COM Integration Question

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ga.johnson121

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COM Integration Question

PostWed May 11, 2022 11:55 pm

Hello,
I am working on integrating an existing ClearCom (MS-232) system with a BMD Studio Fiber system (URSA Broadcast). We have 3 cameras connected via fiber and are trying to get comms with each camera through the fiber and onboard com setup. I got a RTS SSA-424 converter to handle switching from 4-wire to 2-wire com.

My question is wiring the 4-wire side. I found the pinout for the DB-25 connector on the back of the fiber receiver (studio end) and made a cable to connect to that port.
Do I need a seperate 2-wire to 4-wire converter for each fiber setup? Or can I connect all 3 fiber receivers in parallel to the same converter?

I drew up my two wiring options (as I see it), with the separate convert option shown in blue and the parallel connection through one converter shown in red.

If there is another option that I have missed, I would be interested in that as well.

I'm mainly interested in the cameras being able to hear the director, so if this ends up as a one-way system, that would be fine.

Thanks in advance for all the help!

EDIT: looks like the image didn’t work properly. I’ll try to re upload it shortly.
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ga.johnson121

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  • Real Name: Gabe Johnson

Re: COM Integration Question

PostTue May 17, 2022 8:51 pm

Hello, here is the sketch I made of my wiring options. Anyone have any advice on setting up a ClearCom integration?

Thanks!
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Dave Del Vecchio

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Re: COM Integration Question

PostWed May 18, 2022 1:34 am

With 4-wire intercom audio, the talk and listen audio for each user are two separate audio channels. This is the way matrix intercom systems work as well as the audio from the Blackmagic Studio Fiber Converters.

With 2-wire intercom systems (analog party line systems from Clear-Com, RTS, etc.), the talk and listen audio for each user are combined together on a single (party-line) audio channel. So when you interface 4-wire intercom audio with a 2-wire party line system (in the simplest way), the talk audio from a 4-wire user gets added to the 2-wire audio at which point it becomes listen audio on the 2-wire system. And then this audio from the 2-wire akes it's way back to the listen audio channel of that 4-wire user. And having the 4-wire talk audio also appear as full volume 4-wire listen audio can often lead to feedback, echo, and other problems.

Most 2-wire to 4-wire intercom audio converters have some kind of capability to deal with this problem, usually called nulling or null adjustment, to try to control how much of the 4-wire talk audio makes it's way back onto the 4-wire listen circuit. Older 2-wire to 4-wire converters had manual null adjustment controls (which were a bit of a pain to use), on newer converters such as the RTS RSA-424 and DSI-2008, this nulling is done automatically.

What this means, is that if you were to take multiple 4-wire audio talk circuits (such as from multiple Studio Fiber Converters) and wire them together (such as with a y-cable to sum the audio), then run that audio into a 2-wire to 4-wire converter, the nulling of the converter would try to prevent that audio from making it back to the 4-wire listen output. Since there's no way to differentiate audio from each user (each camera operator) any longer, it would all get canceled equally, which would mean that it if one camera operator is talking, it might be difficult for the other camera operators to hear it (though users on the 2-wire system would be able to hear it fine).

To avoid this issue, you could use a separate 2-wire to 4-wire converter for each 4-wire user, which would also allow you to separately set the audio contribution levels of each 4-wire circuit to the 2-wire system.

Note that the RSA-424 actually has two separate 2-wire to 4-wire converters, which could be used to convert 2 analog party line intercom channels to 4-wire audio. And the Studio Fiber Converters also have 2 intercom channels (Production and Engineering) on their 4-wire outputs. So, with one RSA-424 you could interface both 2-wire channels from the Clear-Com MS-232 to the 2 intercom channels of the Studio Fiber Converter for a single user. Or if you are only using a single intercom channel for the camera operators, you could use the RSA-424 to interface that single intercom channel for two different users (you'd just need an XLR splitter for the 2-wire input into the RSA-424).

And if you don't care about talkback audio from the camera operators at all. Then you can just wire the 4-wire audio outputs from the RSA-424 to the listen channel inputs on all of the Studio Fiber Converters (to either the Production or Engineering channel depending on which you want to use). This would allow the camera operators to hear the audio from the 2-wire intercom system, but not talk into it. In that case you'd only need to use half of the RSA-424 input/output ports and wouldn't need to wire the talkback (output) audio pins from DB-25 connector of the Studio Converter.

For more background on 2-wire to 4-wire conversion and intercom system interfacing, these are some good references:
https://products.rtsintercoms.com/binar ... eering.pdf
https://www.plsystem.com/plsys03.htm

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