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Old 21st May 2013, 11:26 AM   #1
diyAudio Member
Join Date: May 2013
Default Party-line / Full-duplex intercom - mixing audio signals

Hi All,

I'm hoping someone can help, apologies if my first post has ended up in the wrong forum. Party-line or full-duplex intercoms allow for the simultaneous transmission and reception of audio from all connected stations, i.e. effectively there is an audio 'bus' and each station takes a feed from it and routes this to its headset, and also adds a signal from its microphone that is then available to the other stations. The connecting wire can be either balanced or unbalanced.

Click the image to open in full size.

My question is: how do you add these signals together? Do you simply connect the outputs of the output op amps/line drivers at each station together onto the wire (via some form of AC coupling [not shown in the diagram above], perhaps?).

Is that what this guy does in this circuit here?: ComClone2 Circuit Diagram
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Old 21st May 2013, 01:52 PM   #2
DF96 is online now DF96  England
diyAudio Member
Join Date: May 2007
You need something called a 'hybrid', which knows the difference between putting a signal out and getting a signal in. In the olden days this required a transformer but op-amps can do something similar. Examine telephone circuits.
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