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Discussion Starter · #1 ·
As described in measurememnts here by Peter Putnam:
http://www.projectorexpert.com/pages/crtalive.htm


"Contrast readings on the VPH-G70Q were on a par with the other projectors, exceeding 80:1 - 100:1 on

half-white/half-black fields and over 200:1 peak."


How can this be?! I'm certain my RPTV, while smaller, has much blacker blacks and much brighter brights than the LT150 (600:1 contrast as measured by Thumper) and Sharp 9000 (claimed CR of 1100:1, and pretty close to my eye after seeing the LT150).


Right after seeing the LT150 I went home and put the same DVD on as I had just seen. The blacks were almost completely so, and the same scene from Twister that had me wondering if it was overcast or sunny on the LT150, was brightly sunlit on my set.


What's going on here? Is there something very different about the way the measurements were done?


Thanks


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Noah
 

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Contrast ratios are notoriously difficult to perform accurately and the method greatly alters the results. Peak contrast ratios are done by measuring light levels with a small white are in the image vs the black background. This tends to differ from ANSI contrast ratio which is done using a 16 rectangle checkerboard pattern which has basically half the screen at white and half at black. The number give by the two techniques can be dramatically different and is affected by projector type.


CRT's can concentrate a huge amount of beam energy into a small patch but not light up half the screen with the same intensity. Thus on a CRT the full on/off or small patch contrast ratio can be very high. On an ANSI measurement the CRT must light up a larger screen area and this often means that the peak beam current isn't as high so the white isn't as bright. The larger proportion of lit up screen also means that light scatter effects tend to lighten the black background. This lowers the measured contrast ratio.


Digital projection will show the same lumens for white no matter what proportion the area being illuminated. This means that the difference between on/off or small area vs ANSI measures tend to be closer together than with CRT. There are still light scatter effects so you still expect ANSI measurements to be lower in ratio.


What is still missing as far as information is the absolute light level for full off black. Without that piece of information the contrast ratio is missing the baseline from which the ratio rises from. You can't just take the quoted maximal light output and multiply by contrast ratio to arrive at the all off light level. The way the numbers are measured simply doesn't work that way unless the measurements where done on purely all on/all off screens.


So basically, it's really tought to draw any good comparisons unless one knows the methods being used, how well the testing environments is controlled to eliminate ambient room effects, equipment light senstivity at very low levels, type of testing done (on/off, small spot, ANSI), and light level of total black. You simply don't get all that information from the manufacturers and despite the advent of ANSI contrast ratio measurement there are still too many variables to make really good judgements based on different published specs.



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Guy Kuo
www.ovationsw.com
Ovation Software, the Home of AVIA DVD


 

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Noah, you keep referring to a contrast ratio of 600:1 for the LT150. I am now running it with the white segment ON for the good DVD prints and according to the post quoting Thumper the contrast ratio in this mode is actually 1000:1!

This puts it pretty close to the new 16:9 projectors, since I assume they measure contrast ratio using the same method.


From what you've posted recently I don't believe you're going to be happy with 2000:1.


This is just a suggestion and in no way is it meant to be inflamatory. I would try different screens with the LT150 (or any other projector) and calibrate the room to eek out the last bit of black level and shadow detail. Don't rely 100% on your calibration disks - I have contrast set just above default and brightness a little higher than that and the image is in everyway * in my environment as good or

superior to my Toshiba RPTV, and of course much larger.


* some DVDs require some changes to contrast and brightness to match the shadow detail of my RPTV, which I use as reference - it faces opposite to my projector screen so I can turn sideways and watch both almost simultaneously.


I guess my point is that we all hope for better units, but let's fine tune what we have - this is an interesting area that lots of forum members aren't aware of or have a negative mind set towards - ie. don't believe it can make that much difference. It can and does!!


Cheers,


Grant
 

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The proof in the pudding is that CRT projectors generally have a much more believable looking image, whereas digital projectors noticably have a tendacy (read me being very polite here) to not have a believable looking image.


Anyone can incorrectly state a measured contrast ratio,and if you in any way believe that a manufacturer of digital projectors is going to make their gear look bad..by doing ameasurement RIGHT, then you are fooling yourself. Of course, the same goes for CRT projector manufacturers. I'm not being partial here. You merely have to look at both typesof units under optimized conditions, and see more than one example of each type. Then, the truth will be easy to see, literally. The CRT has the much more believable image.


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Ken Hotte

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A three chip DLP has a better contrast ratio simply due to the more simplified light path. Hell, a person with a pair of glasses (and modern glasses for people are amongst the best insofar as problems with reduced reflectivity) has contrast ratio problems due to light scatter, surface impedances, interactives, etc.


A LC coupled lightpath, with proper inner surfaces, and polarizer plates throughtout, would go ALONG WAY towards making a digital projector exceed the contrast ratios that CRT has. The overall simplicity and directness of the CRT transmissive sitauation goes along way in creating the better contrast ratio that it sports. The equvilance of the system to that of 3-chip dlp is evident...and show this in the improved contrast ratio of the 3-chip. Less glass, cleaner, more simplistic light path. LC pathways would go far in achieving this greater ratio.


If one where to do a mod on a cheap LCD projector,and just for the heck of it.... seal the lightbox, and LC couple the thing..they would get a STAGGERING increase in the contrast ratio. (as well as gain heat problem immunity due to LC coupling, as wel as eliminate dust problems to a large extent. of course, the big problem become the proper collumnation of the light. It would actually require a complete re-design. oh well. perhaps a 3-chip DLP mod?)Think about it. it would be an easy and cheap experiment. Use 'impedance matched' glycol mixtures, of course.



Have fun.


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Ken Hotte

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[This message has been edited by KBK (edited 09-13-2001).]
 

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Welcome to the next religion. First it was tubes vs solid state, then it was vinyl vs CD, now it is LD vs DVD, and CRT vs digital (or 'lamps' as the 3 gun types lovingly call us).


In my opinion, the marginal improvements offered by CRT (and yes I have seen a few, and no, please don't bring up the G90 paired with a Faroudja 5000 every time - compare products in the same price range), fade to black when compared with the convenience (installation and operation) of 'lamps'. And isn't suspension of belief the name of the game when watching the Fifth Element anyway?


Note - the above is my opinion and is not to be interpreted as gospel. I'll leave that to our analog brethren.



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Sundar

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Discussion Starter · #7 ·
Guy,


Thanks very much for the cogent explanation of contrast measurement issues. I just won't pay attention to them.


Grant,


I'm simply mystified how your LT150's blacks can be better than your Toshiba RPTV's, and by another post stating that the Sharp 9000 did very well on the hand shadow test. Both are very different than what I saw.


Ken,


Sorry to be your nemisis yet again, but:


Passing through a single color filter seems a lot simpler than all the glass involved in splitting the light into 3 colors and recombining 3 optical paths back into one.


I think the root of the problem is not the optics but the fact that the lamp is always on.


And the cinema 3-chip DLP I saw at the San Jose Film Festival had lousy blacks.





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Noah
 

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Since contrast ratio is in fact a ratio of bright over dark, it stands to reason that the very-much-brighter digital projector has the better contrast ratio.


I thought CRTs were known for the black levels - in theory a properly adjusted CRT has a completely zero light output in a dark image area (the electron beam current is cut off). The best digital projector still leaks a little light in dark areas, especially LCDs.


As to whether CRTs or DLP/LCD/DiLAs are better, it depends on how fanatical you are. In a dedicated and purpose-built home theater space, with dark paint and fabrics on the walls, little light is reflected off the furnishings and walls back onto the screen. However, the few real home theaters I have seen strike me as very atypical living spaces - dark walls/floors/furniture, sloping or platformed floors, etc. It's a dedicated space for a specific purpose, and I think a CRT is the best choice for such a room. But I find such spaces uncomfortable and unsuited for conversation and other social activities.


On the other hand, a comfortable furnished room has typically got lighter, even off-white walls, light carpeting, and light colored furniture. In such a space, the blacker blacks of the CRT get washed out by twice-reflected light from the furnishings. I believe in such a multi-purpose space, the very-much-brighter digital projectors deliver better contrast ratios - or spoken another way, tolerate the twice-reflected light with less apparent washout of the blacks, because the whites are so much brighter.


My first projector was a CRT - but I had it in my basement, a cool, slightly moist area with dark wood paneling, chocolate brown rug, and black-painted ceiling. Kinda like a cave.


I keep my current LCD projector in my living room, with comfortable furnishings in a nice open floor plan which makes ambient light control difficult. It has a fireplace, paintings, bookcases, draperies, and attractive furniture. It's comfortable and the exact opposite of the cave.


Although I may not have the blackest blacks going, I have many more houseguests for movies and it's a lot more fun - I could never go back to the dark basement cave.


Gary
 

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Ken, for the sake of argument let's just say that CRT wins in every category. Now with that out of the way, how does one increase black level and contrast ratio with digital projectors? If TI can continue to refine their technology and manufacturers follow Thumper's lead we will see further improvements in the next few years. Until then select the projector that takes you the closest to your desired

"black" level and optimize your viewing environment.


Cheers,


Grant
 

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Noah, I am not saying that the digital projector has better "blacks" than my Toshiba RPTV (as good as or better in every way). I just wonder what my RPTV image would look like if I could blow it up to the size of the projector's image? I somehow doubt that it would hold up all that well. I think I'll try to reduce the projectors image to the size of the RPTV's and see what it looks like.


I also don't think that most DVD transfers call for absolute black very often (I could be wrong). If they don't then the whole argument over blacks may be somewhat moot (at least until the transfers get better)! Seems odd to me also that the comparison between the new Sharp and the Runco CRT didn't reveal obvious differences in black level - maybe these differences are much more apparent on test material as opposed to DVDs. Then again maybe it's a difference in expectations.


Cheers,


Grant
 

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There seems to be a misconception that has taken hold on these forums.

That being that a CRT projector can only be used in a room with a very dark interior. Because if the walls and ceiling aren't dark all this light will reflect back to the screen and wreak havoc with the contrast ratio and black will no longer be black.


I've been using CRT projectors in the same room for years.

My ceiling is white and my walls are paneling (with a color somewhere in the middle between light and dark). My carpeting is beige.

I have never ever experienced a problem with reflected light. Any light reflecting from my room interior is so dimly visible at the screen that it's a non-issue. Black in the letterbox bars (and in the image) still appears to be

black.


Bob



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~ The Sultan of Cheap ~
 
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