Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!henry 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