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From: tjhorton@utai.UUCP
Newsgroups: sci.electronics,sci.med
Subject: Re: Injury via Phone
Message-ID: <4046@utai.UUCP>
Date: Sat, 22-Aug-87 15:14:09 EDT
Article-I.D.: utai.4046
Posted: Sat Aug 22 15:14:09 1987
Date-Received: Sun, 23-Aug-87 10:16:43 EDT
References: <414@gtx.com>
Reply-To: tjhorton@ai.UUCP (Timothy J. Horton)
Organization: CSRI, University of Toronto
Lines: 34
Xref: utgpu sci.electronics:1076 sci.med:2600
Summary: 

Properly designed systems 'should not' be able to injure
(but that's 'should not', not 'cannot').

With digital systems, you can easily design in limits on the maximum
sound level, since there is always filtering of SOME sort anyways.
(this may be 'implicit' in the design, but nevertheless there)
The trouble is, with digital systems, things don't always behave.
"PCM hits" are a class of problem that have plagued some transmission
systems, and can come in in any number of ways, on the switch side.
On the phone side, there's always static discharge, lightning, etc.
But in any case, none of these are ways that a caller can use to
hurt your ears.  Since most phone calls in North America get digitized
somewhere along the line (I don't have figures) this should be the rule.

In analog systems, it's easy enough to use current or voltage limiting 
on the drivers, but I'm not sure that this is as suitable or flexible.
The human ear can tolerate enormous sound levels in general, but it
all depends on the phones and switching equipment involved.  You have
to take into account evan what the handsets can pick up and deliver.

Speaking of lightning... I had the pleasure of doing some work to
design protection systems for phones... to put it simply, it's
damned well impossible to lightning-proof a phone.
You can get lightning hit one or both of tip and ring, thousands
or millions of volts get through to the phone, and any nature of
impedance between the actual strike and the phone.  
I know someone personally who had a 12 inch arc go from the phone's
mouthpiece (in her hand) to a nearby windowframe.  Surprise!

-- 
Timothy J Horton (416) 979-3109   tjhorton@ai.toronto.edu (CSnet,UUCP,Bitnet)
Dept of Computer Science          tjhorton@ai.toronto.cdn (EAN X.400)
University of Toronto,            {seismo,watmath}!ai.toronto.edu!tjhorton
Toronto, Canada M5S 1A4