You said you wanted to use an LCD port. I had to guess what you meant by that, as you were not at all specific. Yes, composite video consists of negative-going composite sync about 1/2 volt below nominal black and positive-going video either digitally (pixel-by-pixel) between fully "on" (about 1 volt above black) and "nearly" black, or analog levels if that's what you require. The video blanking is what holds the black level. Since composite video is most often interfaced at 75 ohms, it's easiest to let the 75-ohm termination at the monitor provide the most negative level, then allow blank/black to be about 1/2 volt above that, with peak video at about 1 volt above that black level. That may not be what you require at all, but you didn't say.

The LCD port is normally that port that "talks" to a character LCD module. If you meant something else, you'll get better information if you specify it precisely.

I don't see how you'd use an LCD port of that type to interface to a video display.

RE