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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Thu May 01, 2014 3:11 pm Post subject: Marquee Placement and Keystone effects |
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Looking back through AVS threads recently I cam across TSE's 8500LC resolving 1080p: http://www.avsforum.com/t/708814/8-does-1080p
This got me thinking how much of an effect that keystone adjustment has on both resolving power and banding. The steeper the pitch of the projector to the screen the more keystone correction is required resulting in a squished trapezoid on the tube face. This ultimately bunches up the horizontal resolution (1920) making it difficult to fully resolve the image and also requires more extensive geometry adjustments to straighten out a grid. Banding is typically introduced by geometry controls with some being more offensive than others (v bow, v key and top/bottom pin).
For those who have set up a Marquee on a test bench and had it centered vertically to a screen were you able to project a uniform image that did not require top/bottom pin cushion or linearity adjustments? Since my PJ is floor mounted I have some flexibility in propping it up to a nearly ideal position and am contemplating whether the effort to do so would yield commensurate results.
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Tim in Phoenix
Joined: 21 Oct 2006
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Location: Phoenix
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| Posted: Fri May 02, 2014 2:09 pm Post subject: |
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Say JB
Any thoughts on leaning the screen forward?
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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Fri May 02, 2014 2:45 pm Post subject: |
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| Tim in Phoenix wrote: | Say JB
Any thoughts on leaning the screen forward? |
LOL..I wish. The screen is a Stewart 4-way electrimask beast and it weighs a ton.
Did you ever do installs with zero offset?
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Nashou66
Joined: 12 Jan 2007
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the big E
Joined: 20 Apr 2013
Posts: 1928
Location: speedwell Tn.
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| Posted: Sat May 03, 2014 3:29 am Post subject: |
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I may do that when I get new home theater seating as I have my screen suspended from the ceiling
I am running my marquee with it pointed directly at the screen with the back propped up to level it out so I can use the most of the tubes
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mr_ro_co
Joined: 08 May 2006
Posts: 1643
Location: Santa Fe NM
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| Posted: Sat May 03, 2014 8:16 pm Post subject: Re: Marquee Placement and Keystone effects |
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| jbmeyer13 wrote: | Looking back through AVS threads recently I cam across TSE's 8500LC resolving 1080p: http://www.avsforum.com/t/708814/8-does-1080p
This got me thinking how much of an effect that keystone adjustment has on both resolving power and banding. The steeper the pitch of the projector to the screen the more keystone correction is required resulting in a squished trapezoid on the tube face. This ultimately bunches up the horizontal resolution (1920) making it difficult to fully resolve the image and also requires more extensive geometry adjustments to straighten out a grid. Banding is typically introduced by geometry controls with some being more offensive than others (v bow, v key and top/bottom pin).
For those who have set up a Marquee on a test bench and had it centered vertically to a screen were you able to project a uniform image that did not require top/bottom pin cushion or linearity adjustments? Since my PJ is floor mounted I have some flexibility in propping it up to a nearly ideal position and am contemplating whether the effort to do so would yield commensurate results. |
Funny you should be thinking about this, as I have been thinking the same. My ultra is hung but I have a unique arrangement that allows for a nearly direct on alignment, which I can fairly easily experiment with, as long as I can still barely walk underneath the projector without hitting my head!
Steve
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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Mon May 05, 2014 8:57 pm Post subject: Re: Marquee Placement and Keystone effects |
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| mr_ro_co wrote: | | jbmeyer13 wrote: | Looking back through AVS threads recently I cam across TSE's 8500LC resolving 1080p: http://www.avsforum.com/t/708814/8-does-1080p
This got me thinking how much of an effect that keystone adjustment has on both resolving power and banding. The steeper the pitch of the projector to the screen the more keystone correction is required resulting in a squished trapezoid on the tube face. This ultimately bunches up the horizontal resolution (1920) making it difficult to fully resolve the image and also requires more extensive geometry adjustments to straighten out a grid. Banding is typically introduced by geometry controls with some being more offensive than others (v bow, v key and top/bottom pin).
For those who have set up a Marquee on a test bench and had it centered vertically to a screen were you able to project a uniform image that did not require top/bottom pin cushion or linearity adjustments? Since my PJ is floor mounted I have some flexibility in propping it up to a nearly ideal position and am contemplating whether the effort to do so would yield commensurate results. |
Funny you should be thinking about this, as I have been thinking the same. My ultra is hung but I have a unique arrangement that allows for a nearly direct on alignment, which I can fairly easily experiment with, as long as I can still barely walk underneath the projector without hitting my head!
Steve |
Given my current screen location placing the projector so that there would be no offset could be somewhat impractical in that I would have to raise it another 28". Even though I have Tim's awesome LVPS fan mods that would place the fans right at ear level of the front row of seating and could also potentially cause some rear seating issues with visibility (though I have to check this tonight). I might be able to drop the screen 6" and that might make this a little easier.
The only way to have zero keystone is to have the centerline of the lens/tube face at the same height as the centerline of the screen as tilting the screen will not change this relationship.
I say if you have the ability to do zero offset, then you should definitely do it.
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AnalogRocks
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Joined: 08 Mar 2006
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Location: Toronto, Ontario, Canada
TV/Projector: Sony 1252Q, AMPRO 4000G
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| Posted: Mon May 05, 2014 9:46 pm Post subject: |
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I talked to Doug Basey about this, he said to be wary about reflecting the light back into your lenses and washing the picture out somewhat.
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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Mon May 05, 2014 9:48 pm Post subject: |
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| AnalogRocks wrote: | | I talked to Doug Basey about this, he said to be wary about reflecting the light back into your lenses and washing the picture out somewhat. |
So what did he recommend as being optimal?
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AnalogRocks
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| Posted: Mon May 05, 2014 9:50 pm Post subject: |
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| jbmeyer13 wrote: | | AnalogRocks wrote: | | I talked to Doug Basey about this, he said to be wary about reflecting the light back into your lenses and washing the picture out somewhat. |
So what did he recommend as being optimal? |
He said there was a reason for the projection angle and to maintain such an angle that the light will not be reflected directly back into your lenses. So maybe lessen the projection angel to 2-3 deg.?
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ecrabb
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Joined: 13 Mar 2006
Posts: 15909
Location: Utah
TV/Projector: JVC RS40, Epson 5010
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| Posted: Mon May 05, 2014 10:09 pm Post subject: Re: Marquee Placement and Keystone effects |
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| jbmeyer13 wrote: | | The only way to have zero keystone is to have the centerline of the lens/tube face at the same height as the centerline of the screen as tilting the screen will not change this relationship. |
I think I know what you're getting at, but your statement is inexact at best, and not accurate if it's taken at face value.
Zero keystone just requires an orthogonal (perpendicular, so the tube face/lens is parallel to the screen surface) projection, with the green lens centered on the center of the screen. I think that's what you meant to say, but the "height" of the projector is really irrelevant. It just so happens that in most rooms, the screen puts the projector at head height, right behind the main viewing position.
But, if you wanted to tilt the bottom of the screen out, so the screen was about 10-degrees up toward the ceiling, it would put the projector back on the ceiling. It would look terrible, but it would work.
By the same token, a floor-mounted, tilted-up projector could yield a zero-keystone install if you also tilted the screen away from the wall at the top, downward, to be parallel with the lenses.
They way I always think of it, is with the screen mounted on the wall, plumb, and the projector perfectly centered, and tilted for zero keystone. Now, imagine a virtual framework that kept the screen and projector locked together so no angles can change, then imagine being able to move that "assembly" into any orientation you want.
Just wanted to add some clarity in case somebody was reading and learning.
Cheers,
SC
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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Mon May 05, 2014 11:54 pm Post subject: Re: Marquee Placement and Keystone effects |
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| ecrabb wrote: | | jbmeyer13 wrote: | | The only way to have zero keystone is to have the centerline of the lens/tube face at the same height as the centerline of the screen as tilting the screen will not change this relationship. |
I think I know what you're getting at, but your statement is inexact at best, and not accurate if it's taken at face value.
Zero keystone just requires an orthogonal (perpendicular, so the tube face/lens is parallel to the screen surface) projection, with the green lens centered on the center of the screen. I think that's what you meant to say, but the "height" of the projector is really irrelevant. It just so happens that in most rooms, the screen puts the projector at head height, right behind the main viewing position.
But, if you wanted to tilt the bottom of the screen out, so the screen was about 10-degrees up toward the ceiling, it would put the projector back on the ceiling. It would look terrible, but it would work.
By the same token, a floor-mounted, tilted-up projector could yield a zero-keystone install if you also tilted the screen away from the wall at the top, downward, to be parallel with the lenses.
Cheers,
SC |
I was referring to the height of the projector in part because of the visual and audio reprecussions. If the centerline across the screen is 48" from the floor then the center of the green lens also needs to be 48" from the floor and parallel to the screen surface to truly have no keystone; correct? However, as jeremy mentioned that could result in light being reflected directly back at the pj. I'm trying to figure out where the ideal location of the pj is for a floor mount to minimize keystone but not have negative ancillary issues. Thoughts?
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CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 6326
Location: Australia
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| Posted: Tue May 06, 2014 12:04 am Post subject: |
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| AnalogRocks wrote: | | I talked to Doug Basey about this, he said to be wary about reflecting the light back into your lenses and washing the picture out somewhat. |
It touches on something simular in the NEC installation manual, however no reason is given. Rear projection setups are suggested to be 0° in that manual, but they dont show it that way or suggest it as an option for front.
I generally always aim to get the angle low as possible, but i havent ever been able to get to complete 0° to see reflection issues.
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ecrabb
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Joined: 13 Mar 2006
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Location: Utah
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| Posted: Tue May 06, 2014 5:37 am Post subject: |
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| jbmeyer13 wrote: | | If the centerline across the screen is 48" from the floor then the center of the green lens also needs to be 48" from the floor and parallel to the screen surface to truly have no keystone; correct? |
Yes, but that holds true only if the screen is plumb (parallel with the wall). If you tilted the screen down (out at the top) about 10-degrees, then the projector is getting close to the floor. Somewhere between 10 and 15-degrees, it actually is on the floor.
| jbmeyer13 wrote: | | However, as jeremy mentioned that could result in light being reflected directly back at the pj. I'm trying to figure out where the ideal location of the pj is for a floor mount to minimize keystone but not have negative ancillary issues. Thoughts? |
I'd imagine the reflected light would be a function of screen gain, screen size, and the throw angle. High-gain, small screen size, and no throw angle would be the worst case scenario. That would put a lot of light back into the lens. Lambertian low-gain (~1.0), a larger screen, and the typical 10-degree or so worth of throw angle is probably the best case, not putting much light at all back into the lens.
The NEC manual probably mentions the 0-degree offset with rear-projection and not with front because with rear-projection, very little of the light is reflected. Rear-projection material is designed not to reflect the project light and instead diffuse it. I don't remember what typical materials do, but I was thinking something like only 15-20% of the incident light is reflected.
Like all engineering, this hobby is a series of compromises. TSE's test with zero keystone is great with respect to resolution, but ignores any other issues related to zero offset in an actual room. OTOH, large offsets would make for large keystone and therefore loss of resolution on the top of the screen (with a floor-mounted projector).
We know the projector would be in an undesirable location with the screen plumb and zero keystone. We also know the screen would need a ton of tilt to put the projector close to the floor with zero keystone. We can test the theories easily without physically moving anything, though. Let's work it the other way… What's the maximum height you'd like to have the projector mounted, say center of lens? Also, what's your screen size and throw distance?
SC
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CasetheCorvetteman
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| Posted: Tue May 06, 2014 7:53 am Post subject: |
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Crabb id never given it any thought prior to this thread but id say that is exactly why they suggest angled front projection, ive got no idea what sort of effect youll get, but the angle given in the diagram seems to sugest the light will be better angled toword the viewing area rather than back at the source.
In my case it works very well for me to have it installed with very little angle due to the ceiling being lower at the projector end, the halos around bright objects do seem to be a little worse though, whether or not this is related i wouldnt know.
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jbmeyer13
Joined: 03 Dec 2010
Posts: 1135
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| Posted: Tue May 06, 2014 4:00 pm Post subject: |
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| ecrabb wrote: | | jbmeyer13 wrote: | | If the centerline across the screen is 48" from the floor then the center of the green lens also needs to be 48" from the floor and parallel to the screen surface to truly have no keystone; correct? |
Yes, but that holds true only if the screen is plumb (parallel with the wall). If you tilted the screen down (out at the top) about 10-degrees, then the projector is getting close to the floor. Somewhere between 10 and 15-degrees, it actually is on the floor.
| jbmeyer13 wrote: | | However, as jeremy mentioned that could result in light being reflected directly back at the pj. I'm trying to figure out where the ideal location of the pj is for a floor mount to minimize keystone but not have negative ancillary issues. Thoughts? |
I'd imagine the reflected light would be a function of screen gain, screen size, and the throw angle. High-gain, small screen size, and no throw angle would be the worst case scenario. That would put a lot of light back into the lens. Lambertian low-gain (~1.0), a larger screen, and the typical 10-degree or so worth of throw angle is probably the best case, not putting much light at all back into the lens.
The NEC manual probably mentions the 0-degree offset with rear-projection and not with front because with rear-projection, very little of the light is reflected. Rear-projection material is designed not to reflect the project light and instead diffuse it. I don't remember what typical materials do, but I was thinking something like only 15-20% of the incident light is reflected.
Like all engineering, this hobby is a series of compromises. TSE's test with zero keystone is great with respect to resolution, but ignores any other issues related to zero offset in an actual room. OTOH, large offsets would make for large keystone and therefore loss of resolution on the top of the screen (with a floor-mounted projector).
We know the projector would be in an undesirable location with the screen plumb and zero keystone. We also know the screen would need a ton of tilt to put the projector close to the floor with zero keystone. We can test the theories easily without physically moving anything, though. Let's work it the other way… What's the maximum height you'd like to have the projector mounted, say center of lens? Also, what's your screen size and throw distance?
SC |
I was taking measurements last night to determine maximum tolerances and here's the specs:
-Screen is 49" x 87" ST130 AT Electrimask-Vertically plumb to the wall it hangs on; no tilt
-Screen centerline height is 47.25"
-Projector lens is 117" from the screen and center height is 21"
-I'm thinking to raise projector another 10"center so lens center is at 31"
I used the calculator in this link: http://easycalculation.com/trigonometry/triangle-angles.php
Here's a quick sketch of what I'm thinking
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ecrabb
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Joined: 13 Mar 2006
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| Posted: Tue May 06, 2014 4:59 pm Post subject: |
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| CasetheCorvetteman wrote: | | In my case it works very well for me to have it installed with very little angle due to the ceiling being lower at the projector end, the halos around bright objects do seem to be a little worse though, whether or not this is related i wouldnt know. |
I suppose since the screen is on the focal plane, the screen would put some light back on the phosphor. Think of the projector as also like a camera; the screen is the "scene" and the phosphor becomes the film.
An even better comparison for us old farts that ever did traditional darkroom photography is an enlarger. A zero-keystone projector is working exactly as the enlarger did. The major difference is that the enlarger's primary focal point (the film) was very dark object while the projector's primary focal point is white phosphor. The enlarger also typically had very slow, very sharp lenses; the CRT projector not so much.
The only time I ever saw obvious, horrible halos was with my 12xx, which is air-coupled HD-8's. Literally if there was a bright point-source light like a moon on the screen, there would be a giant halo about 3x the size, at about 10% intensity, seat-of-the-pants numbers, of course. The G70 was much less noticeable; so much so that it almost seemed like my eyes doing it rather than the projector.
I'd guess most of your haloing is probably the CRT's optics itself. ANSI contrast, by far, is the CRT projector's biggest achilles heel. Huge lens, and a large, white imaging surface.
Should be easy to test, though. Use your HTPC and load up an all-black image with a big white dot. First, look at at on the top part of the screen where it should be reflecting back into the lens, then move the big dot to the bottom of the screen where most of the light would be reflective down toward the viewing position and see whether the halo is any different. If it's not significantly different, it's the CRT optical system itself; if it's much worse up high, then it's the light bouncing off the screen and back into the lens and lighting up the phosphor.
Cheers,
SC
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stridsvognen
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jbmeyer13
Joined: 03 Dec 2010
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| Posted: Tue May 06, 2014 5:39 pm Post subject: |
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| stridsvognen wrote: | http://www.curtpalme.com/docs/ElectrohomeMarqueeUltra8500_8500LC_9500LC_User.pdf
2-10 and 2-11
Here the recomended angle to the screen is 10, and max 15. maybe its a good guide.
Also notice the tilted screen and where the reflection hits. Ill like to try this.. As i have done like most, just put the screen flat on the wall. |
Zero keystone (lens angle matching the screen angle and lens center height at screen center height) results in light bouncing back into the lens which is not optimal. The manual is vague on the screen tilt (>15 degrees) and/or benefits of implementing such a solution. If I place a 5" shim at the top mounting brackets on my screen that would equate to a 5.8 degree tilt angle. Based on my proposed projector/lens location that would yield a delta of 2.2 degrees (8 degrees - 5.8 degrees=2.2).
I'm thinking that tilting the screen would result with some light being bounced back into the lens and that it would be better to go with my proposed solution in the diagram above.
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stridsvognen
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| Posted: Tue May 06, 2014 6:05 pm Post subject: |
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there is only 1 way.. test all options, and go with the best result. 
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