From: RoyMayeda@aol.com
Date: Fri Oct 12 2001 - 06:20:33 PDT
From: RoyMayeda@aol.com Message-ID: <c5.178f7cf8.28f84821@aol.com> Date: Fri, 12 Oct 2001 09:20:33 EDT Subject: Re: LED oddities
Hi, Eric.
I don't have much experience with such things, but I'll take a whack at it.
Might this be similar to the effect of viewing a television set on a video
camera? When the video camera's scanning and the TV screen's scanning are
not in phase, a dark jagged line appears in the image of the TV screen in the
video, even though the screen looked normal to the human eye. I'm thinking
that the LED is flashing at 60 Hz (standard AC for this country). If the
camera is scanning at a different frequency, then it will go into and out of
phase with the blinking LED. When it is in phase, you'll see it as bright,
and when out of phase it appears dark. Since this timing "drifts" gradually
from out of phase to in phase, the transition between bright and dark is
gradual rather than sudden. Analogies that might help are the standing wave
produced when two musical notes are slightly out of tune -- e.g., "harmonic"
tuning of guitars (not sure if that's the correct term -- damping string
about 5-7 frets up, makes overtones with easily heard standing wave), or (and
this is probably more familiar to many students) the drift of the
synchronization of school bus windshield wipers. (For those who have not
seen this, the wipers on most buses are independently driven. The
frequencies are slightly different, causing them to drift back and forth
between moving exactly the same direction at the same time to moving exactly
opposite each other.)
Nope, haven't heard of a fan-fret guitar. Sounds kind of neat though.
Roy Mayeda
Valley HS
Sacramento, CA
PS - Hey, Karen K. You lived in MN for a while, right? Looks like I'm
moving there next year. Any advice? Also, when it becomes definite, I'll
post regarding the opening at my school. Not a high achieving school (rates
a 3 statewide and a 6 in similar schools), but right now my schedule is 4
Chem, 1 AP Chem. The district also pays pretty well. Anyway, more details
as we get closer. Just something to think about for those in the Valley or
thinking of moving here.
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