# phone lines

Discussion in 'Electronic Basics' started by [email protected], May 10, 2005.

1. ### Guest

How can a two way conversation be transmitted along two wires of my
residential phone line?.
Why don't the signals confuse(collide) each other as they travel
simultaneously in opposite directions along these two pieces of copper
wire from the pole to my phone?.

This is not about fancy elctronics,as the same applied in the 1880s

2. ### John PopelishGuest

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.

http://www.du.edu/~etuttle/electron/elect61.htm

3. ### BobGuest

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

4. ### Guest

Thanks for the explainations, John and Bob.
I hate mobile phones too.
Only have one for emergency calls.
Hartly.

5. ### BobGGuest

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.
================================
They would have to add analog sidetone locally...... there is a
significant round trip delay in the digitization and packetization and
sending and receiving process.

6. ### John - KD5YIGuest

Hi, John -

I read that page with interest. I was a bit surprised to note that the
author, in the next-to-last paragraph, refers to Horowitz and Hill and that
their hybrid circuit doesn't work. Do you suppose Win is aware of that? I
don't know how to get in touch with him.

Thanks,
John

7. ### Rich GriseGuest

Post to with a subject line like, Ping Win
Hill - What's this guy talking about?

Tell him Rich the Newsgroup Wacko sent you. ;-D

Cheers!
Rich

He is on and
quite often.