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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Pro AV Forum => Topic started by: Erik Jerde on September 13, 2018, 01:00:23 am

Title: SDI - connecting two outputs together
Post by: Erik Jerde on September 13, 2018, 01:00:23 am
So what's going to happen if I connect two SDI outputs together?  Say something as simple as connecting a video router output port to another output port on the same router?  My gut says it's a bad idea but I can't say why.  I know why you don't connect audio outputs together, I just don't know if the same principal applies to video outputs.  Probably does.

The reason I'm asking is I've got a video patchbay with normaled modules in it and I'd like a couple of the modules to be straight-through but I'm having trouble sourcing the straight through modules.  So, if it won't damage anything or cause problems I can just connect a couple router outputs to each side of a module.  Inserting a patch cable to actually patch them into something will break the normal and then the signal flows just like you'd like it to.  Can't add another patchbay, no space in the rack.

So, like I said, probably a bad idea, but why?  I'm not going to actually do it (on purpose) unless someone knowledgeable on this sort of thing can tell me that it actually is ok.
Title: Re: SDI - connecting two outputs together
Post by: brian maddox on September 13, 2018, 09:00:54 am
So what's going to happen if I connect two SDI outputs together?  Say something as simple as connecting a video router output port to another output port on the same router?  My gut says it's a bad idea but I can't say why.  I know why you don't connect audio outputs together, I just don't know if the same principal applies to video outputs.  Probably does.

The reason I'm asking is I've got a video patchbay with normaled modules in it and I'd like a couple of the modules to be straight-through but I'm having trouble sourcing the straight through modules.  So, if it won't damage anything or cause problems I can just connect a couple router outputs to each side of a module.  Inserting a patch cable to actually patch them into something will break the normal and then the signal flows just like you'd like it to.  Can't add another patchbay, no space in the rack.

So, like I said, probably a bad idea, but why?  I'm not going to actually do it (on purpose) unless someone knowledgeable on this sort of thing can tell me that it actually is ok.

I can't see how plugging two SDI outputs together would damage them in any way, given modern circuit design, but i wouldn't call myself an expert so take that for what it's worth.

That being said, nearly all patchbay modules can have the normaling changed.  Can you just disconnect the normal?  Or barring that, you could always just leave a dummy plug in the bottom connector to 'break' the normal.
Title: Re: SDI - connecting two outputs together
Post by: Erik Jerde on September 13, 2018, 09:46:23 am
I can't see how plugging two SDI outputs together would damage them in any way, given modern circuit design, but i wouldn't call myself an expert so take that for what it's worth.

That being said, nearly all patchbay modules can have the normaling changed.  Can you just disconnect the normal?  Or barring that, you could always just leave a dummy plug in the bottom connector to 'break' the normal.

Audio patchbays are configurable.  Video ones (that I've seen) are sealed units with fixed configuration.  I believe it has to do with keeping impedance right.  It is annoying though!

I've thought about just sticking a patch-plug to bnc adapter into one side to break the normal like you suggest but if having them connected together is a bad idea I'd like to protect against someone coming along and removing the plug.
Title: Re: SDI - connecting two outputs together
Post by: brian maddox on September 13, 2018, 05:56:27 pm
Audio patchbays are configurable.  Video ones (that I've seen) are sealed units with fixed configuration.  I believe it has to do with keeping impedance right.  It is annoying though!

I've thought about just sticking a patch-plug to bnc adapter into one side to break the normal like you suggest but if having them connected together is a bad idea I'd like to protect against someone coming along and removing the plug.

I'm not a video guy, but i've spent a lifetime working alongside them.  I've NEVER heard anyone caution vehemently about taking great care to never plug outputs into each other.  Given that BNC cables are not gendered to indicate signal flow like audio and DMX are, this has to happen during setup all the time.  I just can't see how this could actually break anything.
Title: Re: SDI - connecting two outputs together
Post by: Erik Jerde on September 13, 2018, 11:18:44 pm
I'm not a video guy, but i've spent a lifetime working alongside them.  I've NEVER heard anyone caution vehemently about taking great care to never plug outputs into each other.  Given that BNC cables are not gendered to indicate signal flow like audio and DMX are, this has to happen during setup all the time.  I just can't see how this could actually break anything.

I agree.  I've got the same experience.  Fortunately I located a new old stock box of straight through patch panel modules so my problem is purely theoretical now.  Next time I run into a mfrg engineering rep though I'm going to put the question to him/her.  I figured with most PSW people being audio nerds first and foremost it was unlikely anyone here would have a definitive answer.  Guy's gotta try though right?  :)