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Discussion Starter · #1 ·
I have LCD PJ(XGA) and drive by HTPC.

I have set my HTPC refresh rate to 72Hz for the best movie watching.(according to the forum folks), but I want to know the truth.

Is it true that all digital PJ (especially LCD ) actually can’t display more higher than 60Hz refresh rate?

Is it true that although your digital PJ can accept 72Hz refresh rate (or various refresh rate), actually it convert internally to 60Hz ? And display it in 60Hz?

If all fact above is true, what display will actually benefit from 72Hz refresh rate?
 

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Bubun...


I've heard the same thing! I'm also interested in knowing if somebody has the details on this...


The following is what I think is going on -- but I may be wrong!


An LCD or even a DLP isn't like a CRT at all... CRTs have scan lines and a moving electron beam. An LCD has just a bunch (well, a LOT) of individual elements that all display their appropriate value at the same time. So the fastest the LCD can switch all elements from one value to another would limit what the refresh is.


However, I would argue (although I may be wrong!) that refresh for an LCD is a different creature than refresh for a CRT. Low refresh for a CRT causes you to see flicker because you can actually notice the screen being drawn... remember it isn't all drawn at an instant like an LCD.


LCD would be (and I'm using a . as meaning time and WACk as meaning the image is suddenly displayed)


...................WACK...................WACK.............. .WACK


While a crt would be (where draw is the electron beam movig and done means all the scanlines have been drawn)


draw_draw_done_draw_draw_done_draw_draw_done


So in short a lower refresh for a LCD may not be as bad as a lower refresh for a CRT.


(Please remember I'm making this up from what I think is happening, I'll happily stand corrected if somebody states otherwise...)


...MadMyers
 

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Discussion Starter · #3 ·
Thanks for your explanation.

But it doesn't answer my curiosity.

Is it true that all digital PJ convert all refresh rate to 60Hz?
 

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QQQ - that post only refers to DLPs. For LCD it's quite a different problem as the LCD crystals are physically incapable of refreshing as fast as 60Hz even. So what happens is that either you get tearing - or the internal circuitry of the projector drops frames in order to keep the synchronization.
 

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I'd asked that here , too, a few weeks ago. I got some answers indicating that 60 Hz was *it*, but nothing very detailed.
 

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It depends on the projector. Most DLP's and LCD's convert all incoming refresh rates to 60Hz so will always display 3/2 judder, even if you use an "ideal" rate like 72Hz (which is a perfect multiple of film's 24fps).


Some DLP's have a variable speed colour wheel so can sync to a range of refresh rates.


D-ILA's can sync to any rate between 48Hz and 85Hz and display it without converting to 60Hz.


Cheers,

Dave.
 

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Hi,

Dave is right. Most of the DLPs I have tested lately can only display 60Hz. They would accept other refresh rates but convert it to 60Hz and therefore show the typical 3:2 artefacts. There are a few exception however.

All SIM2 DLPs can display 50Hz (very useful for PAL) without conversion.

At the moment I am testing an HT250. This one can display 50Hz. It can also display 47,952 and is VERY smooth with NTSC HOWEVER you end up with very subtle but still annoying tearing (a thin line moving from the bottom to the top very slowly when there is a pan sideways). In theory the machine can display double frame rate for NTSC film but I will try to find out with SIM2 tech support whether the tearing can be tackled.

Only alternative: display NTSC at 60Hz which is only ideal for NTSC video (bummer).


Regards

Christoph
 

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Discussion Starter · #9 ·
Quote:
Originally posted by clehner
Hi,

Dave is right. Most of the DLPs I have tested lately can only display 60Hz. They would accept other refresh rates but convert it to 60Hz and therefore show the typical 3:2 artefacts. There are a few exception however.

All SIM2 DLPs can display 50Hz (very useful for PAL) without conversion.

At the moment I am testing an HT250. This one can display 50Hz. It can also display 47,952 and is VERY smooth with NTSC HOWEVER you end up with very subtle but still annoying tearing (a thin line moving from the bottom to the top very slowly when there is a pan sideways). In theory the machine can display double frame rate for NTSC film but I will try to find out with SIM2 tech support whether the tearing can be tackled.

Only alternative: display NTSC at 60Hz which is only ideal for NTSC video (bummer).


Regards

Christoph
What kind of test do you perform ?

What tools/programs do you use for this test?

I want to know my PJ capability, hope this test also can be done by myself.

:)
 

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Hi,

a simple way to get a feeling for 72Hz is to use your PC-Monitor and set

it to 72Hz and watch a film with some extended pans sideways.


Then switch it to 60Hz and watch how the stuttering increases (also known

from 3:2 pull down).


Then try to do the same thing with your projector and watch carefully how it

performs. Note: Some DVDs contain stutters of their own so it's alway a

good idea to check with your PC monitor first.


Hope that helps


Regards

Christoph
 

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I think the "pixel perfect" settings for the projector should also be a good indicator. For example, on the Sharp 9000 I believe you have to feed it a [email protected] signal to bypass the internal scaler. This would lead me to believe that the internal scaler converts everything to 60Hz.
 

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Quote:
Originally posted by GScott
This would lead me to believe that the internal scaler converts everything to 60Hz.
Hi,

that's correct. There is an ongoing 'fight' between the customers and Sharp here in Germany (I have to call it that actually) where the users would of course like to have 50Hz (native PAL refresh rate) via VGA to be able to feed the Sharp with a decent HTPC signal.

Maybe Sharp will be able to provide a software update in the near future, although it does not look very good at the moment.


Regards

Christoph
 

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Discussion Starter · #14 ·
Hi Christoph,

Thanks for your advice, but how can I know the actuall refresh rate my PJ?

I read the manual book, it said that my PJ can accept various refresh rate, but maybe the actuall refresh rate it can display only 60 Hz.

Are you sure that all LCD PJ actually display only 60Hz refresh rate?

Does it mean that I should set the refresh rate to 60Hz?

I'm using Vidikron EPOCH D-2200, maybe you have more detail information about this PJ.
 

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Bubun,

the only thing you can do is feed it with 48Hz/72Hz for NTSC and try to find out whether that looks better than 60Hz. You may end up with tearing (upper/lower part of the picture freezing for a split second) or even heavier stutters than at 60Hz. Experimenting may be the only way to find out. Sorry, don't know your specific projector, maybe some one else here does.

Regards

Christoph
 

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Quote:
Originally posted by Bubun


I read the manual book, it said that my PJ can accept various refresh rate, but maybe the actuall refresh rate it can display only 60 Hz.
Almost all PJ's can accept various refresh rates but it does not mean that they don't do any conversion.
Quote:
Does it mean that I should set the refresh rate to 60Hz?
Run test program to make sure. If you don't get any side effects like tearing when using other than 60Hz your PJ doesn't need to be set at 60Hz to get best PQ.
 

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Discussion Starter · #17 ·
Run test program to make sure. If you don't get any side effects like tearing when using other than 60Hz your PJ doesn't need to be set at 60Hz to get best PQ. [/b][/quote]


What test program do you mean?

Is that Video2000?(I'm in the process download it)

I have set refresh rate to 72Hz from the beginning I'm using HTPC, I have very rarely tearing or stuttering.

If I set refresh rate to 60Hz, does it mean I will free from any tearing or stuttering?
 

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Quote:
Originally posted by Bubun
What test program do you mean?

Is that Video2000?(I'm in the process download it)
Yes.
Quote:
I have set refresh rate to 72Hz from the beginning I'm using HTPC, I have very rarely tearing or stuttering.

If I set refresh rate to 60Hz, does it mean I will free from any tearing or stuttering?
Propably you won't see any tearing after that but there might be still some stuttering. Why don't you just give it a try and see.
 

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Discussion Starter · #19 ·


Propably you won't see any tearing after that but there might be still some stuttering. Why don't you just give it a try and see.
[/quote]


I have finished download Video2000.

I will try comparing 60Hz refresh and 72Hz refresh using Video2000.

Are you suggesting to test the "tearing" problem?

Also if I'm not wrong in the predefined custom resolution section (powerstrip), all resolution for LCD is set to 60Hz refresh rate.

Maybe it's true that all LCD PJ only can display 60Hz refresh rate.
 

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If you use an HTPC, go to the shutdown screen. If you see any shimmering in the background, try adjusting it out with the PJs controls (phase, etc). While my Infocus DLP PJ will display 72 hz, I could not tune out the shimmer. At 60 Hz, there is no shimmer.
 
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