Wednesday, April 18, 2007

Burn-in on Sony LMD-230 series HD TFT monitor

It's not a great image (and I had to crank down the gamma to get it even visible) but what you can see here is the residual burn-in of colour bars on a Sony LMD-232 high-definition monitor. I would not have believed it until I saw it at the facility I was working at today. This monitor is two years old and I don't know if this kind of distortion is correctable. It must be the TFT panel itself - there is no way the backlight could take on the effect of bars. You can just see a yellow bar (where the blue bar would be) and where the green-magenta transition is you can see a magenta-green line. I do remember Martin Euredjian from eCinema Systems telling me that if you switch liquid crystal faster than the 15mS recommended by the manufacturer you run into trouble - the transistors become lazy and loose the ability to switch back entirely. This looks like what I saw on this Sony display.
While on the subject I thought I quote Martin on another of his TFT pet hates - grey-grey response times for monitors;
...this brings me to a topic that is being greatly abused by several monitor (and panel) manufacturers. To clarify, "monitor" means a finished product that an end user would purchase. In contrast to this, "panel" means the raw LCD element use by monitor manufacturers in fabricating their product.
This business of pixel response time has to do with how quickly the pixel is able to fully turn on (white) and then off (black).
If you were clicking out Morse code with a flashlight, you couldn't go any faster than the time it took for the light bulb to go from full off to full on and then full off again. For example, an LED flashlight can do this significantly faster than a conventional filament bulb flashlight.
In the context of LCDs, the only response time number that is of any relevance to film/video applications is a measurement taken from black to black. In other words, start with a black display and flash it to full white, returning to black. This is the only number you want a display manufacturer to quote you for response time.
Marketing guys have been known to bend things --a lot sometimes-- and so, it should come as no surprise that some are quoting response times that are simply not achievable within today's constraints. The latest one I heard during NAB is 6 milliseconds. I had people coming to the booth saying "how come yours couldn't do 6 milliseconds, "x" are saying that theirs do".
The critical question here was "Is it from black to black?". When asked, the answer was a resounding "no". People are taking measurements from some level of grey to another level of grey and quoting this as the display response time. You might as well be quoting the time it takes the average person to sneeze, because it would be just as irrelevant. Be aware of this and don't fall prey to contrived marketing.
Best-of-class response time today is somewhere around 15 milliseconds for all LCD's manufactured anywhere in the world. Why? Because there is only ONE liquid crystal fluid manufacturer that supplies fluid to the makers of ALL the good panels. The response time is largely a function of what the fluid can do. Today, the fastest fluid available can do about 15 milliseconds. Remember that and smile as a sales guy quotes you 2 millisecond response time. It might happen.

If you're interested you can read some of the other things he has to say on the cinematography mailing list.
If you're new to TFT displays for television and want a very good overview of how they work check out

1 comment:

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