Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!think!ames!coherent!dplatt
From: dplatt@coherent.com (Dave Platt)
Newsgroups: sci.electronics
Subject: Re: Electrical pulse on telephone lines?
Keywords: ring
Message-ID: <42002@improper.coherent.com>
Date: 14 Dec 89 22:10:28 GMT
References: <4208@crdgw1.crd.ge.com> <4910@blake.acs.washington.edu>
Reply-To: dplatt@coherent.com (Dave Platt)
Distribution: na
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 28

In article <4910@blake.acs.washington.edu> wiml@blake.acs.washington.edu (William Lewis) writes:
>   The same thing happens here (Seattle, Wa., for those who can't see
> the header =8) ) at about 12:00-1:00 at night, every night. (I have
> one of those hyper-cheap phones hanging off of my modem line, and
> if I'm awake and offline at that time, the over-sensitive ringer
> will make a few sickly beeps ... this is a very sensitive ringer:
> it rings from pulse dialing, even ... )

This was discussed in comp.dcom.telecom some time ago.  As I recall,
the explanation is that the telco sends a measured voltage pulse down
the phone lines periodically.  By measuring the current flow and
certain other things, they can check the quality of the phone line
and detect some physical problems as they develop (e.g. insulation
failure, etc.).  The phone-switch in your central office does this
automatically, at a time when the line is idle (usually late at night),
and enters a trouble-report in the database if anything unusual
is detected.

The voltage-pulse used for this purpose is small enough that it won't
cause phones to ring, _if_ their ringers comply with Bell standards.
Many don't.  In particular, cheap phones often use piezoelectric
beepers hooked up in series with a capacitor;  these will often
"chirp" whenever there's a small voltage-change on the phone line.
-- 
Dave Platt                                             VOICE: (415) 493-8805
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