Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Connecting Clear-com to telephone
References: 
[email protected] (Adam Sacarny) writes:

> Hello,
> Sorry to ask another question in such a short period of time. I
> appreciate all of the answers and I just picked up some capacitors for
> recording off of the Clear-com 3-pin XLR plugs.
> My friends and I will be filming a show and for communication we will
> be using a telephone-based intercom system
> (http://www.cownow.com/~ben/theatre/icmart.txt). However, everyone
> else will be on Clear-com beltpaks. How can we patch through between
> the two systems? We've considered some options
> 1) Connect the phone (green and red lines) to pins 1 and 3 of the
> 3-pin Clear-com XLR. 

If you just connect a notmal telephone like this, it will not 
get the operating power it needs. So with a modern phone you will 
not hear anythign because the telephone is not powered. 
Some very old telephones which do not have any active electronics 
in them might work...

> But I would imagine this would create some
> problems, if only because projects like these are never that simple
> :-) But seriously, there would be impedance issues, right? 

Yes. Clear-com system is 200 ohms. 
Telephone is designed for 600 ohms line.

> I'm not
> even sure what impedance *IS*, but I'm guessing it would be a problem
> here :-)

Impedance, denoted Z, is an expression of the opposition that an
electronic component, circuit, or system offers to alternating and/or
direct electric current. Impedance is a vector
(two-dimensional)quantity consisting of two independent scalar
(one-dimensional) phenomena: resistance and reactance.

Yes. And it is not the only problem.

> 2) Connect a phone headset cable (with its 4 pins) to the proper
> respective pins of the 4-pin Clear-com XLR. 

Could work...

> I've read there would be issues with microphone impedance. 

Could be...

> It would be okay (but not ideal) if
> we could just *hear* what's going on over the clear-com system,
> without being able to talk over it. Could this be easily done?

One idea to try: 
Do the recording appriach as discussed eariler. 
Do the connection with the isolation transformer. 
Use 600:600 ohm telecom transformer. Use this 
tranformer output as one phone on the phone intercom system.

I quess that could work. The line level signal on the clear-com 
party line sould be roughtly compatible to the level you can 
feed to a normal phone...

> So we're a bit confused at the moment. Although I don't know a lot
> about circuits I can get in touch with people who do. I also have at
> my disposal a two line phone -- if that helps. Any guidance would be
> greatly appreciated!
> Thanks!
> Adam
> -- 
> Are you doing your part? http://masturbateforpeace.com/

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/