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From: dave%westmark@uunet.uu.net (Dave Levenson)
Newsgroups: comp.dcom.telecom
Subject: Re: How Does a Telephone Receiver Work?
Message-ID: <9849@accuvax.nwu.edu>
Date: 18 Jul 90 18:08:39 GMT
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In article <9841@accuvax.nwu.edu>, dave@mars.njit.edu (Dave Michaels
cccc) writes:

> A telephone handset has four wires going to it, two for mike, and two
> for speaker.  How does the phone merge these two into a full duplex
> pair of wires?  

The telephone contains a hybrid (located in a little box often called
a network) which is supposed to direct the audio energy from the
transmitter toward the line, and energy from the line toward the
receiver.  A little bit of audio from the transmitter is deliberately
'leaked' to the receiver, so that you can hear yourself (which is
often called side-tone).  This leakage path is attenuated so that most
of your speech-energy is directed to the far end.  This attenuation is
called trans-hybrid loss.

> Also, I disconnected the transmitter disc trying to
> make a 'mute' feature on a phone without one, and discovered my friend
> can still hear me (at a reduced volume) from the earpiece ... hmm,
> why?

Your telephone receiver may act in reverse, as a low-level microphone.
Your friend can year the signal it generates due to the above-
mentioned leakage-path working in reverse.  The volume is
reduced partially because the receiver is not very efficient as a
microphone, and partially because of the deliberate trans-hybrid loss.


Dave Levenson			Voice: 201 647 0900  Fax: 201 647 6857
Westmark, Inc.			UUCP: {uunet | rutgers | att}!westmark!dave
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[The Man in the Mooney]