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ADV7842 - banding or quantization of color appearing in video

Question asked by cdarrow@arrow.com on Jan 11, 2018
Latest reply on Jan 24, 2018 by cdarrow@arrow.com

We have 99% of everything working with the ADV7842. We see some banding or quantization of color appearing in video. We believe it is a programming or configuration issue. We can’t seem to figure it out. For example, smooth image transitions from light to dark, any color, contains steps instead of the original smooth transition. It’s as if some sort of clipping or dropping of bits is occurring.

 

We’ve tried both CSC units, the one in the DPP and the one in the CP. We’ve tried tweaking the clamping, gain, and offsets. We’ve followed the example scripts. It seems that the scripts with the EVAL board are mostly 36-bit output. We are 20-bit YUV 4:2:2. One of our theories is: maybe the output formatter has a bug with our combination of color space and pin selections.

 

Tweaking values in the ADV7842 CP section seem to move the “banding” around. This leads us to think this is a ADV7842 configuration issue.

 

We are pursuing an EVAL board to jumper the interface between the ADV7842 and ADV7511 to be identical to the one on our hardware. If we have the same issue, we would then know the issue is probably configuration, as we suspect. And, there is nothing wrong with our board or FPGA. If it works fine on the eval board, we would come away with a known good configuration and would look even harder at our board and the FPGA.

 

On our side, board and FPGA, the interface is well understood, proven, and used elsewhere. We have looked at it very closely and find nothing wrong. Inside the FPGA, immediately after we clock-in the ADV7842 video is a mux prior to any video processing. All other video from other sources is flawless through the same pipeline(s) used by the ADV7842.

 

Attached is an image of a black to red test pattern. Notice the vertical banding. It should be a smooth transition, black on the left, full red on the right. Disregard the moiré effect in the middle. That is pixel to pixel interference with the camera that took the picture.

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