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Application Circuit LT6550 RGB to Ypbpr - missing green

I am working to convert RGBs from an arcade board into Ypbpr component video.  I am following the example schematic provided using two lt6550 to perform the matrix conversion.  After completing the circuit I do not have green, nor any colour using green.  Red and blue appear properly.  Adjusting green values using a potentiometer changes luma/brightness.  I assume green is indeed being conserved in the circuit since luma exists.

Researching I have determined that I need a sync gated with a transistor; as per instructions  Looking at other circuits, I have not seen this being the case and sync is injected directly to Y.  Will the lack of gated sync be the cause of missing green?  If so, I will create and order a new board to prototype with.  I am also using a LTC1983 to generate -3.3v, as per example given in the video suite pdf. I have noticed said chip generates -3v instead of -3.3v.  Will this lack of voltage also be causing issues?  I am attaching my schematic to my circuit.

Thank You

  



spelled lack as lake*
[edited by: pnac at 4:57 PM (GMT -5) on 6 Dec 2021]
  • The -3.0V matter will not be an issue, though dynamic range of signals will be reduced so possibility of clipping is somewhat greater. Make sure you didn't ground the second chip's pin 4, as that's a feedback signal. Also note the expectation of this circuit is to be loaded with 75R on each output to get the proper signal levels. If there is a sync on green, it will show up on Y but somewhat attenuated by the matrix math. Best results are had with the sync going negative and leaving black at zero volts on all inputs. You can easily model this in LTspice to see what the response is to waveforms that you have.

  • Thank you for the response.  I have Y feeding into pin 4 as per schematic.  T.V.s have 75 ohm terminating resistors, so that's not an issue.  I don't have a oscilloscope to test with, I've been taking a more trial and error approach.  My circuit works, minus green.  I don't have sync connected to a switch, so I'm curious if fixing that will fix green.  This is the modification I'm thinking of using.  Before I order a new PCB, I'm curious if my missing green is because of me not switching the sync.  I've attached an updated schematic with improvements, I would like to know if what I did is correct as I will order a new board.

  • The sync switch is the issue, but I'm not sure if I'm correctly constructing it.  Is there a specific transistor to use?  When I control the base of the transistor with the sync, the circuit does not work.  I'm currently using https://www.digikey.ca/en/products/detail/nexperia-usa-inc/PMBTA42-215/763350 for a transistor, is 50mhz to low of a transition frequency?  Alternatively, is using a transistor with the video sync as a switch incorrect?  My transistor circuit was posted above. Is there any application circuit for either 8.5ma current sink or transistor switch?