Correcting brightness levels when encoding RGB

[FusionITR]

I'm using the AD725 to convert RGB to Composite/S-Video and I'm having issues with the brightness level when connecting it to jamma boards (CPS2). I have the circuit hooked up like it shows in the datasheet and it works but the brightness of the picture is way too high. I've tried putting different resister values on the RGB lines going into the encoder but that has no effect.

Anybody have any suggestions with this?

[SeaWolf69]

Hey,


   This is what you need:

http://jrok.com/hardware/RGBv4/index.html

They just came out with this newer model that supports composite, S-video & Component output. It costs a pretty penny, but in the long run its worth it.

SeaWolf69

[NFG]

Resistors on the RGB lines should be all you need.  I have had several chroma encoders and that's all they do.  Sorry I don't have any values handy...

[viletim!]

FusionITR,
Maybe you forgot the 75 ohm resistors to ground on the input?

In any case, for arcade PCB video an input stage like this one is far more appropriate than the one found in the datasheet.

Code Select Expand

video in ---R1---+---C1--- ic input pin
                |
R1=1k          VR1
VR1=1k          |
C1=100n        ---
               GND
Use three for red, green and blue. VR1 (potentiometer) controls the contrast of the video. This input stage is simmilar to what the jrok encoder uses.

Lawrence,
Just wondering...why do you (and others) refer to these chips as chroma encoders? Wouldn't video encoders be a better term? I don't think i've seen an IC responsible specifically for chroma.

[NFG]

These chips create two signals, a luma and a chroma.  I'm not sure why it's called a chroma encoder, but that's definitely what it's doing.  It creates a chroma signal, which is (as we know) a necessary part of creating functional NTSC video.

If you search manufacturer's sites for 'chroma encoder' you will get the results you want.  It's not like I made up the term.  =)