LM1881N question

[Drewman21]

I have a question about the LM1881n chip.  I have two systms one that has sync and one that does not.  I want to only have one jack for the sync/composite line and switch the systems out on my own.  The question is that if i put the LM1881N in line on the sync/composite connection to the encoder it will strip sync out of the composite but will it let sync pass through it if it is powered down or even left on if feed though it?  A simple power switch would be great for me but I don't have a great understanding of these things.  Any suggesttions on what would be a better way to go with still keeping the single sync/composite jack?
Thanks for any help.

Andrew J

[NFG]

The LM1881 does not change a video-less sync signal.

[Drewman21]

So if I run the the LM1881N all the time with either the sync or composite signal I won't get any negative effects from it?  So if fed a Sync signal through the composite input it will not cause distortion to he sync signal upon output?

And would a on/off switch be preferable to shut it down if it isn't needed?  So would would the signal still travel though the chip passively?

How do people do this in a supergun?  I see people put them into them but never include info on how to wire it.  I don't want to sound like a noob on this but I just am not seeing another way to do this while keeping a single input instead of 2.  This is kinda how i have set it up to work and i won't try and force it if it doesn't go that way but it is how it is laid out right now so i want to go with it if i can.

thanks
Drewman21

[NFG]

As I said, it's fine to leave it in there all the time.  It doesn't need to be shut down.

[antron]

so this could be done as a mod on the arcade monitor chassis?
perhaps power the LM1881N with some DC voltage between 5 and 12V that may be available on the chassis?
and one would be able to send it signals from a PC in 15kHz mode or a console?
cool if so.

[NFG]

I put mine inside the receiving end of the DB9 I used to connect all my consoles to a monitor.  It's functionally the same as putting it in the chassis itself.

[viletim!]

Actually, it's not fine to hook up a composite sync signal to the input of an LM1881. See the datasheet under Absoulte Maximum Ratings where it lists the input voltage max at 3Vpp when run with a 5V Vcc. A TTL sync pulse can get as high as 5Vpp (esp true for PC video cards) which can damage the IC.

There are a couple of ways around this that I can think of atm...
-Run the IC from a supply voltage of 8 volts or more. The problem with this is that the IC's outputs will no longer be TTL compatible so you'll need resistor dividers on the sync output(s).
-Put a 1k series resistor followed by a reverse biased 2.7v zener diode to ground bofore the input coupling cap. This (should) let the video pass through unharmed and squash down the TTL sync down to less than 3Vpp before it enters the IC.

[Guest]

thank you for that observation

i'll just put a LM1881 on every console before the manual selector
most consoles seem to have +5V on the RGB video cable to power them.

[Guest]

I put mine inside the receiving end of the DB9 I used to connect all my consoles to a monitor.  It's functionally the same as putting it in the chassis itself.

do you run a PC (MAME) through that too sometimes?

[NFG]

I don't run PC MAME through anything but my PC monitor.

You might consider using a middle adaptor with the LM1881, between consoles and the monitor, that you remove for PC/PCB use.  Then you don't need to purchase and wire up a dozen chips.

[Guest]

I really want a turn-switch solution. those circuits are cheap to build anyay anyway.

thanks everyone!