do lcd screens "scan"?

[supernova87a]

OK, here's another question I've had about LCD screens -- when you look at the electronic connections coming from the screen, there aren't 2 million wires. It's more like (Nrows + Ncolumns).

So does this mean that the screens are scanning to produce the image? At what frequency?

 

[zephyrprime]

<< It's more like (Nrows + Ncolumns). >>


An astute observation. I believe that it must scan and it scans at 60 frames a second for many screens. This translates into ~47 megahertz for a 1024x768 screen. Or maybe ~150mhz if the colors are scaned indivdually.

But come to think of it, I think it may be possible to refresh an entire row at a time in the same way that DRAM refreshes an entire row at a time.

 

[Mingon]

Once the lcd has received the data for what each pixel colour must be the pixels, stay that colour until they are told to change. The refresh rate of lcd's is the amount of time per seconds that each pixel colour can be changed if required. This is why lcd appear to have a higher refresh than they do, but also why blurring occurs during fast movements.

 

[crypticlogin]

<< Once the lcd has received the data for what each pixel colour must be the pixels, stay that colour until they are told to change. The refresh rate of lcd's is the amount of time per seconds that each pixel colour can be changed if required. This is why lcd appear to have a higher refresh than they do, but also why blurring occurs during fast movements. >>



I think this statement is a bit misleading. The 'gates' of an LCD aren't entirely static elements, that is "write once, stay on" type of devices, at least the ones in TFT screens aren't. TFTs can be viewed as a capacitor/transistor combo but the capacitor is leaky and requires an occasional refresh to keep the light transmission constant. Unless the voltage controllers on the screens aren't functioning properly, you won't notice this kind of refresh without the help of high-speed measuring devices. This is the refresh rate that an LCD says it does.

Now you'll say, "but I can see the flickering of a 100Hz refresh on my CRT with my bare eyes!" The TFT has a longer excitation state than an excited phosphor of a CRT tube so even though you can see the CRT flicker, the changes between refreshes on an active-matrix LCD screen are more subtle. In other words, you'll see less change between successive refreshes with an LCD 60Hz refresh than on a CRT 60Hz refresh phosphor. I believe most LCD screens have these types of refreshes at 50 or 60Hz with 70Hz the highest refresh I've ever seen claimed by a manufacturer.

*CHANGING* a TFT pixel from one state to another is a slower transition and is the cause of the ghosting images. Upping the refresh rate of an LCD won't reduce the amount of ghosting simply because the TFT gate simply can't react fast enough, not today's TFTs. It's like telling the driver of a car to accelerate to 60 miles/hour from rest once every second -- you can tell him "drive at 60MPH" till you're blue in the face but the car just isn't going to reach that speed any sooner.