Electronics > Projects, Designs, and Technical Stuff
RGB to YPbPr (YCbCr) conversion and bright picture issues
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MaarioS:
I'm working on a specific RGB to YPbPr (component) converter. It is supposed to connect old consoles to a modern flat 240p supportive TV. However I have an issue with luma Y output part. Here is the schematic of the current version I created:


YPbPr formula is R 0.299 + G 0.587 + B 0.114 and resistors R1-R3 are supposed to do this part

Sometimes the picture is too bright on the top, especially if the picture is black, the picture gets bright on the top and gets even brighter after time. I noticed when I put R6 75ohm resistor as pictured, the brightness decreases a little as it should, however if I lower the resistor value even more, the picture gets screwed up overall.

Right now I'm clueless on how to fix it. Does anyone have any ideas?? I can take a photo of the brightness issue if needed. Many thanks in advance

EDIT: here's the real photo of the effect (some reflection is shown but the main issue is still visible)
flynwill:
Do you have a scope?   If you do this should be straightforward to debug, without one pretty difficult.

I would hazard a guess that there is something terribly wrong with the DC levels of the R, G & B signals or possibly they are not blanked properly, the net result is the level of you Y output is shifting shifting significantly during the vertical interval.

Is is possible there already is sync on the green channel?

Audioguru again:
Video is supposed to have DC coupling so that the black level is always correct. This circuit has an output capacitor that messes up the DC brightness levels.
Many, many years ago high cost TVs used a "DC restorer" circuit because they did not know how to DC-couple a video amplifier.
tooki:

--- Quote from: MaarioS on November 07, 2019, 09:21:17 pm ---I'm working on a specific RGB to YPbPr (component) converter. It is supposed to connect old consoles to a modern flat 240p supportive TV.

--- End quote ---
Given that we are starting to see TVs with no analog inputs at all, wouldn't it make more sense to use an RGB-to-HDMI converter? They're readily available, from simple off-the-shelf ones, all the way to FPGA-based ones made specifically for console gaming.
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