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Again, completely wrong . . .

Posted by Martin419 (A) on December 22, 2005 at 10:55:38

In Reply to: Martin NO CD or SACD in exceeds 1.2mm, DVD's are two .6mm sandwitches and DO NOT exceed 1.2mm. posted by Teresa on December 21, 2005 at 18:20:54:

You wrote:- " NO CD or SACD in exceeds 1.2mm".

This is totally wrong, as is your bit about "small variances due to warps, uneven surfaces and minor disc defects " -- accounting for the +.3mm margin above 1.2mm.

So please stop making things up. Instead do some actual research.

You can start here:-

http://www.chipchapin.com/CDMedia/cdda1.php3

Look at the diagram, and note that the "polycarbonate plastic" alone can be 1.2mm. On top of that you can add these three things:-

1. Aluminium;
2. Acrylic;
3. Label.

In short, the modal average thickness of all CDs ever made will logically be somewhat higher than 1.2mm (unless items 1,2,& 3 above have no thickness at all -- which is impossible!)

Quote from that web page:-

"The thickness of a CD can vary between 1.1 and 1.5mm. The biggest part of this is clear polycarbonate (nominally 1.2mm). There is a very thin layer of reflective metal (usually aluminum) on top of the polycarbonate, and then a thin layer of some protective material, topped by a label or some screened lettering."

It's as simple as that.

Now read this:-

Summary of CD Physical Specifications
The table below summarizes some of basic physical specifications of the 12cm compact disc. We'll be referring to nearly all of these in the following pages. A "micron" is a micrometer.

Diameter 120mm ±0.3mm
Substrate Thickness 1.2mm ±0.1mm
Total Thickness 1.1mm - 1.5mm
Substrate Index of Refraction 1.55 ±0.10
Reflectance of Metal Layer through Substrate 70% minimum
Laser Wavelength 780nm ±10nm
Numerical Aperture of Laser Optics 0.45 ±0.01
Track Pitch 1.6 micron ±0.1 micron
Scanning [Linear] Velocity 1.20m/s - 1.40m/s (±0.01m/s as recorded)
Channel Bit Rate 4,321,800 bits/s
Data from ECMA-130 CD-ROM Specification

In particular, on some lines you will clearly see the universally understood engineering tolerance notation " ± " .

Note that this symbol " ± " DOES NOT appear after the third line i.e. " Total Thickness 1.1mm - 1.5mm " whereas " ± " DOES appear on other lines where tolerances are specified.

And if you want even more detail, you can download the PDF:-

http://www.ecma-international.org/publications/standards/Ecma-130.htm

This post is made possible by the generous support of people like you and our sponsors:

The Cable Cooker

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