LM1881 Based sync stripper/inserter

sync

#1

I’m working on a video ring mod intended for composite signal based on the ad835 IC. In order for it to work properly I’d need to separate the sync signal from the composite video and I was intending to do that with the LM1881 sync separator. From what Lars posted on an old post on the video circuits facebook group I understood (and maybe I’m wrong) that in order to re-insert the sync signal you first need to black-clamp/dc restore the glitched out signal, then clip the black and white levels and generate a blanking signal using lm1881 back porch out and an analog switch using a 0 V reference, and, just then, with another analog switch, using the composite sync out of the LM1881 as control signal to put the sync in the previously blanked 0 V portion of the signal using a -286 mV reference voltage.
Also I’d need buffering stages before the lm1881, after the black clamping and a 2x gain one before the output
Now I am wondering
A: if I got this right
B: where and how to split the signal so that it goes both into the lm1881 for sync extraction and the ad835 for the actual processing before all the dc restoration, black and white clipping, blanking and sync insertion.


#2

Hey there!!
So, there are a lot of steps in between input and output in the type of circuit you’re working on. Have you checked out the LZX Cadet module schematics? They are open source and available for download on our website, and present a structure for doing what you want to do.

Check out the Cadet 3 schematic. This shows a basic framework for DC restoration of external video. The Cadet 2 schematic shows a structure for taking the DC restored signal (after modifying it using for example a 4 quadrant multiplier like AD835 or the setup in the Cadet 10 Multiplier), clipping black/white levels, blanking, and then encoding to RGB (including sync insertion, which is a function of the AD724 IC.)

If you are doing a composite video in/out circuit, there are some missing pieces you’ll need to fill in. Do you have a block diagram or schematic of what your circuit looks like so far?