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Discussion Starter · #1 ·
I understand why CRT's are full of circuitry that more or less depends upon a constant speed.


But what about new digital LCD, DLP, or Plasma displays? It seems they could be made like video cards, with a buffer that just sits there until you tell it to flip to the next one. Up to a certain point this would avoid judder and timing issues. If you wanted 24 FPS you could send that many, with no flicker.


The reason movie theaters show each frame multiple times is because there is a brief black screen in between. But that doesn't apply here, does it?


Do any displays do this?


- Tom
 

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Maybe for backwards compatibility? They're going to be receiving a clocked signal coming from the PC so they might as well use it.


I don't believe DVI can handle something like this, you'd need to come up with a whole new standard.
 

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I don't have any experience with actual displays, but I write signal processing software that is based on the same principles used by all A to D and D to A devices.


Just like when converting analog to digital, it's often much easier to oversample than to design matched bandwidth filters with proper roll-offs.


There are cases like film projection where this idea really does make sense, and cases like digital displays where its possible to only cause a state change in the display when the underlying pixel data changes. How often you try to detect that state change could be one type of refresh. How often the underlying pixel data is used to change the state of the display device could be another type of refresh (I assume that pretty much all digital displays do something like this).


Tying the ADC to the analog refresh rate just makes everything so much easier since the video buffer is always a synchronized frame of video data. Allowing the display device electronics to work at a different refresh rate is important for cost in the case of many types of projectors (DLP and fixed RPM color wheels come to mind) and performance in the case of early plasma displays. The switching time of the plasma pixels was not fast enough and that could cause ghosting similar to passive LCD, so driving the display faster the switching time of a pixel doesn't do you any good.


Current plasma displays are much better in this regard and support higher display refresh rates. Even modern LCD have fundamental switching times that affect the final refresh rate. I recently came across an article that presented a method of over driving the LCD pixel to accelerate the state transition and thus increase refresh rate and contrast. 2X 3X 5X color wheels on DLP is another type of over sampling but the sample rate doesn't have to be a multiple of the analog refresh rate, just a multiple of the underlying physical display device refresh rate. In the case where the refresh rates don't match there can be problems with tearing or jitter, but that's part of the magic of display and I assume that that kind of stuff are closely guarded trade secrets within the different companies.
 

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Discussion Starter · #4 ·
So no one is aware of any multisync digital displays yet with any sort of digital input that just keeps the same frame displayed until it gets a vertical sync signal (or equivalent)?
 

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trbarry, - I could be wrong but I thought

that ~most~ LCD, DLP, dILA type displays do

exactly what you describe. They have a native

internal refresh rate and accept incoming "data"

into a buffer.


I suppose that many of them would write into

the "active" buffer so you could get some issue

if your PC output signal is a mismatch to

the native display scanning rate.


What I wonder is which ones have a "double

buffer". One where the incoming data goes

into the inactive buffer until the vert

refresh hits and then it would swap over

so that the active scanning is from the

newly filled buffer and new data now goes

into the now inactive buffer.
 

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PVR,


I think you're right, and I don't think these displays can actually do this fast enough, hence the tearing effect that is visible when alot of flashing white light is displayed. I also noticed that changing resolutions not only create scaling artifacts when not native, but don't quite have the same "tone".


Kei Clark

Digital Connection
 

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Quote:
I'm not sure that it is a speed thing as I get tearing at 50Hz.
Ouch!


I mentioned speed because earlier on in the LCD technology, manufacturers were known to state this to be a problem when displaying video, when there is alot of movement (action shots, pans, etc.) Obviously the technology has improved.


I notice the tearing effect when I watch 1080i, although not 720p native material, and when I set DVD playback at 72Hz vs. 60Hz. It's obvious that the programmed timing maybe suspect, as this often differs from one model to another.


Kei Clark

Digital Connection
 
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