Why are LCDs 18-bit only?

[figgypower]

This is my first time posting on the Hihgly Technical forum, so if I'm in the wrong spot, sorry about that.

Now, I've always thought that LCDs are 24-bit, but in the last month or two I've found out otherwise. From browsing the Dell Inspiron forums I've found out that color LCD displays used in laptops are not really 24-bit, but 18-bit. You can find one such posting at "http://delltalk.us.dell.com/messages/message_view.asp?name=insp_video&id=zwpmw". I've looked this up in some other sources, and this is true - for laptops, but not their desktop variant. From experience I know that Photoshop editing is much easier on my CRT monitor and it seems to be a pain on my laptop, especially noticeable when you're down to editing single pixels. I've looked up how LCDs work at "http://www.howstuffworks.com/lcd.htm" and I just don't see why 24-bit LCDs on laptops are not possible. If most LCDs can work with 256 level of brightness per pixel, doesn't that add up to 16,777,216 for seperate RGB components? Is cost a factor? That doesn't make much sense for me either, since desktop LCDs cost less. I can't see why form factor matters either; they seem to be made in the same way with similar... for that fact why are desktop LCDs fatter?

 

[Adul]

maybe it is an issue with power draw? that is the only major difference I can think off.

 

[Peter]

Not all are ... DVI transmitters transport 888 RGB data (24-bit). What the display then uses is another story. Many are 666 (18-bit), and some of the really large ones use a two-pixels-per-clock mode, thus cutting their otherwise very fast clock in half but also halving the color depth to 444 (12-bit).

VGA chips use a number of tricks like micro-dithering and multiframe color approximation to bring the number of rendered colors back up a bit ... but if you see an 18- and a 24-bit panel side by side, you'll hardly notice unless you deliberately look for differences. The 2x12-bitty ones do look worse, but there it's a choice between no picture at all and a picture with little color depth.

Some DFP units even take 36-bit input, but very few graphics chips actually can serve that ... and across DVI, it's impossible anyway. So if a notebook manufacturer chose one of those very fine panels, you'd get more color depth than your VGA even produces. However, the LCD transistors don't reproduce that too well anyway.

What you really need is a thorough gamma and color correction for the LCD.

regards, Peter