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Very long cable audio link

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Hi Folks

I have to produce a full duplex audio connection system capable of operating over about a mile of cable. It will basiclly act as an intercom between a number of points.

There will be a number of tap in points (about 10 in total) any ONE of which can become a "listener" and any ONE of which can become a "talker" at any one time.

There's a bit of signalling too between talker and listener during conversation but it's just switching on and off relays at either end under push button control of the users.

Before anyone makes the suggestion, no I can't use radio and no I can't use the telephone network.

ok, so looks to me like we need some nice balanced line transmitters and receivers. Anyone got any reference designs for this sort of thing they'd be willing to share...or any advice from similar projects in the past??

One issue that I haven't resolved by myself is the question of whether or not to transformer couple the transmitters and receivers.

Any thoughts would be most appreciated as time is short
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