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Author Topic: Non perf screen Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 03-22-2012 05:51 PM                     Is it at all possible to put a non perf screen in front of all 3 screen speakers. Will there be enough hf coming through. We have an Eiki projector that is reflecting the light off the perfs into weird patterns on the screen. I put white cardboard against the front of the screen and it disappears. I also put it behind the screen right up against it and it also disappeared. There is no room above the screen for speakers.  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 03-22-2012 05:56 PM                        Assuming that the Eiki is a video projector, you have a moire issue. The pixel array is interacting with the screen perfs and causing weird circular patterns. It is the same sort of effect that one gets when folding a window screen in half. This is obviously very distracting to the audience. I have yet to find a good way to predict if this will happen with a given screen and lens and video projector combination. If anyone knows how to do this, please tell! It does not happen with LCoS or CRT projectors, only DLP or LCD (because of the fixed pixel array). The quick and dirty fix is to put up a white test pattern, zoom out the lens until the moire issue goes away, and then electronically scale down the picture to fit the screen. Another not-really-correct fix is to defocus the lens slightly. A better fix is to replace the screen with one with a different perf pattern and hope that the problem does not recur.  |  IP: Logged Paul H. Rayton Expert Film Handler Posts: 210 From: Los Angeles, CA , USA Registered: Aug 2003 posted 03-22-2012 06:38 PM                    We are assuming that the Eiki projector of which you speak is a video projector of some sort, right? Eiki also made 16mm film projectors of various sizes and types. If you're talking about one of their video projectors, just as an idea you might see if you could move this thread over (or close this one and re-start a new thread) in the "Digital Cinema Forum" section. I'd think you would likely find more people who may have encountered this issue over there, since it's so clearly associated with such types of projectors.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-22-2012 08:08 PM                     quote: Scott Norwood It does not happen with LCoS or CRT projectors, only DLP or LCD (because of the fixed pixel array). Correct me if I'm wrong, but since when doesn't LCoS use a fixed pixel array? The Moiré patterns or aliassing effects you're witnessing are probably happening, because your pixels are about the same size or smaller than your perforation. I guess, a screen with smaller perforation (holes) would probably be the best solution. A screen without any perforation with speakers behind it seems like a very bad idea to me, because your screen will become one large membrane.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 03-22-2012 08:31 PM                     Defocusing ist still the best option if moiree is the issue. - Carsten  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 03-22-2012 09:41 PM                     Sorry guys. It is a video projector. You are right on Scott. Defocusing it slightly does improve it to a point. Going through the zoom range does not though. The only fix I see is getting a non perf screen.  |  IP: Logged Bobby Henderson "Ask me about Trajan." Posts: 10973 From: Lawton, OK, USA Registered: Apr 2001 posted 03-22-2012 10:15 PM                     quote: I have yet to find a good way to predict if this will happen with a given screen and lens and video projector combination. If anyone knows how to do this, please tell! I'm sure there's some way to be able to predict if moiré will be a problem by doing some complex geometric math. Having dealt with moiré in print, I think the easiest solution would be creating perforated screens that are more d-cinema friendly. Movie screens typically have perforation patterns that are strong with vertical and horizontal angles. That sucks when the pixel grid pattern is horizontal and vertical. Basically, you can run into the same problem that often occurs when scanning a half-toned newspaper image (or anything half-toned) into Photoshop. The acquired image is prone to be riddled with moiré. This problem can be solved by rotating the image 15 or 30 degrees; it often disrupts the moiré pattern by taking it out of alignment with the pixel grid. Movie screen companies could do the same thing: make the screen perforations in a pattern 15 or 30 degrees off horizontal-vertical axis. I have seen ads for "video friendly" screens that merely make the perforations much smaller. But they still have the standard pattern with the horizontal and vertical alignments. Tiny screen perforations might be good for the crude 2K pixel grid. But as projectors improve to 4K or perhaps levels well beyond 4K the moiré issues could easily reappear. With the perforation pattern taken off the horizontal-vertical axis moiré shouldn't be an issue regardless of perforation size or projector resolution. I wonder if this is something I could patent?  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 03-22-2012 11:03 PM                     This might sound weird but try tilting the projector. (i.e. Turning it on its roll axis.) The idea is to stop the pixels from lining up with the holes or, at least, stopping them from lining up so often. Depending on the ratio of the size and spacing of the pixels to the size and spacing of the holes in the screen, you might only have to tilt the projector a couple of degrees. If you want to know the math for figuring out Moire patterns, check Wikipedia: http://en.wikipedia.org/wiki/Moir%C3%A9_pattern  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-23-2012 05:23 AM                     quote: Bobby Henderson I wonder if this is something I could patent? Maybe you could invent screens with some kind of random perforation .  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 03-23-2012 09:06 AM                     The sound will be very bad with speakers behind a non-perf sheet vinyl screen. Is this a "microperf" screen? Normal perfs are fine for normal cinema sized screens with decent seat-to-screen distances and "digital" video projection. The pixel size is large enough that each pixel covers several perfs and the brightness difference between 9 perfs and 10 is minor. You may see some subtle moire patterning with white field projection but it is not objectionable with normal movie images. With a small normal perf screen the problem is severe. The brightness difference between a pixel covering no perfs and one covering one or two is definitely noticeable. A microperf screen has closer spaced smaller perf holes so the pixel size to perf pitch gets back to what a larger screen would have. These perfs themselves are less visible with seats close to the screen - common in small screen venues. AMC has a lot of rooms with the speakers above the screen, and with the bass bins below the screen and the horn above. This sounds OK up to a point - most people don't notice it but if you're aware of the location you start to notice the sound coming from way up there. Even if you can't put the entire speaker above the screen, can you put the HF driver up there with the bass below or behind the screen? Screen attenuation is frequency dependent, LF goes right through and HF gets progressively attenuated as frequency rises. The screen material mass is the culprit. At low frequencies the sheet vibrates slowly so not much energy is used accelerating its mass. At higher frequencies the energy used to accelerate the sheet mass to vibrate it faster uses up more and more of the sound energy so less is left on the "outside" as actual sound. You could also use a woven screen. These offer low acoustic attenuation and are available with anti-moire surfaces or a tilted cutting to misalign the weave and the pixel axes.  |  IP: Logged Bobby Henderson "Ask me about Trajan." Posts: 10973 From: Lawton, OK, USA Registered: Apr 2001 posted 03-23-2012 01:43 PM                     quote: Marcel Birgelen Maybe you could invent screens with some kind of random perforation. That would indeed be another worthwhile solution. In printing, one can use Stochastic screening instead of an evenly distributed half tone pattern. Stochastic screening uses frequency modulation to create a somewhat randomly distributed dot pattern. This same model could be applied to the perforations of a movie screen. Some care would have to be taken with regard to the size of the perforation dots and their distribution. In print work Stochastic screening will typically use more dots than a traditional half tone pattern. With a movie screen the goal is trying to maintain the white appearance of the screen while letting the sound pass through it. The screen needs enough dots to allow this, but not so many that the screen takes on a gray tonal appearance and reduces image brightness. Like others, I think solid screens are a bad idea. AMC tried it with their Torus screens, which required speakers to be positioned above and below the screen. The solid screen in combination with the cubic nature of stadium seated theaters created noticeable problems with echo, not to mention the other audio problems that are going to be there anyway.  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 03-23-2012 01:52 PM                        The reason why this does not happen with LCoS (at least not to my knowledge) is probably that there is no visible pixel "grid" and that the space between the pixels is either nonexistant or at least sufficiently invisible as to not interfere with the screen perforations. Also, LCoS has always looked a bit "soft" to me (I don't know why this is...maybe someone can explain that), which may be part of the reason why the pixel grid is not visible. I am surprised that the screen manufacturers have not made an issue out of this (by offering screens that are designed eliminate or reduce the problem). It seems like the whole issue is the dirty little secret of the video projection industry that no one wants to talk about or even admit is a problem.  |  IP: Logged Bobby Henderson "Ask me about Trajan." Posts: 10973 From: Lawton, OK, USA Registered: Apr 2001 posted 03-23-2012 02:27 PM                     Given the somewhat commercial nature of this web site (Film-Tech sells a number of cinema products) I wonder if Brad and his F-T colleagues ever considered marketing movie screens. A moiré-proof movie screen might sell pretty well. Obvious questions: how do movie screen companies perforate their screens? I'm wondering about the process and how difficult or easy it would be to simply rotate the pattern 15 degrees.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 03-23-2012 02:38 PM                     Stewart does rotate their perf. grid to avoid Morie. Since their perfs are, more or less, human applied...they are also more random and thus don't show morie like others. Random patterns have the issue that they are MUCH more visible than a constant pattern your eye can't fixate on. MDI's original mini-perf was an example of this. Morie gets the most annoying when the pixel density is close to the perforation density. A popular misconception is that micro-perfs reduced Morie...they actually make it worse in most cases ad that puts the pixel density on the same order as the perfs. Sometimes just zooming the lens a little bit one way or the other will get rid of the offending tiger-stripe.  |  IP: Logged David Graham Rose Expert Film Handler Posts: 187 From: Cambridge, UK Registered: Sep 2002 posted 03-23-2012 03:51 PM                     Greetings All I would say no. From Sawston goodnight David  |  IP: Logged

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