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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.electronics
Subject: Re: Full duplex intercoms?
Message-ID: <7846@utzoo.UUCP>
Date: Mon, 30-Mar-87 20:52:50 EST
Article-I.D.: utzoo.7846
Posted: Mon Mar 30 20:52:50 1987
Date-Received: Mon, 30-Mar-87 20:52:50 EST
References: <1922@ihlpl.ATT.COM> <562@puff.WISC.EDU> <3972@fritz.UUCP>, <18003@ucbvax.BERKELEY.EDU>
Organization: U of Toronto Zoology
Lines: 16

> Why is it that most intercoms and speaker phones are half duplex? They usually
> require a push to speak button.

Basically, because the manufacturers are cheap.  A half-duplex intercom can
be just an audio amplifier, a speaker, and a switch or two.  To make it work
full-duplex, you need some way to prevent feedback, which gets more complex.

> Does anyone know of any intercoms which are
> full duplex, i.e., both parties can speak at the same time with no button to
> push?

Not commercial ones.  Circuits for such things have been published here and
there.  Not inordinately difficult, as I recall, but non-trivial.
-- 
"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology
the dust.  Which shall it be?"	{allegra,ihnp4,decvax,pyramid}!utzoo!henry