So that's what it's for...

A project log for It's evolving: Aiphone video intercom

Modifying an old video intercom to bring it up to the 21st century.

FlorianFlorian 07/10/2021 at 03:020 Comments

Using LTspice XVII I simulated the part of the circuit of the display module I didn't understand what its purpose was. For the composite video signal source, I used the NTSC voltage source published here by Bordodynov

And here is the simulation results:

So it's look like this circuit is adding a small offset to the original signal, making it completely positive (no more signal falling under 0V). As I know nothing about video signal, I assume there is AC and DC coupling for video composite signal and that small module display works only with DC coupled signal?

To help me understand better, I looked for some litterature and found those:

From what I understood, the composite video signal coming out of the mainboard should be AC coupled and that small circuit in the display module does a DC restoration before feeding the composite signal to the LC749880T (Image controller LSI for LCD-TV).