Best way to do RCA sync thru?

I’m designing a module(1) that’s going to have RCA Sync In (to a LM1881) and I’d like to have RCA Sync Thru also, but I’m not sure the best way to accomplish this. I had been planning on using the portion of the Cadet 1 schematic that does this. Video/sync in, terminated by 75R to ground, and it goes to a LM6172 buffer via a 499R (and to the LM1881 via a cap), and the buffer outputs the thru signal:

But recently I noticed that Syntonie’s VU001 handles Sync Thru a little differently… Input is directly tied to output, a jumper can terminate input to ground if you want, and input goes to a LM6172 buffer via a 499R and continues to the rest of the circuit:


So, I’m curious if @creatorlars or @syntonie (or anyone else who understand video electronics!) would care to comment on benefits/drawbacks to the different methods. One thing I like about the Cadet method is that users never have to deal with a termination jumper, since the input is always terminated. I don’t have anything that syncs via RCA, so I can’t test for any noticeable differences in behavior.

(1) I’ll be talking about my module more soon, but as a teaser, it’s a simple Arduino Nano-based module that’s being designed to be a multifunction cool-lowres-digital-shapes/text generator. Hopefully some people are interested in it, but even if they’re not, I won’t mind… I’m mostly designing it for myself since it’s something I really want.


Both approaches are valid. Generally there is a buffer when it’s a frontpanel jack, but maybe not one if the sync is just going to an LM1881 and meant for backplane distribution. The primary advantage to the second circuit is that you do not necessarily need U1.2 (the opamp buffer) if, for example, you are terminating the sync at the LM1881 and not going further with it. That may be just perfect for your project’s needs.


Do you mean not going any further with it in my own module but the thru output can still be used by other modules (if the termination jumper isn’t set)? If so, then that would indeed be perfect, since it’ll let me reroute one of the trickier parts of my PCB!

(edit to clarify this question:)
Do you mean I could do this?

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Hi @joem and @creatorlars, the buffer (U1.2) can probably be bypassed and the circuit will work in a similar manner. I’ve added it so the front RCA input and the sync in on the back have independent input load (so you can set the sync termination without affecting the front input termination), so it avoid having the signal at the front RCA to be unterminated when using it as a distribution amplifier. Also, felt bad having this extra op amp with no use :upside_down_face:

The schematic you posted looks right, no need for an op amp if you send the signal to the sync extractor directly and doesn’t require further processing.


@syntonie I would have done the same!
And yes, @joem that looks like a fine drawing to me! Just don’t forget your power bypass cap for LM1881. :slight_smile:
**Very important: ** If you do this method, your sync signal input net (the one shared between RCA jacks and Jumper_3 and 100n cap) should be as short as possible. Jacks should be adjacent to each other, termination resistor and lm1881 input pin should all be together in a cluster. Don’t let the sync run thru an interconnect or travel across the board.


Since no-one else has commented on this aspect of your post yet: yes, interested in your module! I have a similar idea on my “get around to this one day” pile too :slight_smile:


Thanks @creatorlars and @syntonie! This has been extremely helpful!

Funny you should mention that… I did forget it in my initial schematic drawings and caught when I made a breadboard prototype, but then forgot to update the schematics and only caught that a week ago! So I’ve forgotten it twice so far! Ha!

I’m going to do a proper post about it sometime when I have time, now that I’m far enough along with it to hopefully not abandon it.

It had been on my “get around to this one day” pile too, but then when I became unemployed during the first month of pandemic lockdown, I had all the time in the world to work on it! :slight_smile: