Xbox 360 Discs/HD DVD Gonna Get Scratch Proof Discs? - High-Def Digest Forums
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Old 09-01-2007, 06:20 PM
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Default Xbox 360 Discs/HD DVD Gonna Get Scratch Proof Discs?

I was wandering if anyone knows if the xbox games and the HD DVDs are gonna get that scratch proof for their discs like what the Bluray discs have?
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Old 09-01-2007, 10:35 PM
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Originally Posted by BluRayDvd View Post
I was wandering if anyone knows if the xbox games and the HD DVDs are gonna get that scratch proof for their discs like what the Bluray discs have?
I wonder if Blu-ray is going to start producing thicker discs like HD DVD does, then they won't need the scratch protection.
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Old 09-01-2007, 11:58 PM
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Ok, let's dispel this confusion:

The reason Blu-ray needs a hard-coat is not that the discs are thinner than HD-DVDs. Actually, they're same thickness. In fact, HD-DVD, Blu-ray, CD, and DVD are all 1.2 mm thick. The laser for Blu-ray has a higher numerical aperature, meaning that the beam focuses over a wider angle. Both HD-DVD and Blu-ray use the same pit width of 0.25 μm, so in order to get a wider angle (higher numerical aperture), Blu-ray had to put the data closer to the surface. Here's a little diagram:

Code:
             0.25 μm
           |----------|
Pit         _________      _
           \    |    /     |
            \   |   /      |
             \  |  /       |  Distance from surface
              \ϑ| /        |
               \|/         |
Surface                  
of disc   

Numerical aperture = n sin ϑ
n is the index of refraction, which is like 1.45-1.65 for the normal plastic
The way they make Blu-ray with the hard-coat is quite different from what they do with (HD) DVDs. DVD and HD-DVD look like this:

Code:
|---------0.6 mm top layer---------|
|very thin aluminum recording layer|
|-------0.6 mm bottom layer--------|
The top and bottom layers are made of plastic. When you stratch the disc deeper than 0.6 mm, then you damage the recorded data--and that's when you get read errors. That's why DVDs are immune to slight scratches. The laser can focus through them as long as the data is still in tact.

To add a hard-coat, you'd add thickness to the disc, which changes how far the laser has to focus slightly. Since everything is on the order of microns, a very thin coat could throw off everything, and the laser wouldn't be able to get a good read on the disc.

Blu-ray discs could look like this:
Code:
|---------1.1 mm top layer---------|
|very thin aluminum recording layer|
|-------0.1 mm bottom layer--------|
using the same plastic as DVDs and HD-DVDs for the top and bottom layers, but that leaves the data vulnerable to small scratches. Instead, what they do is this

Code:
|---------1.1 mm top layer---------|
|very thin aluminum recording layer|
|---------0.1 mm hard-coat---------|
The hard-coat is much harder than the plastic you'll find on the bottom of a DVD, so that solves the small scratch problem.

HD-DVD could use a hard-coat if they reduced the thickness of the bottom layer, but that would increase costs. They would have to manufacture two plastic layers (of two different thicknesses!) for each disc, and then apply a hard-coat as well. Blu-ray would only be making one layer and the hard-coat, which saves money because it should be cheaper to make one of something but make it thicker than to make two of the same thing each half the thickness, since the latter requires twice the machine time.

So to summarize: Because you can't make the bottom layer thicker and still use the same laser, Blu-ray can't make "thicker discs" (i.e., make the bottom layer thicker), and HD-DVD can't just toss a hard-coat on there without changing their entire manufacturing process. One of the alleged benefits of HD-DVD is that the production method is identical to DVDs, and if you take that away, you'd have to reconfigure all the manufacturing equipment, just like Blu-ray required.

Last edited by Aurora; 09-02-2007 at 02:14 AM.
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Old 09-03-2007, 12:59 AM
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^
Nice info.
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