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Re: [AV Media Matters] Digitizing Audio and Video-ReallyBigPost
Hi Jim et al,
What this IT guy was getting at, was that if you shelve a HD, the disc
will eventually lose its' magnetism, unless you spin it up from time to
time. I have heard this once before, perhaps it is just FUD, as Richard
mentioned, but I would like to know if there Is any truth to it?
Best,
Alyssa.
Moderators Comment:
No, there is no truth to that other then perhaps considering earths'
magnetic field over centuries - or a more likely culprit are those
really strong magnets placed on some bookends used on library
shelving..... - but there are other more important reasons not to
consider shelving a HD and trying to recover the data years later as I
detailed in my moderators comment to Jim Wheeler the other day.
I would suggest to all interested to learn a bit about coercively and
magnetic recording. It is covered in any decent book on recording
technology or magentics. It is a fundamental concept in this area. What
is coercivity you ask??
coercivity
SYLLABICATION: co.er.civ.i.ty
NOUN: The intensity of the magnetic field needed to reduce the
magnetization of a ferromagnetic material to zero after it has reached
saturation
Hard Drives have high coercivity - so it will take a strong magnetic
field in close proximity for a period of time to really cause a problem.
On Monday, June 2, 2003, at 06:19 PM, Jim Wheeler wrote:
Alyssa
There is NO way that spinning or not spinning a disk can effect the
level of
magnetism of a disk. I wonder where that idea came from?
Also, the spinning video heads on a videotape recorder are fixed in
the
scanner and do NOT move! I ran an experimental scanner at 800 rps and
nothing went
wrong. Think about that; in a single second, that scanner made 800
revolutions! Most scanners run at a very slow speed of about 30 rps.
Be careful with information from someone who is speaking out of their
field.
There are many "old wives tales" that keep getting repeated.
Computer tapes do not use redundant recording because that would
double
the
amount of data to be recorded. Instead, error correction is used. At
Ampex,
we were using Miller-squared error correction about 30 years ago and
then
Reed-Solomon about 20 years ago. New error correction algorithms are
always being
developed. The Ampex digital videotape machines I helped design, all
have
about three layers of error correction.
As a test, I removed about a square inch of oxide. During playback, I
had to
look at a large monitor in a dimly lit room to see the area with no
recording. That demo convinced me that digital recording was here to
stay.
DAT is a consumer format so DAT machines do not have sophisticated
error
correction. DataTapes use sophisticated error correction because they
MUST
preserve the data of banks, insurance companies, etc.
BEST
Jim Wheeler
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