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Date: 02/26/02 at 8:43 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk

In Reply to: I think you are not correct russlk posted by Waqar Zaka on 02/26/02 at 5:45 AM:

Hi Waqar,

Ordinary analogue telephones use only two wires to carry a full-duplex conversation. The way it is done is this:

The transmitted signal voltage is impressed on the two wires, usually via an audio coupling transformer.

In the received signal path, the transmitted signal (from the transmit signal path) is subtracted from the signal received on the two wires. That cancels out the transmitted signal, so that all you have left is whatever has come from the opposite end. This can be easily done using op-amps.

Modems that connect to an analogue telephone line use the same technique. I once used a (300 baud!!!) modem chip from OKI semiconductor, and the data sheet showed how the transmit and receive paths were implemented. The data sheet is at

http://www.oki.com/semi/datadocs/doc-eng/msm7510.pdf

David Ashby

Follow Ups:

  Thankyou dashby Waqar Zaka   Posted at: 02/26/02 (0)

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