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Discussion Starter · #1 ·
Just a warning for those considering this combination.


I have an LT150 mated with a 16:9, 8' wide microperf Grayhawk. The moire is pretty bad. Don't know how to quantify, as I have never seen it before, but it is far more bothersome to me than the rainbow.


Different size screens should change the effect.


jeff

 

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Just as a matter of interest, did the dealer who sold you the screen warn you about possible Moire with a digital projector? Such microperf screens have long been reccomended for CRT projectors only, and you should have been told this.


Gary
 

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Jeff,


I am actually a little surprised to see this, moire is most commonly found on high resolution LCD projectors because the more visible screen door interacts with the perfs. I didn't expect to see a DLP with the same problem!


If it is moire you will see different coloured bars on the screen, the bars will probably be curved and (in my experience) they run vertically. What will really expose the problem is moving the projector back and forth. If the pattern changes (waves shifting and changing colour) when you move the projector it is likely that you are seeing moire.


The solution is usually to get a Cygnus IMX de-pixelating lens which will also help with any visible screen-door. You should definitely give Stewart support a call because this is not normal, they are aware of this problem with the Sony 10HT (although this is the first occurrence I've heard of with the LT150) and they should be able to help you. The Panamorph or ISCO should also help reduce the effect (or possible eliminate it), if you were planning on getting an anamorphic lens.


Regards,


Kam Fung
 

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Discussion Starter · #4 ·
When I purchased the screen, my intent was to use an 8" CRT I had purchased used. Unfortunately this projector didn't work out (long story). So the vendor didn't have the opportunity to warn me.


I am fairly certain this is moire. The lines I see are horizontal though. Very hard to describe, but look like fairly narrow (about 2-3") wavy lines across the screen that are somehow different in brightness. Moving your head back and forth on the horizontal plane accentuates the problem.


I did call and discuss the situation with Stewart support (which is great!). They don't have an LT150, but did reproduce it with an LT154. THey were aware that it's an LCD and a DLP is likely to be different.


I considered the Panamorph, but it's about as expensive as the projector, generates other geometry issues, can be difficult to set up, and is not readily available.


The IMX lens is also expensive (about $900), and there is no model with direct support for the LT150. One of their existing models may work.


I finally decided to give up the microperf. I'm likely to upgrade to a 16:9 native DLP in a year or so, and would hate to see the problem continue. Unfortunately, this means center channel below the screen.


jeff
 

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I think it's a good move - microperf screens do in fact attenuate the critical midrange and high frequencies that give clarity to dialogue. Commercial theaters use screens with larger perfs to minimize attenuation, and the extra punch of horn type speakers to overcome such attenuation as remains. Unfortunately, the smaller the perfs, the more absorption of sound - I don't like the way microperf screens make most speakers sound in a home theater.


Be aware that most horizontal format center speakers are a compromise intended for RPTVs and large direct-view sets. The horizontal driver layout tends to cause undesireable frequency-response variations due to driver cancellation effects. The net of this is a horizontal format speaker sounds different from a vertical format speaker, even if it is constructed from the exact same drivers. I use and advocate three identical vertical format front speakers, either directly below or above the front projection screen, and tilted to aim the tweeters directly at the viewer's heads. This works out because your binaural hearing localizes sound in a horizontal plane quite well, but hardly at all vertically. I find that once you darken the room, the sound is localized to the screen quite well.


Gary
 

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I have found that good receivers and pre amps can provide wonder full phantom sound(no center). Keep in mind your front speakers will be performing 140% their normal 5.1 load and must be of exceptional quality to handle the increased complexity of the signal. You also must remain in between the speakers, preferably within the screen boundaries or you will notice a shift away from center(the real benifit of a center speaker). Another solution is to use a pair of center channels on each edge of the screen, same rules apply and you benifit by doubling the center channels output. Use a pair matching the L+R speakers. I really enjoy the uniformity of tonal qualities you get when using a well matched set of speakers, also any change in speakers within a system cause jumps in the sound when it pans between the unmatched speakers.


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Nick Ozanich
 

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I have an XGA DLP projecting onto Da-Lite AudioVision microperf material (about 100" wide) with no moire effects. The attenuation actually was not bad at all, and I was able to compensate for the very slight roll-off using the equalization controls on my Sony 777ES receiver. It sounds great and the speakers are completely out of view on a wall-mounted shelf behind the screen.
 

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I had settled on getting a Stewart microperf screen for my home theater which I am about to begin construction of. Projectors which I am considering include the Seleco 200DM and the Sanyo PV60. Based upon the above comments, I wonder if I should reconsider. Does anybody have any experience with either of these screen/projector combinations?
 

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Just the general comment that microperf screens do in fact audibly affect the sound. Since the average HT speaker has smoother and wider frequency response than the commercial theater speakers, the differences are sometimes dramatic. I don't know if any digital receivers allow frequency equalization of only the speakers behind the screen and not the others, anyway I've never seen such a feature. Boosting the attenuated frequencies to the front speakers would accentuate the same frequencies in the surround speakers, is my guess.


The audio is very important to me, so the microperf is not a compromise acceptable to me, as I mentioned before the speakers dissappear in a darkened room. Also, I have tried the phantom center channel and it can work very well, but the "sweet spot" got really small in my smaller room, there really was only one seat that got balanced front sound. Of course my screen is only 72" wide and the RF and LF speakers only that distance apart - larger rooms would have larger "sweet spots".


If you (or your spouse) prioritize the benefits of microperf above the audio, you may be able to largely compensate for high frequency screen attenuation by removing the speaker grilles from the speakers behind the screen, and leave them on the surrounds - if you are lucky, the sound may more or less balance out.


Gary
 

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I am getting ready to begin construction on my theater.


It will feature a 120" wide 2.35 aspect MicroPerf (Stewart) with a digital projector of some type (construction will be about 10 mos so I have time to consider the PJ).


Anyways, sound is an important consideration.


Did I read that Stewart makes some sort of EQ designed for their perforated screens? Anyone know?


Otherwise a parametric EQ would undoubtedly do the trick.


On the video side, this moire 'thing' associated with the digital PJs worries me. I wonder if it is only associated with the PJs of lesser cost or smaller color wheels?


Jeff


[This message has been edited by thebland (edited 09-25-2001).]
 

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THX certification requires each micorperf screen ship with a center channel EQ that compensates for the audio attenuation caused by the screen. Also, the EQ has a 12vdc trigger to turn the EQ on/off automatically for retractable screens installations.


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Randy

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Randy, it sounds like you know the answer! Are you saying that what is required for a microperf installation is a THX-certified receiver and speakers? Also, must you only put the center channel behind the screen, or can you put all three front speakers behind as in a commercial theater?


Gary
 

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Gary, you don't have to use THX speakers and/or receiver to use the THX-EQ supplied by Stewart Filmscreen with THX-microperf screens. However, you must be able to insert the EQ between the line-level output of your surround processor and your amplifier; the EQ has one RCA input and one RCA output.


The EQ is one channel only. I don't see why you could not specify two additional EQs with your screen order if you intend to place all speakers behind the screen.


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Randy

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Good to know. Thanks Randy.


Just to add, I believe THX recommends that all three front speakers be in the same plane. Thus, three of those EQs would be required if all fronts are behind the screen.


Any idea of cost for these things?


Jeff
 

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OK, I guess I couldn't use that solution - my surround receiver is not THX certified, nor does it have such line level outputs and inputs. The solution you describe sounds more like one applicable to a seperate processor and power amps. I'll keep it in mind for the next house when I build the HT room from scratch. Thanks!


Gary
 

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I"ve been using an LT-150 with a 122" wide Grayhawk Microperf THX screen. The screen does come with a unit to try to equalize the effects of the screen.

My theater was designed by Dennis Erskine and I asked him about the EQ. He said most people don't even bother using it. My room is nearly done but my audio and video set-up is still in a temporary hook-up.


I have a Proceed AVP processor and AMP2 powering 2 Legacy Focus speakers for the front. I currently don't have a ceenter channel but I am contemplating a third Focus for the center or going with 3 of something else. At any rate, I have not noticed any significant loss of sound quality with the microperf screen. My speakers do sound different but that is because they are in a completely treated room. Thus, I have no complaints with the sound effects of the screen. When I get my system finally completely set-up, I'll try the Stewart EQ in it to see what changes occur.


As for the moire effect, I have not seen any of the effects described. However, I am a novice in the video realm and have yet to experiment much with the projector.
 

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I agree with you. I have a very identical setup (DaLite 16:9 Audiovision Microperf screen with about 100" wide, SIM2 XGA DLP HT250)


Everything is fine! (And I need it, because I can't position the speaker in front or besides the screen)


Best

Visar
 

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Discussion Starter · #18 ·
The effect is dependent on the width of the screen. My screen is 8 feet wide. It may be that at 122", the effect is gone or minimized.


The easiest way to see it is to put up a large white field - use Avia or an HTPC. Stand at viewing distance. The image should be a very flat and even white. The moire pattern is evidenced by wavy lines of varying brightness, almost like wavy hotspotting.


Are you getting adequate brightness? That's an awfully big screen for this projector with the brightness loss from the Grayhawk and microperf..
 

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My speakers are behind the microperf screen. You must remember that there are sound effects from a non-perforated screen. I'm not sure of the exact effect, it might be slap echoes. Thus, one way or another a sizeable screen, micro-perf or not, has some sonic effects.


My room is a dedicated theater room and has no windows. The screen frame is the Velux black and the fabric of the entire front wall is black as is the carpet on the proscenium. The wall fabric is dark blue/plum. The room is very dark and I think this helps with the LT-150 which has 800 lumens. I gave the unit a try because of the Dell deal. I can always use it in another area of my house, for work or sell it. Truth be told, I probably will upgrade the PJ as soon as I think something significantly better is available at a decent price. For now, the picture is perfectly acceptable even though I'm running a non-progressive DVD player via composite. I have 2 RGBHV lines run in the room but not where I can use the LT-150 in a temporary mode. I'm also close to ordering an HTPC thru cellarcinemas (anybody have + or - feedback on them?). I hope the picture will improve a great deal with those 2 changes.
 
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