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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.periphs
Subject: Re: terminal noise
Message-ID: <4750@utzoo.UUCP>
Date: Sat, 8-Dec-84 19:36:02 EST
Article-I.D.: utzoo.4750
Posted: Sat Dec  8 19:36:02 1984
Date-Received: Sat, 8-Dec-84 19:36:02 EST
References: <36@mot.UUCP>
Organization: U of Toronto Zoology
Lines: 21

It's vibration at the horizontal scan frequency of the display.  Any CRT
terminal will have a fair bit of energy running around at this frequency,
since it's got to bully an electron beam into going from the end of one
scan line to the beginning of the next in a few microseconds.  This means
the horizontal deflection circuitry has to go from "+MAX to -MAX" in that
length of time.  Particularly with magnetic deflection, which is normal
for CRT displays, this is hard work.  So there are quite energetic pulses
present at that frequency.

Given that there are magnetic materials here and there, and that the
deflection yoke itself may well be responding mechanically to some small
extent, there is an inherent noise source at that frequency.  What varies
is how intense the noise source is, how well the surrounding bulk of the
terminal absorbs the noise, and whether anything resonates at that exact
frequency.  We've got Ampex terminals which seem to have something in the
case or innards that can resonate at the horizontal scan frequency -- minor
mechanical disturbances (e.g. giving the terminal a good thump) can flip
the terminal from noisy mode to quiet mode.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry