Anyone with LED full color video screen experience

[techie]

Would like to share the experience / information regarding

1. RGB oval LED selection
2. LED drivers
3. Control electronics
4. Software

 

[Romanp]

I have some experiencein this issue.
You need RGB Clusters.

I used TI and Toshiba led drivers

 

[techie]

How do you control the intensity of the RGB channels. At such high refresh rates and only software PWM intensity control to play with, how can you accomplish this.

 

[Romanp]

You should devide VGA 640x480 resolution on small modules with resolution 64 x 16 RGB pixels. Such module will work with refresh 25 -30 KHz. It is ok, is not it

8 bit data serialized for led drivers. You get 8 x 30 KHz friquency.
Another issue is how to separate part of data that is intended for this module. Do you understant.
Each my module had FPGA, frame memory, microcontroller, led drivers and LVDS inerface. But the most expensive there are RGB clusters.

 

[yager]

As Romanp said: cluster, TI/Toshiba driver are based componets.
To control the R/G/B LEDs total on/off time to simluate full color.
Assume frame rate as 100Hz, R/G/B 256*256*256 colors, full screen as 640x480 dots (could be more clusters as 1 dots):
you will find how fast need to scan 1 dot!
So we split it into small stand alone pieces such like as 64x32,
each part takes care its own area and receives R/G/B data from mother board.
The mother board we used:
Video decoer and/or ADC(s) convert video/VGA signal to Scaler,
capture the digital data from Scaler and send them out to daughter/driver borads.
Why scaler? we use it to process some effects (intensity, dithering, gamma correction, white balance...) by daylight or not...
And scale to real screen size (the most cost is LEDs, a few customers will buy a really big size).
Software? The main job is: select input source and record the billing.
That's all.

 

[spit]

I have experience in this, design from scratch a system to implement this. Yagger post reflects much of what I came accross, since I had no no video experience when I started, my approach was to abstract the transmission of video data to led display modules, to allow scability of image. I designed a protocol to allow the reconstruction of video data, (a partitioned frame of data) from a digital stream with display module having enough intelligence to collect data from a stream and pass on the rest to modules downstream, followed by on command to rasterized the present store video information. FPGAs were used throughout, with off the shelf video decoder and video scaler device all of this command via microcontroller talking to a PC. There is an excellent application note on the HP web - in their LED section, on the subject of displaying video using leds.

Spit

 

[techie]

==> How do you control the intensity. Teh TI and Toshiba drivers aer all on/off type. So you have to use PWM for intensity control. However, it means that in order to establish a PWM control, you have to update all drivers easch PWM clock. At 60Hz frame rate and 256 levels of brightness, the PWM clock comes out to be 65usec. So each 65usec, you have to update all the LED drivers. That sounds near impossible to me since all the LED drivers are serial. So if you have cascaded 16 drivers in serial fashion, you require to shift 256-bits + overheads during 65usec. Am I right?

==> What FPGA do you recomment for this application. Any cost idea. I am new to FPGAs but have a lot of experience in microcontrollers. Can micros do the job here.

 

[techie]

HP LEDs. Can you please mention the HP website for LED application note. Is it the same HP that makes printers etc. I could not locate the LED section.

 

[Romanp]

VIDEO/VGA LED Display

Why scaler? we use it to process some effects (intensity, dithering, gamma correction, white balance...) by daylight or not...
And scale to real screen size (the most cost is LEDs, a few customers will buy a really big size).

About gamma correction, white balance. All these issue are very important. For gamma correction I used LUT withing the FPGA.
The value getted after gamma corrector forms the maximal PWM resolution !!!
PWM. It is a single way for control of the led intensity.
Do not warry. The value is less then 16 bit. I has 10 bit. But I did not see this screen outdoor in shine day

 

[techie]

Romanp, to do gamma correction and white balance, in addition to the intesity control, how many bits of total PWM did you use. 10-bit or 16-bit. Also what is the size of FPGA required for this pupose.

 

[Romanp]

Romanp, to do gamma correction and white balance, in addition to the intesity control, how many bits of total PWM did you use. 10-bit or 16-bit. Also what is the size of FPGA required for this pupose.

Dear Techie,
This issue is a tradeoff between module friquency and minimal requirements.
I guess that for gamma and w/b correction it is enough 10 bit. But it is resonable to make an option for additional number of bits.
What about the FPGA, I used 10K30 in 1999 year.
I advice you to finish your design (FPGA) on system level. After it you will see better what you need.

 

[techie]

can you please let me know the approximate cost of an 10k30 chip.

 

[Romanp]

I do not know its price now.
I can estimate the price of LED clusters.
If each one costs aboute $1 (In reality it will be more) only for one module 64 x 16 you will spend $1024. FPGA can be $30. You need not any huge Virtex or Stratix that costs about $800 each

 

[techie]

I guess you are right roman. $30 are nothing compared to the whole cost of the system it controls. Since I am new to FPGA, can you suggest what development tools I would need to start of its development

 

[Romanp]

Last time I work with Xilinx ISE 5.x
I think it will be enough for this design.
You can write in VHDL or Verilog. It does not matter.
Also tools of Altera are user friendly.

 

[techie]

I discovered a company making LED driver chips that have built in PWM intensity control of 10-bits. If that is taken care of by the chip, dont you think the rest of the design can be handled by a microcontroller with ease.

 

[SAITO]

Dear romanp.

I have 2 quastion.
One is how to make 16bit pixel data.
Another one is how to control color and brightness of
each LED cluster. How to obtain Same color and brightness.

 

[techie]

Saito, I quite dont understand your first question of 16bit pixel data.
To control color and brightness, you use PWM to control the current that each color LEDs receive.

 

[lassyy]

I am designing a color LED driver IC , It belong a power IC . The pole is How to get a controled and low RSon constant current source. Now, have not any solution yet. anybody have the expirement?

 

[SAITO]

techie

1st quation meaning really 16bit . Generally decoder output is 8bit, but
after Gamma correct by LUT. Actually ADConverting is not 10 or 16bit.

2nd quastion is meaning, actually LED have a problem brightness and color variation problem in same current and same Pulsewidth(by data).
Specially green color LED Color Variation width is very big.
Then Big LED Screen maker is doing color correction each pixel.

I want know how to this color correction by what method and algorithm.