John Popelish said:
As far as the copper is concerned, the two signals are just added
together. For a simple phone loop, you hear both your own voice and the
other person's voice in your ear piece. The tricky part cane in when the
lines got long enough that amplifiers were needed. Then the two signals
had to be separated, so that one amplifier boosted the signal going one
way, and another amplifier boosted the signal going the other way. I
think the first fancy electronics invented for this purpose was the hybrid
transformer, that performed this separation.
Perhaps this page will help:
http://www.du.edu/~etuttle/electron/elect61.htm
This hybrid, sometimes called a transhybrid or 2-to-4 wire converter is a
nifty little gadget. Its basic operation relies on the concept of being able
to predict what the transfer function is from the telephone's microphone to
the phone line -- while not driving the line with a zero ohm source
impedance (which would kill the incoming signal from the person you're
talking to).
In the simplest case, you assume that the phone line has some fixed
impedance -- say 600 ohms. If you then drive the line with a 600 ohm source
impedance driver then you know that the signal driven onto the line will be
exactly V/2 (assuming V is at the low-impedance side of the driver). Since
you know you're putting out V/2 on the line then you can simply subtract a
V/2 copy from the line and use that difference signal as your receive
signal.
In this perfect case, you see zero (-infinity dB) of your own signal in the
receive path when you're transmitting. However, in the real world, you
cannot precisely predict what the line impedance will be. In practice, while
using a simple impedance modeling network, if you get 10dB of rejection
you're lucky. There are some fancy signal processing techniques that can do
better than a simple passive (or opamp based) circuit, but they're
(obviously) more complex.
Good phone always add back some of your own transmit signal back into your
earpiece. This is called sidetone. It replaces the signal, from your mouth,
that was blocked by placement of the phone to your ear. The cell phones that
I've used don't do this. To me, it sounds very unnatural. This is one of the
reasons I can't stand using those little fuggers.
Bob
