What is inside Telephone Set?

[pardeep]

I'm breaking head on telephone encryption in local loop. The local loop has only one pair of conductors forming a current loop. The telephone system works in full duplex mode. I fail to understand how it separates incoming and outgoing speech from the voice modulated current stream.

Thanks
Pardeep

 

[klug]

Let say, you are subscriber B.
You have A - incoming, B - outgoing signal.

Line=A+B

the trick is that you produce outgoing signal B. So, you have clean B that is produced by you.

To get clean A, you need :

A=Line-B

That is all.

There are two ways to make such substraction: by transformer or by circuit in operational amplifiers. Old telephones are using transformer differencial system.