Dan Dugan wrote:
>
> >The walkman style minidisc recorders get
> >considerable time out of AA batteries. They also are somewhat more
> >immune to the humidity than cassette.
>
> Walter, in the MD recording system, does the head contact the disc,
> as in floppies, or ride on an air cushion, as in hard drives?
Some of both or neither, it's different. If you look at the mechanism it
mechanically resembles a floppy drive. The recording surface is never
contacted, except by a laser beam and the laser is quite a bit farther
away than the head on a hard disk. You don't have the problem of dust
getting caught between the head and the disk. The back side of the disk
is contacted by a head who's only purpose is to put a magnetic field at
the front surface. A minidisc only records on one side of the disk, like
CD-R or CD-RW, though those differ in other ways.
Like all kinds of MO disks, the record surface is coated with a phase
change material, a sort of plastic. This change can only take place when
the material is heated enough, well above any environmental temperature,
even a closed car in the sunlight won't do it, and then exposed to a
magnetic field. To write a bit, a laser heats a spot up above this temp,
and a magnetic field is applied to cause the phase change we want. The
spot cools and the phase is now fixed, for more than 50 years if not
rewritten. To read the spot the same laser is used at much lower power
to read. In the case of minidisc it's reading a polarization shift. This
all may sound slow, but there are MO disks that can do this at speeds in
excess of 10 megs/sec. The MD can do it at least at 4X speed, I have a
deck that can transfer CD audio to MD at that speed.
The magnetic head on the backside does not have to make the kind of
contact that you do with a floppy writing head. It does not provide the
spot shape, the laser does, it only provides a relatively crude magnetic
field. The closer it is the less power needed. So, they skim the surface
with it making light contact. The rated life of a MD is based on how
long it would take that head to wear away enough of the base material of
the disk to cause problems. This comes in the form of a specification
for the number of read/write cycles and the rating is more than a
million read/write cycles for high quality media like the HHb discs. The
magnetic head in the recorder never wears out enough, or at least not
before something else does. I wore out a couple sony walkman MD's before
going to the Portadisc. And what goes on them is the tiny switches under
the tiny buttons. It's a fairly easy operation to replace the switches
as they are in groups on tiny circuit boards. My oldest walkman still
works, and it's to the point that the plastic pause button is nearly
worn completely through. It's had the switches replaced once and is
something like 5 years old.
In case you have not seen what a minidisc looks like, go to:
http://www.minidisco.com/
and go to their blank media and get more info on one of them. The case
around the disk that you see is, unfortunately, not completely sealed,
it's open a small amount around the hub on the backside, like you have
with floppies. But it still provides considerable protection. People
have run over these things in a parking lot and still had them play.
They've dropped them in mudpuddles, rinsed them out with clean water and
still got the recording back. They are tough.
Now I don't recommend abusing a minidisc, you are far safer taking care
of the equipment and disks. They are immune to any ordinary magnetic
field, though some bored college students have managed to expose them to
enough to cause damage by rubbing strong magnets directly on the disk.
(the only thing more dangerous to equipment than a bored college student
is a 2 year old!) Their real enemy is dust and dirt. There is error
correction in the MD system, but there are limits. I've used relatively
simple precautions with MD's over the years and have never had a disk
failure, and I've never had the machines fail to record. I generally
keep my recorders in a closed Cordura pack when not in use, I always
keep the unused disks in the shrinkwrap until use, and after use keep
them in the original slip cases and those in a pack or a plastic box.
People who cart them around loose in pockets do experience TOC problems
from time to time, same with folks that don't care for the recorders. If
carrying a spare disk when recording, I'll have it in a pocket that's
closed with velcro. Remember a spare disk is still in the shrinkwrap. If
I need to carry a recorded disk around, I'll normally slip it in it's
slip case into a ziploc bag and into the pocket. And put it back the the
main pack as soon as I get back. I normally put a disk in the recorder
and don't remove it until it's full. There is no harm in leaving the
disks in and risk of dirt each time they are removed and put back in. On
the same vein if the recorder has gotten dirty or dusty (or wet) on the
outside, I clean it off as soon as possible. And not with something that
will push the dirt in. And I always make sure it's clean before changing
disks. A reasonably clean soft cloth will do the job.
I also record on new disks always, and when recorded I use the locking
tab. And the only disk editing I do is to write a index number into each
disk for cataloging. I do title tracks on the ones I use for music
listening, but don't bother with the nature recording ones, just write
the assigned track numbers into my field cards. Disks are cheap. One of
the first lessons about digital is that survival depends on being very
paranoid about disk failure. Stack the deck as much as you can.
Because of the way I care for my recorders, I never have to clean the
heads. You don't dig into the mechanism if you do, (though with care you
could) you just use a cleaning disk. It's main function is to clean any
dirt off the laser lens. Those who abuse their machines often have to
clean the heads several times a year.
As I noted, these are definitely a better choice on the practical side
here in hot and humid (and sometimes very dusty) Georgia. I originally
went to MD because I was tired of failures of cassettes and their
recorders due to these conditions. MD's are not perfect, just a whole
heck of a lot better and safer.
Walt
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