How are retail DVDs created?
There is much discussion about the factors that differentiate brands of recordable disc, but how exactly do the major home video studios create pressed DVDs? How does the science work and how does it differ from the creation of DVD-Rs?
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Retail DVDs are replicated with pressing equipment.
Pits and lands are read within the groove, or spiral "road" that the laser reads from, as the disc is spun at high speeds. When a burn writes outside of the groove (or even when a press does not fall within the groove), you get bad discs, because the data cannot be read. DVDs can't offroad! On really crappy DVD recordable media, using a magnifying glass, you can sometimes see mottling of the disc surface, on a burn. The burned pits are not precise, but rather fades across a larger area outside the groove. I've added a photo of one several posts down. Remember that the disc is spinning, and to stay inside the groove, the disc needs to stay as flat as possible, and not wobble. There is some built-in tolerance for how much wobble is allowed, both in burning and reading. The data is not written in a straight line, it wiggles a bit. As long as the laser can pick-up data within the groove, it's fine -- even if the burned data appears to be burned by a drunken laser! Beyond this, remember your physics! The outer edge of the disc is spinning faster than the interior. It's also wobbling more. This is why so many cheap discs "coaster" towards the end of a disc, especially in that last 400-500MB worth of space. The dyes used in write-once discs, and phase-change crystalline metals used in re-recordable/rewritable discs, reflect light using a combination of the material itself, and the foil above it. While the light does reflect, it is not as reflective as the pressed discs. Some dyes, such as the metallic AZO dyes used by Mitsubishi, use a metallic base that assists in the quality of reflection -- one of the reasons their discs are so well-liked and perform so great. Cheap dyes with many imperfections, on the other hand, impede light reflection too much, resulting in the many read errors. Ritek's organic dye is infamous for poor readability, as seen from various tests and reviews online, both by research bodies and home amateurs. Dual-layer pressed media is essentially two platters on top of one another, with a semi-transparent spacer between them. The extra layer does not really affect disc reflectivity. Double-layer recordable media is a bit more complicated, because there are multiple write surfaces, with various levels of translucence and transparency. I don't really want to explain that if I don't have to, can be hard to explain using regular English. SuperMediaStore has a write-up, but it's still fairly wordy and techie. Disc reflectivity can tank pretty badly on recordable dual-layer/double-layer media, especially when the materials are substandard quality. Double-layer recording is also complicated further due to the inconsistency in construction among manufacturers, and compatibility among burners. By this, I refer to inverse stack process vs 2P process. Most all burners are fine with the original 2P discs, but the inverse stack discs will either burn poorly or fail to burn at all in a large number of burners out there in use. However, inverse stack is cheaper to produce. Further complicating this already complicated issue is the fact that the cheap production method is being used by the typical low-quality and mediocre-quality blank disc manufacturers, so you have discs with multiple possible problems!
As you're probably understanding now, burned media are quite a different critter from your basic pressed disc. |
OSTA has a decent write-up that isn't too complex, with more information on non-pressed disc construction.
From http://www.osta.org/technology/dvdqa/dvdqa13.htm Quote:
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The British duplication/replication service WizBit has a decent write-up on the glass mastering process.
From http://www.wizbit.net/cd-dvd_product...ass_master.htm Quote:
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If you're still in a reading mood, the respected optical disc testing firm DaTARIUS released a really good paper on dual-layer/double-layer recording. See the attached PDF.
However, I would like to point out this incorrect statement: Quote:
Anyway, good read, VHS statement aside! :) . |
And risking overkill, here's some more! Check out these videos:
"Reload page to view video." Also http://www.youtube.com/watch?v=ncU_A76W3P8 http://www.youtube.com/watch?v=XWGBa05-qz4 http://www.youtube.com/watch?v=Qqlgtvwjml4 Whew! :) |
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Here's a photo of a disc with mottled and splotchy dye:
Attachment 380 If you look closely, you can also see some zonal burning rings. The faster the burn got, the worse the dye reacted. This photo doesn't really show the full effect, it's covered in speckles. There was also a bad spot easily visible in the dye, near the outer edge: Attachment 381 If you darken and filter it, you can see the problem goes deeper than is easily seen by the naked eye: Attachment 382 The burn is crap -- a coaster. |
Holy crap, thanks Admin! Now to find the time to go through all that ;)
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