I remember a college assignment where I had to design a sound reinforcement system with a multiple console setup for a stadium sized venue and I was quite stumped as to how the whole thing was done at that point. After much research, asking around and reading about it I managed to gain a basic understanding of the entire signal flow, now about 5 years on I have seen most of these setups and done quite a few shows with such setups. Here is my understanding of the signal flow and setup of MOH and FOH both analog and digital.
From the point of view of stage, not much changes except that all the inputs which would get patched into the console snake now get patched to a Splitter. The splitter takes in a single input and gives out multiple transformer isolated balanced outputs(“copies”) of the signal for use with monitor consoles, broadcast OB van consoles, recording outputs etc. Most commonly splitters give out two splits, one for the Front of House (FOH)) console and another for the Monitors On House (MOH) console.
As seen in the picture on the right the inputs get patched as standard into the snake however there are two outputs of the signal given out, one called the Main and the other called the Split output. The difference between both is that only the main output allows passing of phantom power from the console to the mic. If both splits were allowed to pass phantom power that would lead to two separate feeds of +48V(dc) adding upto +96V(dc) which would cause your mic to get fried for sure!
Generally the main out is given to the console with priority such as FOH console which mixes for the audience and the split out is given to the lesser priority consoles such as the MOH, or Broadcast etc. When a mic needs phantom power it is upon the FOH engineer to turn on the phantom power so that the MOH engineer also receives the mic signal.
Various other splitters such as the DS-800 by XTA Electronics allow upto 4 separate splits of the signal with transformer isolation as a fixed feature. However due to the higher number of splits each unit only takes in about 8 inputs, which means for a 55 input situation there would be a requirement of about 7 DS-800’s which is as expensive as a console!
Front Of House(FOH)
The FOH console would receive the first split from the splitter which would mean that phantom power requirement are to be met from the person operating this console. Not significant changes happen when it comes to the mixing procedure since analog splits allow consoles at MOH and FOH to work independently.
Monitor Of House(MOH)
The MOH console receives the second split out from the splitter and will depend on the FOH operator only for phantom power to be turned on for condenser mics, in all other cases MOH operation is entirely independent from FOH.
Communication Between MOH and FOH
Since there are some discussions about the weather and about the neighbours dog that need to take place between FOH and MOH a communication medium is required between both the consoles. The good old way of screaming your guts out over the PA might work in small venues but in a stadium with a monstrous PA it might be quite futile. Cellphones are another common medium but may not be useful if you are doing a show on some beachside in the middle of timbuktu, cellphone networks may not be reliable, as for walky talkies lets not even get to those since they never work as expected, unless you bring your own set.
The best way to work for communication is usage of the good ol’ talkback mic being routed out from the console and being returned through the stage snake and fed to the monitor console as an input and a similar thing done by the monitor console operator. My preference has always been to send it through the direct out on the channel of talkback and the talkback coming from the other console can be directly routed to the console wedges for uninterrupted communication. Some people prefer their talkback being sent through an aux for some reason that i can’t comprehend…maybe its an analog habit.
The digital side of things
Digital setups allow usage of the console stage box to be used as a splitter and allow separate feeds for MOH and FOH without having to run the bulky Concert Snake and another snake for returns. One single fiber-optic or BNC line can carry upto 96 channels of digital audio with control data and the without any significant noise interference. Most of these stage boxes from leading manufacturers such as Midas, Digico, Soundcraft, Avid, offer a split option on the stage box where u can connect another snake and get a split of the signal. The rest remains similar, i.e. the FOH console controls the preamps on the stagebox. So if the FOH operator increases gain on a certain channel the Monitor console signal will be increased by the same amount.
This may seem quite stupid but is the way digital works. This would of course create havoc for the monitor engineer as changing gain levels during show results in monitor mix changes and brings on the wrath of the egoistic artist. Most consoles to counter this have created an option called trim which is the next knob after the gain, which allows you to compensate for such changes. I don’t if the other consoles do it automatically but Digico allows automatic compensation of the gain level. Midas on the other hand i think offers independent feeds but I can’t be quite sure about that.
Hope this was informative and clear.Here is an image that will sum it all up from the analog point of view. Thanks for reading. If you liked this article please do recommend it to your friends and share the word.