LM1881 sync stripper board from RGC

[ASDR]

This one:



https://www.retrogamingcables.co.uk/SYN ... B-STRIPPER

It's missing the 0.1uF & 680K components on the RSET line as specified by the data sheet (https://www.ti.com/lit/ds/symlink/lm1881.pdf).



These components are also not present in their SCART cables with integrated sync strippers



Other boards seems to have them, i.e.



Is there any downside to not including these components in the sync stripping circuit? Is there an issue with the circuit by RGC?

[Guspaz]

Per the datasheet, rset is for controlling the output vertical sync, and vertical sync is derived from composite sync (rather than the other way around). So if you're not using vertical sync, as neither that red board nor any SCART adapter would, you don't care about rset.

My guess is whoever designed that second (green) board didn't actually read the datasheet and just did whatever was on the "typical connection" diagram on the first page.

[ASDR]

Per the datasheet, rset is for controlling the output vertical sync, and vertical sync is derived from composite sync (rather than the other way around). So if you're not using vertical sync, as neither that red board nor any SCART adapter would, you don't care about rset.

My guess is whoever designed that second (green) board didn't actually read the datasheet and just did whatever was on the "typical connection" diagram on the first page.

Alright, you only need to worry RSET if you actually want to use it as some kind of sync separator. Weird that they're present in all of RetroRGB's sync stripping circuits (https://www.retrorgb.com/syncstripper.html) even though none of them use the vsync output.

Thanks for clearing that up!

[matt]

It's also been noted that this circuit should have a 75 ohm terminating resistor on the input side.

[ASDR]

It's also been noted that this circuit should have a 75 ohm terminating resistor on the input side.

Ok, it doesn't have 75 ohm termination internally? I have cables with strippers from RGC and thefoo83 and neither of them do that, nor do the two boards pictured above from RGC and VGP. Never had issues with 'reflection', though.

I assume this would be a correct circuit:



The RGC board is also missing the decoupling cap.

[matt]

Yes, the chip usually works fine without the 75 ohm resistor. I didn't use it for many years and never had a problem either. But it's been noted by folks with some expertise that it should be included. Tim Worthington made a fairly in-depth post on this forum a while back that's a good read.

[ASDR]

Guess it's like the decoupling cap in Tim's circuit above, you might be fine without it. Or like not having a fuse or an RF shield in your console, will probably be fine without them, too. If I ever design my own board I'll certainly include the termination resistor.