Looking at my [landline] phone jack, I only see 2 contacts. One of these must be the ground reference, leaving only one line to transfer bidirectional audio AND key-presses. I am aware that dialing is done with DTMF (modulated sine waves at certain frequencies), but how can incoming and outgoing audio be transferred to the phone?
Thanks!
baum
baum:
Looking at my [landline] phone jack, I only see 2 contacts. One of these must be the ground reference, leaving only one line to transfer bidirectional audio AND key-presses. I am aware that dialing is done with DTMF (modulated sine waves at certain frequencies), but how can incoming and outgoing audio be transferred to the phone?Thanks!
baum
A landline telephone line does not ground either of the two leads, called tip and ring. In fact the teleco company has monitoring equipment on their lines to detect for improper grounding of either lead should that happen.
Interfacing signal onto a phone line requires special isolation circuitry. It can be done but the circuitry has to be protected from things like the 90AC 20Hz ringing current, etc. There are approved interface module designed to make proper interfacing of signals to and from a phone line. There are also laws that only 'approved' interface components to be used depending on country.
Lefty
So one wire is RX and one is TX?
baum:
So one wire is RX and one is TX?
No, the two wires form a current loop that carry audio frequencies in both directions at the same time. That is why special circuitry is use to be able to couple signals onto and out of line. Special balancing requirements must be met to make sure the proper signal levels are used to not unbalance the line. Interfacing onto phone lines is not a simple task that requires special approved components and design.
You are best off searching for proven existing designs or modules that do all the complicated parts.
Lefty
okay. i get it. Thanks!
The two-wire copper connection for plain old telephone service (POTS) is usually referred to as the local loop. Google has a lot on those keywords.
The local loop is a current loop. The central office runs a 48-volt potential on the circuit. The signals are sent in both directions by varying the impedance in the circuit, and thus the current flow. The fact that both the transmit and receive signals are superimposed on one another has the effect that you hear yourself in the earpiece, which is okay for voice, but substantially complicates modems and digital encoding. Look up echo cancellation for all the gory details.
Look up echo cancellation for all the gory details.
Nah. I'm good. 
Thanks!