Need video signal expert (and help)

[charliez]

I have RGB signals, and a composite video signal. I need to separate the sync and combine it with the G(reen) signal.

I have found the LM1881 datasheet, but am unable to figure out how to recombine the sync signal back to the G(reen) signal.

Can you point me to a schematic?

Thanks!

 

[Nigel Goodwin]

I have RGB signals, and a composite video signal. I need to separate the sync and combine it with the G(reen) signal.

I have found the LM1881 datasheet, but am unable to figure out how to recombine the sync signal back to the G(reen) signal.

Can you point me to a schematic?

Thanks!

For a start, why do you want to do this?, and what are the two units in question?.

 

[charliez]

I have a video display screen that takes NTSC RGB with the sync on the green signal.

The source video is already NTSC RGB, it also has a composite video output (which includes sync). The source video comes from a VGA to RGB scan converter.

I've searched on www.epanorama.net video circuits and articles, but it doesn't quite have what I need.

I also have this trace of an existing working board, but I don't know what the transistor is (PNP, NPN, MOSFET, JFET, ???)

 

[jem]

Hi:

Here is my take in this. First of all, I would suggest using a GS4881 instead of the LM1881. Unless NatSemi has changed the design of the LM1881, they have terrible temperature drifts. Mind you that this is from my experience of about 12 years ago!

Second, I do not think your circuit can work. The big reason is C2. In order to add Sync on Green, you need to clamp the Green signal to a known DC level, and the sync has to be below that clamped black level. Using C2 (AC soupling) destroys this DC reference.

Now, if, as I suspect, the Green signal from your scan converter comes from a DAC, then it should be ground referenced. In this case, you can try this circuit. It is an adaptation of a circuit I designed about 12 years ago to convert a standard VGA signal (RGBHV) to RGB SOG without using any external power. In most video cards, the RGB signals are ground referenced, and the syncs are TTL levels (positive or negative). Basically, the circuit uses the backporch signal from the GS4881 to generate a small negative voltage, which is then superimposed on the Green video.

Hope this helps,

Jem

 

[charliez]

Many thanks Jim, I'll try this out !