Quick clarification, please, or please point me to a previous post. In video mode, the default setting for the R G B LED duty cycles for white is 40% R, 45% G and 15% B. Given that the number bits per RGB color channel depends on the timing of the mirrors, how can you get 8 bits for the B channel if its duty cycle is only 15%? When does the reduction in duty cycle start to impact on the number of bits available for a given channel?
I work in color vision research for which it is important to get 8 bits per channel without any corrections or dithering, etc, etc. Which mode is best for that?
Thanks you and apologies if I am going over old ground.
Hi Andrew Stockman,
Welcome to DLP&MEMS forum.
My apologies for missing this thread.
The 40%R, 45%G and 15%G parameters are used to get a decent 'white' color. In "video mode" all three LEDs turned ON sequentially to reproduce the color. What it means is that at 60Hz frame rate or 16.67mSec duration the data corresponding the each channel is shown as per duty cycle parameter, in this case it is
16.67mSec * 0.40 = 6.668mSec (Red data display time)
16.67mSec * 0.45 = 7.501mSec (Green data display time)
16.67mSec * 0.15 = 2.5005mSec (Blue data display time)
DMD high switching speed along with LED switching makes it possible to show all 8bit of Blue channel color. DMD is PWM display device so each 8bit of channel information is loaded one bit by one. The duration of display of each bit increases by 2x amount from LSB to MSB. So in case of blue channel almost 2.5005mSec/2 = 1.25023msec is occupied by MSB bit and each lower bit displayed 1/2 of the remaining.
So what it means is even at 15% or 2.5005mSec allocated time it is possible to show all the 8bits of channel.
Now the 'video mode' as such uses dithering & degamma corrections to create a better image. If you are interested in allocating equal dutycycle for all the three i.e., Red = 33.33% , green = 33.33% and blue 33.33% plus disabling all image correction blocks then it requires a customized built on the hardware.
We will be able to support this only after we understand your application and opportunity as such. You can send me friend request. we can take it offline.
Thanks. So, the LSB for the blue video is 0.00942 msec for a 15% duty cycle, which is within the specs of the device?
I can turn off the degamma correction and change the dutycycles to 33% in the Light Commander software. What I can't do (but I want to do) is turn off all the dithering.
Have sent a friend request so we can continue offline,
Mathematically yes. There are other factors involved such as DMD load time, so each bitplane needs to be loaded before display so there will be 8 loads in blue 8bit data display. So this time also consumed in display time.
A simple experiment will get you the precise numbers like, send a RGB888 video where only LSB bit of Blue channel is ON and remaining all 23 bits OFF. If you put a photo-detector you will find the display time of this bit.
The "video mode" mode was meant for basic video functionality. For user we have provided just DutyCycle and Degamma setting.
Thanks. I see. I can try that.
But is it possible to turn off spatial and temporal dithering? If not in software, in hardware?
And if it isn't possible, what is the dithering algorithm?
The closest you can go would be set DC of R G and B to 33% each respectively. Then enable degamma but set the degamma value to 1.0 (Linear). That is the max we can do. Now, the binary weighting i explained earlier i.e., from MSB to LSB is 1/2 from its upper bit is little bit different in case of "video mode" here it is not strictly 100% binary weighted one. I thought to update to you on this.
The other way possible but it is monchrome structured light, that is input RGB888 data is interpreted as three 8bit frames. But you will not be able to reproduce all colors in a frame.
You will find more from your experiment. Let me know if you need any more info.
Thanks for this thread, it has been useful in understanding how the DLP works in video mode.
Regarding the duty cycles, when they add up to more than 100% , what actually happens?Because from what I read and understand, the duty cycles determine the percentage of time each LED is switched on.Also, does degamma and dithering also occur after compile? as in does the values/settings change?
1. More than 100% basically means a overlap configuration i.e., more than one illumination turned ON simultaneously. What it also means is that in an allocated frame time there are instances where more than one LED is ON. For example: 15% Overlap sequence, 85% of the time where only one illumination is ON and the remaining 15% of the time more than one illumination is ON.
2. Degamma and Dithering enabled and applied if you are running DLP LightCommander in Video Mode. The degamma adujstment allows editing the parameter in Video Mode.
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