Re: Magnetized tapes

Jim Scholtz (
Fri, 29 Dec 2000 13:24:22 -0800 (PST)

HI Paul, I'm not a audiotape professional but I am an authorized JVC VCR
repairperson and I think that I can give you some pertinent information
about your magnatization problem. Found below at your question points.
Jim Scholtz.

At 12:00 PM 12/29/00 -0800, you wrote:
>This is a question about audio tapes, not video. Many people on this
>list deal with spoken audio as well as video, so I'm hoping someone can
>help with this mystery.
>My library is finding many audiobooks on cassette being damaged by some
>unknown source of magnetism. This results in the sound fading in and
>out. I am almost 100% sure that it is happening due to staff error. We
>use magnetizing wands to activate the security strips in books and do
>this by moving the wand along the spines of carts of books ready to be
>re-shelved. I am guessing that occasionally an audiobook gets stuck in
>with the books and gets exposed to the magnetism by a staff person who
>isn't paying attention. But... I'm not finding any damage to
>videocassettes from magnetization.
>1. Is there another likely source of magnetization that users of
>audiobooks could be exposing our tapes to?
I presume that you are using a magnetic security system like Knogo or 3M
and have a pass-around system for audiotapes rather than having patrons go
through the gates. If not, that might be a cause. All tapes won't
necessarily be effected due to the speed at which the patron/tape is passed
through the gate and the angle of incidence of the tape to the magnetic
reading device (I know that angle of incidence is used for radio frequency
gates, but it applies here as well.) The sound on videotape is recorded in
2-ways - embedded on the azmith between videorecording bars and as a linear
sound track. On professional systems, the sound track is recorded on the
top of the tape, under the videotrack. Most often when a video is
subjected to magnetic interference, the bottom track, or control track
(operating much like 16mm film sprockets to assist in advancing the tape -
electronically of course) is adversely effected. Audiotape is much more
suceptible to wear and tear, de-magnitzation, etc. It is thinner, has less
tracks, is recorded in a linear (not azmith fashion) and tracks are not
embedded (usually only 2 tracks per side - 4 tracks (1 side; 2 on top, 2 on
bottom). You should also check to see how old and how many plays
(circulations) the audiobooks in question are/have. We find perceptable
sound damage after about 125-150 circs. Hope this information helps.
Jim Scholtz.

>2. If a videotape were exposed to magnetization, what would be the
>expected damage? just to sound or also to image quality?
Videotape would exhibit control tracking problems that the automatic
tracking control of most VCRs would be unable to track out. This damage
would be random, not throughout the entire tape. Sound track damage would
only occur in very severe cases - video damage only if you held a magnet
over the tape.>

>Thanks, Paul
>Paul Duckworth
>Collection Development Coordinator
>Springfield-Greene County Library District (Missouri)