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Color/Grayscale LCD Fundamentals

I'm trying to understand how color and grayscale LCD controllers work.. bear with me..

Looking at a couple of LCD panels, they have row and column drivers, with 1 bit per pixel, usually organized as R-G-B, R-G-B, etc. This implies "hardware" support for 3 bits-per-pixel. However, controllers discuss 8,

12, or 16 bits per pixel. I assume this is done by PWM techniques across multiple frames, but given the scan rate of most panels, even this seems impossible. If I have a panel with a max scan rate of 120Hz and I connect it to a SHARP ARM/controller with 65536 color levels, the best I could get with my "technique" (without flicker) would be 2 color levels, which still doesn't work out..

So.. my question is this: how does an LCD controller get 4096 or 65536 colors from a LCD display? Similarly, how does an LCD controller get 4 or

16 levels of gray from a LCD display? I'm reading through data sheets but somehow I'm just missing something very fundamental..

Thanks!

Reply to
Ian McBride
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Well, basically you've figured it right: they use PWM (in STN and DSTN models at least, TFT is another world). With a 60Hz refresh rate and 16 gray scale levels you would get around 2Hz of traditional refresh rate. However it doesn't mean that the LCD pixels are turned on and off twice in a second. They still turned on and off at 60Hz and that's what you experience as flicker. On the other hand STN and DSTN displays are pretty slow so they smoth out these fast transitions and that reduces flicker even further.

Regrads, Andras Tantos

Reply to
Andras Tantos

Hi Ian,

For passive-matrix panels, you are basically correct - multiple color levels are synthesized by "PWM". It's more technically referred to as FRM - Frame Rate Modulation. Dedicated LCD controllers have a kind of lookup table in them that determines how often a pixel value of a specific intensity will be turned on. There is a counter inside the controller, which is incremented every vertical sync pulse, which is used [effectively] to lookup inside a 2D table that cross-references the color intensity against the current "frame sequence number" and yields a single bit which indicates whether this bit should be on or off for this particular frame.

For example, imagine a simple 8-frame sequence to handle 2 bits of color information. For color value b00 (off), the output for frames 1-8 would be

00000000. For color value b01, the output might be 00100100. For color value b10, the output might be 01101010. For color value b11, the output would be 11111111.

This technique is quite well documented in the datasheets for Epson's SED135x series parts (now renamed S1Dsomething). They also list their actual FRM tables, which as I recall are 64 sequences long.

For active-matrix panels, the controller supplies of bits of color information which are DAC'd directly onto the drive transistor for the subpixel.

Reply to
Lewin A.R.W. Edwards

Howabout animated graphics on passive-matrix panels? It would seem that the more things were in motion, the fewer effective levels of color you would be able to display.. or does the eye continue to average it out into something coherent? .

Reply to
Ian McBride

It depends on the frame rate of the animation vs. the frame rate of the controller. But the short answer to your question is that yes, animation screws with the FRM process, and this is just one more reason why passive panels don't work as well as active for animation, movies, etc.

Reply to
Lewin A.R.W. Edwards

Are you sure it's not expecting *analog* RGB?

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Reply to
ZO

Passive panels with analog inputs are rare or nonexistent.

Reply to
Lewin A.R.W. Edwards

I've never seen an analog passive panel. Using a DAC to set intensity is something that is done on TFT panels however. All implementations using more than 8 colors on STN (that I've seen) are done by the FRM technique described above. STN pixels are piggishly slow, with response times measured in the high 10s and low 100s of milliseconds. This is why frame rate modulation work so well, but STN panels are terrible at displaying moving graphics. One big issue that TFT fixed was response time, now the pixel intensity could be programmed with an analog DAC, allowing very fast reacting pixels.

-Jim

Reply to
Mood

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