I've been reading on how old landline telephones work (POTS, as they call it) and found out it's quite a bit more involved than I had expected.
This circuit comes from the wikipedia article "Telephone", and I have a few questions.
First, what I've got so far. Please tell me if I got anything wrong:
There is a pretty high DC voltage (48 V) between both wires (called tip and ring). I still don't know which one is higher though. Wikipedia says the voltage is negative with respect to earth to avoid corrotion but I don't know which one is connected to ground, if tip or ring.
The voices are modulated on the AC current, not on the voltage. This means that the current along the loop should be
i = I_DC + k1 * x1 + k2 * x2. In that equation
I_DCis the current resulting from the DC offset on the line,
x2are the voice signals of each user (they could be air pressure at the microphone) and
k2are constants to convert to current.
With that in mind, I'm trying to understand how this circuit works. I have a few questions:
At audio frequencies, the impedance of the speaker should be fairly resistive so the fact that it's modulated current makes no difference at all. The current drives the speaker just as well as a
R_speaker * ivoltage would. Is this correct?
You get feedback on the speaker, right? The microphone signal goes through the transformer, I think I should hear myself but I never payed attention to that, so I don't remember (also it's been a while since I used a landline phone).
Related to the previous questions, Wikipedia mentions that A3 is a hybrid coil, so that the outgoing signal doesn't overpower the incoming signal. What I think this means is that A3 adds up both signals, but since the current from A2 goes into the middle tap, it's level is reduced by the turns ratio on the transformer, since the incoming signal uses the whole coil. Is this correct?
A2 is the microphone, represented as a variable resistance, which I don't understand. My first guess would be that the variable resistance generates AC current due to the DC voltage applied to the resistor. Is this correct? However, most modern phones use electret microphones, which just generate a small voltage proportional to the sound pressure. How is this implemented on a modern phone? Do they feed a current source with the amplified voltage from the mic? And how is it implemented on an old phone? What kind of microphone works as a variable resistance?
What are the typical amplitudes of the AC currents?
These are all the questions I can think of right know. I will update if I think of anything else.