolby preferred the placement of the non-sequential filters inside the light box. I know they can be placed at the end of the light path. I suspect it is just so that they can sell "the same automated solution" to everyone. For a price.

[beer]

The explanation can't be that they are afraid of the luminous flux and prefer to rotate them, as the law of conservation of energy say that a filter the aprox. size of the wheel placed in front of the lens would be equivalent and able to cope with the same light, not to mention that in front of the lens is going to receive oodles of less light and heat to begin with, and probably better spreaded through the surface of the aprox. same-sized filter.

If you want to check out the numbers yourself, you can go here for some measures:

http://www.barco.com/projection_systems/downloads/Stereoscopic_technologies_overview.pdf

There are duty cycle numbers for zscreens, active shutter glasses, time-sequential infitec (one-projector Dolby) and dual passive infitec (dual projection Dolby).

Posted by Steve Guttag (Member # 268) on 01-19-2010, 06:33 AM:

Putting anything after the lens is not preferred. Anything in front of the lens comes off of resolution and contrast so Dolby is correct in leaving the filter between the light and the imagers.

If you think about it...the only thing that is different for Dolby in a dual projector system is that the filters need not spin (and are, of course not split)...the desire to have the filters be able to be in or out of the light path remains.

Steve