EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: Poe on October 13, 2015, 02:29:17 am

Title: Modern LCD control
Post by: Poe on October 13, 2015, 02:29:17 am

I'm trying to learn more about modern LCD monitors and how they're controlled.  Eventually I'll tear one down and poke around for wave forms and such.  If anyone could point out my mistakes or link to a source, or a teardown someone else has already done... that would be wonderful.

I think I understand up to the LVDS.  After that point in the control chain, I'm lost. 

Originally I thought the driver IC controlled grey-scale for each unit/sub pixel via PWM. 
i.e. ON = Light, OFF = No Light

This is apparently incorrect. 

Now from what I gather, each pixel's plate charge is adjusted, which in-turn 'twists' the light more or less, which in-turn increases or decreases the amount of light going through the polarizer... right?

Each LCD driver IC adjusts each pixel charge to ~8 bits of accuracy via it's OWN DAC?!  That's a lot of DACs. 

Although from what I gather, each DAC is only 4bits and is simply like a fine adjustment?  The most significant 4bits select a rough voltage level from ~16 preselected (resistor set) voltage levels.  This rough adjustment is largely responsible for gamma correction?  The 4bit DAC simply adds a small voltage offset?

How far off am I?  If that's how they adjust gamma, then how is it changed in software?

I read that most modern TN panels are only 6 bits versus the 'true 8bit' IPS panels.  Is this simply because TNs are faster and they can get away with more dithering?  Is it simply a limitation of the LCD driver circuitry and a TN panel could have eight bits of greyscale?

Title: Re: Modern LCD control
Post by: amyk on October 13, 2015, 04:12:09 am

Yes, there are many DACs in parallel in the source drivers.

Give these documents a good read...

http://web.nchu.edu.tw/pweb/users/fansen_wang/lesson/7043.pdf (http://web.nchu.edu.tw/pweb/users/fansen_wang/lesson/7043.pdf)

http://www.ti.com/litv/sgls099 (http://www.ti.com/litv/sgls099)
http://lcd-module.de/eng/pdf/zubehoer/hx8218a.pdf (http://lcd-module.de/eng/pdf/zubehoer/hx8218a.pdf)

http://ee.cust.edu.tw/ee/91-93%E5%90%84%E9%A1%9E%E8%AC%9B%E5%BA%A7%E8%B3%87%E6%96%99/051117%E7%A0%94%E8%A8%8E%E6%9C%83/%E8%98%87%E7%B4%94%E8%B3%A2/LCD%20Source%20Driver%20Design.pdf (http://ee.cust.edu.tw/ee/91-93%E5%90%84%E9%A1%9E%E8%AC%9B%E5%BA%A7%E8%B3%87%E6%96%99/051117%E7%A0%94%E8%A8%8E%E6%9C%83/%E8%98%87%E7%B4%94%E8%B3%A2/LCD%20Source%20Driver%20Design.pdf)

http://www.ti.com/lit/pdf/sgls116 (http://www.ti.com/lit/pdf/sgls116)
http://www.microtipsusa.com/pdf/HX8643A.pdf (http://www.microtipsusa.com/pdf/HX8643A.pdf)

Title: Re: Modern LCD control
Post by: Poe on October 13, 2015, 12:55:13 pm

Thanks.  I'll give them a read.

It's amazingly more complex than I would have thought.

P.S. Do you happen to have any info on large format RGB LED displays (like jumbotrons) or OLED displays as well?  Thanks again.

Title: Re: Modern LCD control
Post by: amyk on October 15, 2015, 02:45:24 am

For OLEDs you can look at driver IC datasheets (like this one (https://www.eevblog.com/forum/projects/oled-voltage-levels/?action=dlattach;attach=174049)). Mike has some good videos on large multiplexed LED displays:

https://www.youtube.com/watch?v=6iHTbGsnI8Q (https://www.youtube.com/watch?v=6iHTbGsnI8Q)

https://www.youtube.com/watch?v=Sq8SxVDO5wE (https://www.youtube.com/watch?v=Sq8SxVDO5wE)