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On Thu, 11 Jan 2018 23:52:22 -0000 (UTC), Diesel wrote:

Char Jackson
Fri, 01 Dec 2017
16:18:56 GMT in alt.windows7.general, wrote:

On Fri, 1 Dec 2017 10:37:37 -0500, Wolf K
wrote:

On 2017-12-01 09:29, Mayayana wrote:
That's my hesitancy about USB sticks and
hard disks. Magnetic storage. I don't really
understand how it works, but it seems that
it has to be less durable than grooves in plastic.
And maybe it's susceptible to magnetic fields?
I don't know.



Cosmic rays?

The Earth's magnetic field is strong enough to degrade magnetic
storage over time. That's one reason VHS/Beta tapes become
unusable.

Mayayana mentioned grooves in plastic within the context of
backing up data to DVD. Obviously, there are no grooves... ;-)


No, but, there are small pits when the laser burns the disc. [g]

For consumer writeable media, my understanding is that there's a dye
layer sandwiched between the two outer plastic sheets. When you hit the
dye layer with a laser, it darkens. That's later seen as a digital "0".
Areas that weren't hit with the laser stay translucent, and are later
seen as a digital "1".

The reason a darkened area is considered a "0" is because there's a
reflective layer just above the dye layer, so when the laser is in read
mode and focuses on a specific area, if it can shine through the dye
layer and get a reflection from the reflection layer, it's a "1". No
reflection, because the dye has been darkened, is interpreted as a "0".

The part about "pits" and "lands" refers to commercially pressed optical
media. Consumer media doesn't have those characteristics.

And, if air is ever allowed access to the material where your data is
actually stored, sandwiched between those two layers of plastic that
make the cd/dvd, bitrot will take hold and eat your data over time.

In addition, the presence of that dye layer in writeable media is why
direct sunlight kills them. Over time, sunlight darkens the dye layer,
turning the 1's to 0's.

--

Char Jackson

Dell computer with no input
 

On Fri, 12 Jan 2018 00:15:07 -0500, Paul wrote:

Optical media uses pits. Although there's going to be
a difference between pressed and burned media.

If you see pits and lands, it's commercially pressed, typically from a
glass master.

If you see a dye layer, it's burnable in a consumer level burner.

Ne'er the twain shall meet. Commercially pressed media don't have a dye
layer, and burnable media don't have pits and lands.

--

Char Jackson

Dell computer with no input
 

Char Jackson
Fri, 12 Jan 2018
16:55:30 GMT in alt.windows7.general, wrote:

For consumer writeable media, my understanding is that there's a
dye layer sandwiched between the two outer plastic sheets. When
you hit the dye layer with a laser, it darkens. That's later seen
as a digital "0". Areas that weren't hit with the laser stay
translucent, and are later seen as a digital "1".

Yep.

The part about "pits" and "lands" refers to commercially pressed
optical media. Consumer media doesn't have those characteristics.

Correct.

And, if air is ever allowed access to the material where your data
is actually stored, sandwiched between those two layers of plastic
that make the cd/dvd, bitrot will take hold and eat your data over
time.

In addition, the presence of that dye layer in writeable media is
why direct sunlight kills them. Over time, sunlight darkens the
dye layer, turning the 1's to 0's.

Yep. The bitrot issue happens on commercial discs too, if the
pressing process had a problem in some way and the seal doesn't hold.




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