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Author Topic: Digital projection and perforated screens
Evans A Criswell
Phenomenal Film Handler

Posts: 1579
From: Huntsville, AL, USA
Registered: Mar 2000


 - posted 02-22-2002 10:49 AM      Profile for Evans A Criswell   Author's Homepage   Email Evans A Criswell   Send New Private Message       Edit/Delete Post 
Just now while writing a post on the "Movie Reviews" forum for Return to Neverland, I thought of something that may be a problem for digital projection that has never entered my mind until 5 minutes ago.

I've noticed on several occasions that perforated screens with a rectangular array (or any other regular array) of holes tends to create the appearance of a pixel structure in the image if one gets too close. On 3 or 4 occasions, when sitting close to a screen (4th row or closer), when the focus was perfect, the array of holes have caused the projected image to have a "digital, pixelized" appearance.

Couple this with a projected image that already has a rigid pixel structure of a given resolution. The pixel structure of the projected image and the structure of the perforations of the screen will create "beat patterns". Suppose the projected image has a resolution of 1280 by 1024 and the screen has an array of holes that is 1300 by 1050 (I just made those up). This will show up as a beat pattern that is made up of 20 vertical waves going across the width of the screen and 26 horizontal waves going up and down the screen. This may or may not be noticeable, but at certain combinations of resolutions, coupled with other irregularities caused by keystoning, this could be distracting if the wrong combination of screen perforation pitch and digital projection resolution were combined.

Has this possibility ever been investigated or mathematically analyzed? I'll bet that unless a screen is very small, the existing screens have a small enough "performation pitch" for this not to be a problem with current digital projectors, but as resolutions increase, could it be more of a problem?

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Evans A Criswell
Huntsville-Decatur Movie Theatre Information Site


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Ian Price
Phenomenal Film Handler

Posts: 1714
From: Denver, CO
Registered: Jun 99


 - posted 02-22-2002 01:57 PM      Profile for Ian Price   Email Ian Price   Send New Private Message       Edit/Delete Post 
Having been involved with projecting video on to screens at the Telluride Film Festival these past 4 years and attending many digital shootouts at various trade shows, I can tell you this. So far, with any resolution of video, there are always fewer pixels than holes in the screen. Also the pixels have always been larger than the holes. So if you are close enough to the screen to see the holes, you are way too close to enjoy the video image.

Now with a small enough screen and a very high-resolution projector, this may become a problem.

There are ways to make a screen acoustically transparent and visually opaque. By that I mean a screen without holes that still passes the high frequency information without too much attenuation. I believe the Japanese came up with the black fish-scale screen that had this characteristic. I have never seen one, but the concept is intriguing. Two other ways are to reposition the speakers above or below the screen, this has annoyed some people in the past. Think AMC and the “Suck Screen.” Or to make the screen so thin that it is mostly acoustically transparent. One idea that is fantasy but intrigues me is to make the screen the sound transducer itself. The question about this is would deep bass make the image vibrate too much? Remember, the larger the transducer's surface area, the less it has to move.

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David Stambaugh
Film God

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From: Eugene, Oregon
Registered: Jan 2002


 - posted 02-22-2002 02:03 PM      Profile for David Stambaugh   Author's Homepage   Email David Stambaugh   Send New Private Message       Edit/Delete Post 
Ian, you mentioned AMC and the "Suck Screen". Not to sidetrack this thread, but is that the Torus screen? I think that was the official name they used. Other than the sound not coming from directly behind the screen, what was wrong with the Torus design? Just curious. If it was solid, not perforated, wouldn't apparent resolution be higher?

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- dave
Look at this! His chin strap has been cut!

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Evans A Criswell
Phenomenal Film Handler

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From: Huntsville, AL, USA
Registered: Mar 2000


 - posted 02-22-2002 02:20 PM      Profile for Evans A Criswell   Author's Homepage   Email Evans A Criswell   Send New Private Message       Edit/Delete Post 
quote:
Having been involved with projecting video on to screens at the Telluride Film Festival these past 4 years and attending many digital shootouts at various trade shows, I can tell you this. So far, with any resolution of video, there are always fewer pixels than holes in the screen. Also the pixels have always been larger than the holes. So if you are close enough to the screen to see the holes, you are way too close to enjoy the video image.

I'd bet that this phenomenon I'm describing would only be a problem if the ratio of the screen performation pitch to the projection material "pixel pitch" were in the range 0.5 to 2.0 , probably being most bothersome with values closer to 1.0 .

You're right that if you're close enough to the screen to see the holes, you're too close, but artifacts created by the style of interference I'm describing could be visible even from the auditorium rear.

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Evans A Criswell
Huntsville-Decatur Movie Theatre Information Site


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Ian Price
Phenomenal Film Handler

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From: Denver, CO
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 - posted 02-22-2002 02:22 PM      Profile for Ian Price   Email Ian Price   Send New Private Message       Edit/Delete Post 
Yes, not to sidetrack the discussion, AMC's Torus Screen with HIT Sound System was developed to improve screen brightness with no increase in lamp wattage. The Screen was non-perforated and stretched over a frame in a box with fans to "suck" the air out, therefore creating a compound curve in both directions, left to right and up and down.

In one regard they were successful. The screens were brighter and more evenly illuminated. But the downside was that the speakers were positioned above and below the screen and usually sounded very shill. The other problem was the parabolic reflector that worked so well for light also worked very well for sound. The result was that you could hear people chewing their popcorn five rows behind you with perfect clarity and also could completely understand conversations happening 20 rows back. I also think that the sound reflective screen was responsible for the sound being shrill.

Ok, now back to the discussion about Pixels and Holes. I am sure that if the right convergence of pixels in perfect focus and the right quantity of holes happened, that there might be an interesting phenomenon. But I'm sure that it's probability of this happening is about the same as the 34 pound terrier theory. (Its thought that the natural frequency of the Golden Gate bridge is such that on the right day, no wind, certain temperature and humidity, that a 34 pound terrier walking at a precise gate, would exactly match the natural frequency of the bridge thus bringing it down.) Nice theory but the parameters would never be matched.


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Aaron Haney
Master Film Handler

Posts: 265
From: Cupertino, CA, USA
Registered: Jan 2001


 - posted 02-22-2002 02:53 PM      Profile for Aaron Haney   Email Aaron Haney   Send New Private Message       Edit/Delete Post 
I recall reading that digital installations might use "microperforated" screens to help aleviate this problem. There may already be some such screens in place. I doubt there will be much interference anyway, as Ian pointed out, but why take chances?


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

Posts: 3836
From: Albuquerque, NM
Registered: Feb 2000


 - posted 02-22-2002 03:32 PM      Profile for Paul Mayer   Author's Homepage   Email Paul Mayer   Send New Private Message       Edit/Delete Post 
Haven't noticed moire interference patterns due to the screen perfs on any of the DLP presentations I've seen so far. It is a potential problem as Evans has noted, but on theater-sized screens at current resolutions moire is not visible--the projected pixel size simply swamps the perf pattern.

Home theater screens are another story. There has been discussion of this on the AV Science forums in the case of very high-end home theater installations using perf screens with HD DLP or LCD displays. Since these screens are much smaller, the ratio of perfs to pixels is much smaller, getting into the range where moire becomes noticeable. Solutions have included changing to a solid screen or a micro-perf screen, changing the screen size, critical leveling of the projector to the screen, slight de-focusing, or decreasing the image resolution. Anything that increases the pixel to perf ratio will work.

BTW much more noticeable than perfs vs. pixels is the use of electronic picture geometry adjustments to eliminate keystoning and the like. All but the most high-end of HD image scalers produce quite visible moire as soon as the video raster no longer maps to the pixel array.

Paul
Feeling just duckie!
Hey! Some feel chipper, I feel duckie!
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Nic Margherio
Film Handler

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From: St. Louis MO, USA
Registered: Dec 1999


 - posted 02-22-2002 07:00 PM      Profile for Nic Margherio   Email Nic Margherio   Send New Private Message       Edit/Delete Post 
Continuing on the sidetrack...

All that Ian pointed out about the Torus screen is accurate, however, I feel that the worst characteristic of it is the geometric distortion it imposes on the picture. Sit anywhere outside of the "sweet spot" in a Torus auditorium and you will notice severe visual distortion of the image, especially noticeable during credits and up-down camera pans.

From a maintenance standpoint, the screen "sucks" too.

I reacall once when an HVAC problem created a pressure imbalance in the auditoriums. The poor little "sucky" motors just couldn't handle the stress and gave out. This left us with a billowing mass of fabric to pass for a screen. As much as they suck when they do suck, they REALLY suck when they don't suck.

There is a little sensor in the middle of the screen that senses the distance between the fabric and the frame. This is used to control the shape of the screen by telling the motor "to suck or not to suck." (Didn't Hamlet say that?) This sensor has been known to go bad from time to time, and, as you may imagine, is not very fun to replace.

You must dust a Torus screen very regularly or else dust will collect disproportionately on the bottom of the screen (the part that faces upward) and become suprisingly noticeable. Not that screens should not be kept dust-free, but it is just another strage quirk about it.

AMC has not installed the screen for years now, and they are not without advantage; try getting 19-20-19 on a conventional flat screen!
You can do it quite easily with a Torus.

For the most part though, they really do suck.


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Paul G. Thompson
The Weenie Man

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From: Mount Vernon WA USA
Registered: Nov 2000


 - posted 02-22-2002 07:19 PM      Profile for Paul G. Thompson   Email Paul G. Thompson   Send New Private Message       Edit/Delete Post 
Evans, I have noticed the same thing with the LED projectors we use for screen ads. The picture looks great until about a distance of 20 feet. Then, the quality just drops off to very poor.

I wonder how they are going to get that problem corrected.

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Steve Kraus
Film God

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From: Chicago, IL, USA
Registered: May 2000


 - posted 02-23-2002 03:47 PM      Profile for Steve Kraus     Send New Private Message       Edit/Delete Post 
Not really a technical comment but as we're on the topic of digital projection, AMC, and suck screens, I was going to take a peek into Return to Neverland running in DLP last night at the AMC Gigaplex while awaiting another show and was surprised to see the room vacant after the 7pm show had let out. I guess Buena Vista doesn't allow splitting a screen but you'd think they'd have had a 9:xx PM show. They had late shows with it on film on another screen. Strange they wouldn't want to crank out another DLP showing.

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David Baum
Film Handler

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From: Brussels, Belgium
Registered: Aug 2000


 - posted 02-23-2002 06:16 PM      Profile for David Baum     Send New Private Message       Edit/Delete Post 
hi

do any of you have experience with HARKNESS HALL PM180+ microperforated screens (0.5mm diameter) ? what is the holes arrangement ( delta or square ) ?

they say these are good for close viewing, up to 2meters.
Don Stewart ( but this is about home theater screens ) says that 1280x720 or XGA projectors must be used with screens of 92" or larger ( diagonal ) to avoid the "moiré" patterns created by interraction between pixels and holes.
thanks

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999


 - posted 02-24-2002 12:19 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
I'm sure you meant the MP (microperf)

The HH microperf is a bit odd in it's arrangement...is it dense in one dimesion be regular spacing in the other (looks kinda like stripes). The perf size is tiny though, especially as compared to conventional perforations.

The next smaller version I believe you mentioned was the plus as in MP plus...now that has a standard delta formation but it looks more like a scrim. The perfs are very tiny and they disappear when one is only but a few feet away. However, you better not have anything even remotely reflective behind the screen because it would be almost translucent.

Next week, I'm going to be doing some experiments with the mini-perf screens so I might be able to provide some more information.

I can say watch out for the MDI mini-perf...while the holes are indeed smaller than the standard perf, they tend to stand out and have a very odd pattern (not likely to morie) but are way too visible for close viewing...I can see the perfs at 20 feet away...it is almost like they high-lighted the perfs despite them being about 50% of the size of normal screen perforations.

Steve

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"Old projectionists never die, they just changeover!"

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David Baum
Film Handler

Posts: 90
From: Brussels, Belgium
Registered: Aug 2000


 - posted 02-24-2002 04:40 AM      Profile for David Baum     Send New Private Message       Edit/Delete Post 
hi Steve


MDI mini perf ?

yeah, I meant MP.

cheers

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Carl Martin
Phenomenal Film Handler

Posts: 1424
From: Oakland, CA, USA
Registered: Feb 2002


 - posted 02-24-2002 06:14 AM      Profile for Carl Martin   Author's Homepage   Email Carl Martin   Send New Private Message       Edit/Delete Post 
i consistently see perfs from 20' away in brighter scenes. across the board, not just in one theater. of course it's worse on small screens in small auditoriums. i've sat in the back row and seen perfs. for a smaller theater i'd much rather get rid of them. if speakers were paired above and below the screen, wouldn't they average to an apparent source somewhere between them on the screen?

carl

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Gordon McLeod
Film God

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From: Toronto Ontario Canada
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 - posted 02-24-2002 01:01 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
That averaging would only occur for a small percentage of seats

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