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Path: utzoo!watmath!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: net.dcom
Subject: Re: 2-line phone and modem problems
Message-ID: <293@kitty.UUCP>
Date: Sat, 10-Aug-85 09:32:36 EDT
Article-I.D.: kitty.293
Posted: Sat Aug 10 09:32:36 1985
Date-Received: Sun, 11-Aug-85 08:24:13 EDT
References: <427@ucdavis.UUCP>
Distribution: net
Organization: Recognition Research Corp., Clarence, NY
Lines: 40

> my roomate recently bought a special 2-line phone; it
> seems to be causing problems with my modem.  when this
> phone is hooked up, i receive voluminous amounts of your
> standard modem type garbage ("{i", "|}", etc.).  i'm
> convinced that this phone is the cause, because by simply
> disconnecting it the garbage stops.
> 
> has anybody else experienced this?  what exactly is causing this?
> is there a solution which doesn't include tossing this phone?

	If I had a dollar for every model of electronic single and two-line
telephone presently on the market, I'd be a millionaire...
	Since I don't know the specific telephone you are having trouble
with, I will take a guess at the problem based upon past experience.
	Some of the 'fancier' electronic telephones having built-in dialers,
call timers, etc. are self-powered from the telephone line; i.e., in an
on-hook condition they 'steal' several mA of current from the telephone line
to power a DC/DC converter which in turn charges a small nicad battery. This
provides a continuous source of power for the telephone electronics.  The DC/DC
converter could be generating high frequency noise which is being transmitted
back over the telephone line and hence into your modem.
	Another possibility is that even if the telephone is not telephone
line-powered but uses an AC transformer, there may be RFI imposed on the
telephone line due to shoddy microprocessor design (many of these telephones
use a single-chip microprocessor to support the various features).
	As far as a solution is concerned - I don't have one.  About the only
possibility would be to build a low-pass filter for use in between the
offending telephone and telephone line.  That would be a rather tricky thing
to do, since there would be substantial constraints on such a circuit design
so as not to impair the normal telephone operation.

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