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Discussion Starter #1
I'm currently in the process of researching a projector for use with an HTPC (which should come as no surprise). I will want to run at various rates especially 72Hz and 75Hz


I'm a bit confused about how all the different technologies handle different refresh rates so could you correct any mistakes in the following.


1) LCD

Handles different refresh rates properly, some may exhibit tearing over a certain frequency.


2) DLP

??????

How does the speed of the colour wheel interact with the ability to display differnt refreash rates.


3) DILA

all inputs are displayed at 75.1Hz


John
 

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Since no one answered yet, I'll take a crack at your questions:


1) LCD tend to prefer 60 Hz but they do support other rates. My cheap Sony VPL-CX1 XGA LCD ($1500) did perfectly without any tearing on this refresh rate of 60 Hz on my AMD1.3/Geforce2 GTS Pro HTPC using Powerstrip and 12.4x Detonator drivers. At 70 Hz or higher tearing can be slightly seen on pan scenes.


2) DLP may or may not have fixed frequency of 60 Hz on color wheel but will still support different refresh rate. My LT150 XGA DLP ($2300) perform flawlessly at 60 Hz but still image can occasionally be a little jittery (computer text or games). At 70 Hz, computer text is rock stable but video can exhibit minimum tearing on pan scenes. Thus, I flip mine to 60 Hz for movies and 70 Hz for computing (rare occasion as bulb is too expensive :) ). Some DLP color wheel will rev up or down depending on frequency but I think 60-72 Hz is still considered optimal. Some advocate 60 Hz for video-based DVD's and 72 Hz for film-based DVD's. I'm too lazy to flip back and forth and I think 60 Hz is perfect for 99% of my movies.


3) DILA--don't know but I assume it's similar to LCD with may be a different Hz sweet spot :)


Remember to autoadjust after every frequency change to let PJ sync. perfectly (Phase or Clock may be off otherwise giving suboptimal imaging). Monitor driver is usually "Plug and Pray" although my PC recognizes my LT150 as LT150 without installing any special drivers.
 

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Discussion Starter #3
I suppose what I'm really asking is can ecah of the display types display VGA signals properly at 72 and 75.


By the sounds of it LCD may be yes DLP maybe yes DILA no.


Has anyone tried running their LT150 at 72Hz with an HTPC are the pans smooth.


John
 

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John - The answer to your question is YES all of the display technologies you mention, LCD, DLP and LCoS, (D-ILA is a brand name for an LCoS implementation) can, given the correct front-end electronics, sync to and perfectly display 72 and 75 Hz inputs. Unfortunately, most projector specs (monitors and TVs too for that matter) tell the frequencies and resolutions the projector can recognize (sync to) as oppose to how AND how well the included front-end electronics will handle the frequencies and resolutions. :(


You asked about the LT150: I attached my LT150 to my DScaler equiped PC at 72Hz and the picture exhibited tearing. I changed to 60Hz and the picture exhibited no tearing.
 

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Another example to Huck's point is the VT540 which cannot sync 72 at all, but does very well at 60 and 75.
 

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Discussion Starter #6
Huckster


Thanks for the info.


I'm vaugely under the impression that there is (on some technologies) a difference between the front end and back end when it comes to refresh rates.


I think the LCoS system has a fixed output refresh rate that is used regardless of input rate.


I've become very sensitive to 3:2 pulldown after working in the judder geatures in Dscaler so running at 60Hz isn't really an option.


Have you tried syncing to 48Hz on the LT150?


John
 

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I only run my Davis DLS8 at 72Hz. It looks great. I haven't personally tried 75Hz, but I believe that the Davis models can sych successfully up to 85Hz. I'll try it tonight when I get home.


Tony
 

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I'll add my contribution, which is that my projector, the Davis DLX650, needed a new software revision to fix tearing ('L'). But this tearing fix only works at 60hz. So while this projector can synch at various rates, only 60hz is without tearing.


Mike
 

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I have mentioned this in a previous thread- it seems from the many threads on this subject of digital projector refresh rates that each projector model has a different "optimal" refresh rate, depending on the optimized refresh rate for its display buffer.


How do you know what a given projector's "optimal" refresh is? Basically, trial and error. Do you see tearing? Try another rate. Or find out what refresh rate is specified for calibration.


My XGA LCD projector's Service Manual tells a technician to input a 60Hz 1024x768 XGA signal from a test signal generator for all calibrations and adjustments.


I interpret this to mean that 60Hz is the designed-in optimal refresh rate for my projector's display buffer. This rate seems to have the least tearing artifacts, agreeing with this interpretation. Whether this is the optimal rate or not, I'm not sure. But I'd guess there's a reason Mitsubishi chose 60Hz for calibration and adjustment procedures on my projector model.
 

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John,


The best way to verify tearing is to write a little Windows program that just draws a tall white vertical block (say 10-20 pixels wide, and the height of the display) on a black background. Then animate it bouncing left and right across the screen at the monitor's refresh rate (make sure you sync!), moving just a few pixels per frame. I've been meaning to do this, but haven't gotten around to it.


Then hand it out on this forum and let all us HTPC users try it out at different refresh rates. This type of animation (high contrast, vertical bar) will exhibit tearing in the most obvious fashion. I suspect that with this experiment, you'll find out best which projectors can truly handle alternate refresh rates.


A lot of people don't notice tearing unless it's an extreme case, while others are very aware of it. As far as I can tell, most LCDs run at a constant refresh, and even if they support other refreshes, they are just sampling the signal at the higher rate, and displaying it at their fixed rate.


Aaron
 

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Aaron:


Great idea! I'd be willing to write this, if you want, and if you can articulate a little more what you mean. So, if you're testing 75hz, for example, you're saying to move it 75 times a second, a few pixels each time? What do you mean by "be sure to sync"? What about running at exactly the refresh rate - the actual refresh rate is rarely exactly an integer - correct?


- Dave
 

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John,


Check out the JVC D-ILA White Paper available

at:

http://www.jvc.com/prof/Attributes/c...&feature_id=12


The JVC D-ILA can handle signals with refresh

rates from 50 Hz to 78 Hz, and the projector

refreshes the D-ILA chip at a rate that is twice

the rate of the input signal.


Greg
 

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Download Video2000 (21 MB) free from www.madonion.com and it'll test video playback, moire, refresh, etc. After I used it I deleted it as it takes up too much room. I found 60Hz best for my VPL-CX1 XGA LCD and LT150 XGA DLP.
 

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Discussion Starter #14
Greg


thanks for the link


I thought DiLA was optimal at 75.1 but there seems to be no explaination of that.


the program is a good idea, I'll think how to best implement it.


John
 

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Well, the "right" way to do it would be to use DirectDraw and page flipping. You have to be careful not to introduce any artificial tearing by drawing out of sync with the monitor beam.


For example, to animate the bar you have to blank the screen and draw a new bar. If you do this in the middle of a refresh cycle, you'll introduce artificial tearing. By page flipping, you can render to the back buffer and then swap buffers on the vertical sync to guarantee you're not introducing any tearing artifacts that way.


And yes, you should definitely animate once per refresh cycle, i.e., 75 times per second for a 75Hz rate. The easiest way is to just render as fast as you can, then wait for the page flip to occur, which will be synced with the refresh.


Aaron
 

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John - I cannot convince my graphics card to output 48Hz so I haven't tried that rate. :(


There CAN be major differences between the capabilities of the front-end and back-end of projectors. Some of this comes about when one manufacturer builds the optical path (back-end) and another buys that design and bolts a front end to it. If the integrator doesn't fully understand the limits and capabilities of the back-end, there can be a BAD mismatch. By the way, a product that WE would consider a bad mismatch may sell like hot-cakes!


I know people here are fanatics for picture quality (that's why I come here) BUT many of the projectors discussed here are designed and optimized for Powerpoint. In that buisness presentation market, it is hard to describe a situation when anything other than 60Hz refresh is required or where tearing would be a product killer. Instead, LUMENS are king and picture quality (along with any other consideration) is a distant 4th or 5th. :)
 
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