Anyone with LED full color video screen experience

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Junior Member level 1
LED Display Design

I Design LED Full Colour Display Board. There is a driver chip from Texas wich has programable constant current sink, used with the brightness clock registers will control intensity of LED. Also for display matching there is LED segment register for individual LED element matching. I have DSP processor recieving video data for display then this DSP sevices 5 Driver chips which can drive 768 Pixels. Each pixel being 3 RGB LEDs. 8)

SAITO

Member level 1
What is a LED Drive IC from TexasInstrument. TLC5911 or anyone? Please explain for DSP processor purpose function.
I want know more detail.

Generaly video processor is output 10bit RGB after Gamma correction for 8bit RGB. Is this meaning realy 10bit. What is this for theorical mean. And,next Colour and Brightness uniformity problem is how to method or algorithm. Arcording to explaning of BARCO/Lighthouse LED Screen maker, methode is each pixel and colour sensing and this x,y,Y data be writen to a eeprom in a rear LED Module, at time screen
power on data send to controller. And controller is correcting video data RGB by each pixel/each colour x,y,Y data. If you know a detail method, teach to me plaese.

techie

To my understanding, each module should have its own control board with one LUT for gamma correction and one LUT for brightness matching. The RGB video data will be received a 24-bit data with 256x25x256 levels for R,G & B channels. For each pixel and each of its RGB channels, this incoming 8-bit data will be multiplied with gamma correction value first (using LUT, since the relation in non-linear) and then multiplied with intensity variation factor to compute the correct level of current required for each LED separately.

But all this needs huge computing power. I am being convinced that an FPGA with LUT is the only solution. It is definetly out of scope of any 8-bit microcontroller. Maybe a DSP with good horesepower can do the job. But since the computation is very simple and very fast, an FPGA looks a better option.

As far as the 8-bit, 10-bit, 16-bit discussion goes, It depends how much accuracy you want. There are now 3 level conversions that come to my mind
1. Gamma correction
2. Intensity matching
3. Overall brighness controll for day/night time viewage
All 3 conversions require some mathematics and gamma correction is a non-linear function. At any stage, you might want to upgrage you data width from 8-bit to 10 or 16 bits.
Any ideas that these 3 corrections should be done in which order

Junior Member level 1
There is lots of ways to do this but I will discribe what I have Done.

First think of the display like like this, A master controller at the front with a video source of what you want to display.

Second the display itself effectivly made up of lots of small areas each one with its own processor controlling a number of pixels.

Ok so we have a video source which we will assume is digital 8 bits of Red Green & Blue, but for better control we need 10 bit this will give us a better grey scale at lower brightness levels ie at night when we turn brightness down we don't loose colour depth. To achieve this our master controller must convert 8 bit to 10 bit this is done with LUT curves. So all data now sent to display from master controller nas been Gamma corrected to 10 Bit.

Now each section of the display waits until its section of data arrives and then a DSP will program the drivers with this recieved data. The driver chips from Texas have the 10 bit registers for the video data so it is easy.
For display brightness all sections will recieve a value from master controller then this is used to program current sink register in driver chips.

Hope this helps.

techie

Thanks roadster. Things are getting somewhat clearer now. Can you also shed some light on the following

1. Your TLC chip has got individual dot correction feature. If this feature is not avaialable ( I am using Toshiba chips), then is it possible to do some sort of dot correction within those 10-bits. For example, that I send the 10-bit per pixel-color data from master CPU to the panel CPU. The panel CPU then does another LUT to correct the variation between dot brighness. Do you think that the video performance will be degraded or not.

2. If I am using 1-chip (16-led) to control 4 pixels, each having 2Red+2G+2B LEDs, then since for white balance, I have to operate the 3 colors at different levels and I operate red at 14mA max, Green at 12mA max and blue at 20ma max, then I have also to use these factors into account when calculating the pixel brightness. Do you think that if I also do this thing in software, then the performance of 10-bit will be acceptable or not.

Junior Member level 1
Techie.

The driver chip I used was Texas TLC5911 look at the data sheet for this device it will help you to understand better. We drive at upto 80mA because the LED's are driver with pwm so you have to have more current.

Regarding dot correction and white balance I have used Toyoda Gosie LED's and have found that these devices already come with the the approriate output levels for a good white balence.

What I meen the general rule is you need a lot of green then red then blue. If you look at the data sheet for Oval 4mm Leds you will see the greens have the brightest output followed by red then blue.

As far as DOT correction in our display we have found there is not really a need because generally the LED's are very well matched so if you don't mix the bins you will be fine. 8)

techie

Thanks roadster. That is good news that Toyoda LED match well. I was planning on using a non-branded (small company) Taiwan LEDs. They cost red $0.07 (350mcd) , Blue$0.20 (350mcd) , Green \$0.20 (1000mcd) in large quantities. Can you suggest which Toyoda model you chose and what is the approximate price.
techie

Junior Member level 1
Techie

What size displays do you intend to build ?

What is your end market for them ?

Will they be used indoors & outdoors ?

:twisted:

techie

Roadster, Here are my specs of display

128x96 pixels display with 1pixel = 2R+1G+1B LEDs. 16mm pixel pitch (or 8mm for red), The red LED's are addresses separately and are placed diagonally in the pixel and placed at equal distance to all the other LEDs in the same and the next pixel. This gives a virtual resolution of almost twice the actual resolution.

The displays is for outdoor, daylight.

Romanp, I have to give it >6000cd/m2. It requires a pixel lumniance of 1536mcd. The LED brightness I gave in my last post does give that much performance.

techie

Question:
When they say that the screen is giving >6000cd/m2, does it mean that each color gives that much intensity or that the overall intensity is that much.

SAITO

Member level 1
Genellaly, The 6000cd/m2(nit) means a screen luminous intensity white
balance condition of screen.

SAITO

Member level 1
Do anyone have a experiance for LED color correction methode or
algorithm. I have try it but very difficult. Anyone help me.
This Color correction means for each pixel color differancial and
luminunce differencial. Not mean the Whole screen color correction.

techie

Saito, do you mean that each of your LEDs have significantly different luminosity. This is unlikely if you purchase all LEDs in a single lot and dont mix the boxes. but if you have purchased some stock lot without any grading, it might be the case.

techie

Do you mean that R and G and B are not matched with each other to produce white balance. Or you mean that one R is not matched with another R.

motopiza

Junior Member level 3
high-tech outdoor advertising in Times Square possible.

h**p://www.xilinx.com/publications/xcellonline/xcell_51/xc_es-tsquare51.htm

Using FGPA and CPLD.

SAITO

Member level 1
Question:

Recently LED Screenmaker anounce for LED Video screen Grayscale function.

14~16bit Grayscale for each color and reflashrate 1000Hz etc.

I dont' understanding how to LED Brightness PWM Controle above high spec.
Would you mind teach to me PWM Controle methode and algorithme ?
Or, Anyone have a CPLD(FPGA) HDL Source code , and send to me by mail.

I have a PWM Controle CPLD Compiled file but this jedec file not comprehension.
Attach this file.

techie

Can you upload the schematic of the Pwm control module.

I have some LED drivers that include the PWM function within. Some Drivers even have 3x8 channel and PWM function all within a chip. But these are quiet expensive. The FPGA remains the cheapest.

Can someone upload a reference schematic.

Bus Master

Full Member level 3
Hi,

I also need an example FPGA reference design for an RGB PWM LED driver.

Yours,

RichardH

Newbie level 5
Hi

Thanks

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