From a 1921 AT&T text.

You have two parallel two-wire LONG LONE phone circuits. How can you use these wires to carry an additional DC Telegragh signal with NO interference to the existing phone lines?

(P.S.: There were some REALLY smart cookies 100 years ago!)

 

Twisted pairs?
Individual shielded pairs?

 

METALLIC POLAR-DUPLEX

 

METALLIC POLAR-DUPLEX

You're close. How was this circuit isolated from the phone lines?

 

You're close. How was this circuit isolated from the phone lines?

I could lookup the exact details (I don't remember) but I worked in some really old TCF operations that had some really old comm equipment in the 70's that had the capability to multiplex DC teletype/telegraph signals on audio 'dry loop' landlines.
https://clearedjobs.net/job/tech-co...ci-ndash-dahlgren-va-quantico-virginia-202903
https://miscpartsmanuals1.tpub.com/TM-11-5895-1012-10/

 

Are you referring to the "phantom pair"? Where the AC signals are transformer coupled into the two phone signal pairs, and the DC voltage is sent between the two pairs?

 

Are you referring to the "phantom pair"? Where the AC signals are transformer coupled into the two phone signal pairs, and the DC voltage is sent between the two pairs?

Yes...the phantom circuit. A few years later, they used the phantom circuit for another voice circuit, saving 50% of the wire used

 

Yes...the phantom circuit. A few years later, they used the phantom circuit for another voice circuit, saving 50% of the wire used

Not a lot different from Power over Ethernet!

 

I could lookup the exact details (I don't remember) but I worked in some really old TCF operations that had some really old comm equipment in the 70's that had the capability to multiplex DC teletype/telegraph signals on audio 'dry loop' landlines.
https://clearedjobs.net/job/tech-co...ci-ndash-dahlgren-va-quantico-virginia-202903
https://miscpartsmanuals1.tpub.com/TM-11-5895-1012-10/

A typical TCF with some of that old equipment we visited.