Feed RGB+sync from video chip to YPbPr input

Discussion in 'General Electronics Chat' started by hang-on, Dec 28, 2014.

  1. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    Hi,

    In the SEGA Master System sits a video chip (Sony CXA1145). It outputs R,G, B and sync from separate pins, so I can easily get RGB/SCART for my TV. But I also want component video (YPbPr). As a first step, I'm going to tinker with the stuff, and so I ask: If I feed sync and, let's say, G to the Y input on my (other) TV, I can expect a picture - but with messed up green colors, right?

    I know the math:
    Y carries luma (brightness or luminance) and synchronization (sync) information. Y = 0.2126 R + 0.7152 G + 0.0722 B

    I'm looking for a minimal way to get a YPbPr picture from an RGBS source - colors are not important in this first step.

    I'll eventually do it correct (I'm learning about voltage dividers and op amp math from this site). But for now, I'm interested in breaking down the project in basic, hackable steps.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  3. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    Yes - thank you! I have compiled a collection of these circuits in another forum: http://www.smspower.org/forums/SMSIIRGBYPbPrVideoMod-t15048.html

    I'm trying to find a way to build such a circuit up in smaller, more 'hackable'/tinker-friendly steps. That is why I'm looking for advice on a minimal setup to start by.
     
  4. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    My goal is to ultimately achieve something like Ron H's circuit discussed here: http://forum.allaboutcircuits.com/threads/signal-math-with-op-amps.79779/

    If I just want to make Y (not Pb and Pr) as a first step, could I build the portion of the circuit inside the circle? (See attached picture).

    I suspect there is something wrong with my approach to sync, though. When I feed the Y signal from the video chip (it does also carry sync), straight to the Y RCA input on my scaler device, I get the wobbly picture (attached). So I suspect I need to attend to this sync issue, before I proceed further. There is nothing in Ron H's circuit that addresses this sync issue, right?

    Have anyone seen this kind of sync issue before?
     
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