RE: [AV Media Matters] cassette tape shelf life?

[kbradley@nla.gov.au]

	The statement below is a myth.

	>> Earth's magnetic field will cause signal to noise degradation
over a
	>> long period of time unless the magnetic recordings are stored in
a
	>> magnetically shielded container.

	>Do you have any data on this or reference to any scientific
studies.
	>I've never seen any definative studies on this.

	The earth's magnetic field can be measured at around half an oested.
Knight's (I think) study published in 1972 (in IASA's phonographic bulletin)
showed a DC field as high as 50 oesteds causes measurable third harmonic
distortion in standard coercivity reel tapes (which is still a long way
short of erasure), and similar results from ac fields of 5-10 oesteds.
Magnetic fields do not build cumulatively higher fields, so the one thing we
are probably safe from is accidental erasure of magnetic tapes in the
earth's magnetic field.  However, ac motors and lighting ballasts at very
close range can generate problematic fields, but they do drop off at the
inverse of the distance squared.

	Of course, geologist's claim that there have been major reversals
and shifts resulting in large short term fields, however
	one of the risk areas I haven't planned against in managing an
archive is geological upheaval.  I expect I'll have greater problems to deal
with!!

Kevin Bradley
Manager: Digital and Audio Preservation Resources
National Library of Australia
Canberra   ACT 2600
AUSTRALIA

Tel:  +61-2 6262-1381
Fax: + 61-2 6262-1653
Email: kbradley@nla.gov.au
NLA home page: http://www.nla.gov.au
Change is inevitable, except from a vending machine

Moderators Comment:
I was going to comment but I am glad that Kevin did it for me. Any reading
on retentivity will clear this up quickly.
jim