phone lines

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:

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

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

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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.

Post to news:sci.electronics.design, 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

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

He is on news:sci.electronics.design and news:alt.binaries.schematics.electronic quite often.