[moved] How does erasing RW discs work?

vbimport

#1

Hello! I just want to ask, if the erasing of a RW disc is some special action or just writing zeros all over the disc. I have a suspicion, that my BenQ 1620 cannot erase DVD±RWs properly…


#2

The “quick erase” just writes a small signature to the start of the disc, indicating that the disc is “empty.” This doesn’t actually remove the data from the disc. It only makes it appear empty so it can be written again.

The “full erase” does physically clear the entire disc, and it takes as long to do this as it does to burn an entire RW.


#3

OK, that’s clear to me. But I wonder how it does the “clearing”.


#4

I can’t be totally sure about how the “clearing” is phyiscally done, but I would expect that it just writes a single bit over the whole disc such that all data is gone. Whether or not this returns it to the same state as when you first used the disc, I really do not know.


#5

This scan is fully erased & burned MKM A02. But I think that it should be all the way under 100 and I guess that all those other high areas are just badly erased. The media hasn’t single scratch.



#6

I did a google a while back on the subject, found some interesting material. I think what you’re asking about is what goes on in the media, right?

Here’s what I understand of it; more knowledgeable members might have a few corrections in my narrative.

The “phase change” material is in a layer just next to the reflective aluminum plate. It’s sensetive to temperate changes, but not so that any room temperature levels should alter it. When the material is heated to a certain target temperature and held there for a certain duration of time, it reverts to it’s liquid crystal state, which makes it fairly transparent - the signature of a “1” - being reflective.

During writing, a higher temperature for a shorter duration changes this into a solid state, and it obstructs light, creating a zero.

Times are very short, but are much longer than what’s required to “color” the ink on R media, which is one reason RW media is always slower.

Anyway, like a previous post said, the area representing the links to the directory are written with a “signature” indicating it’s empty, and the directory section may be written over - but erasure is a series of “1”'s not “0”'s (though I suppose either would work).

Obviously the laser can be controlled to revert the material to liquid or solid states at will during a write operation, else it would have required a full scale erasure to be reusable. In either case, it would seem clear that an erased disc is not exactly the same as a unused one. It’s likely that what was once a zero would have a tiny remnent of it’s former self when converted into a 1, dependent on aim accuracy. This isn’t enough to cause read failures (we hope), but perhaps enough that an “erased” disk might be recoverable in a lab (partially).


#7

Thanks! But the question of my drive remains. So erasing is simple writing of the same bit all over the disc. Is it possible, that the drive cannot write RWs properly while it can write ±R perfectly? Noone else here on the forum has problems with RW, so I guess it is worth trying to return this one to the shop for RMA…


#8

I have no problems with my two BenQ DW1620 drives and writing DVD+RW discs.


#9

I’ve been cutting DVD “tests” to RW to double check the compilation. I have a bunch of DivX videos for my son, transferred from VHS or recorded from cable, and some of odd layouts. I play the video a few times from the RW until I know it looks correct before I burn a +R. I’ve erased and re-used a handful of INFODISC A10 4X +RW’s for this without problems. One of the three stand alone players doesn’t take RW’s, but the other two can’t seem to recognize any difference.

My BenQ rebadge (NU DW163) hasn’t complained a bit, and quality scans remain above 96, even though the media itself isn’t top shelf.

I usually do a “quick erase” - takes around 20 seconds at 4x.