cd_read_side.jpg (8K)

Chip's CD Media Resource Center:
CD-DA (Digital Audio) 1

Image from Disctronics

CD-DA (Original Flavor: Audio)

Compact Disc - Digital Audio(CD-DA) was the original CD specification for digital audio as developed by Philips and Sony in 1980. The specifications were published in a document known as the Red Book which has continued to be updated. As of this writing, the latest version of the Red Book was released in May 1999. To this day, engineers will often refer to Red Book or Redbook Audio, meaning the ubiquitous 16-bit, 44.1kHz, stereo digital audio format defined in the Red Book.

The current Red Book specification is available directly from Philips but, regrettably, it will cost you US$100 and you'll have to sign a confidentiality agreement. What Philips calls "an approximation" of the Red Book specification is the international standard published by the International Electrotechnical Commission (IEC) as IEC 908 (now known as IEC 60908), however this currently costs US$224! Fortunately, much (not all) of the information is repeated in the ECMA-130 CD-ROM specification, which can downloaded in PDF form at no charge.

CD Physical Structure

Perhaps the thing we most take for granted about the Compact" Disc is it's size. CDs are indeed "compact" compared to their older brother the Laserdisc: 30cm in diameter, the size of LP. CDs can actually be made in several diameters, but the only really common size is 12cm. The Red Book also defines an 8cm size, the so-called "CD Single", but they evidently haven't caught on (have you ever seen one? I have not).

But despite having it's diameter rigorously specified in the Red Book and elsewhere, the actual size and shape of a CD can be remarkably flexible. In the last few years we've seen rectangular "business card" sized CDs, and other shapes are also possible. It's true that these don't meet the specification. But they work. Fundamentally that's because CDs are played from the inside out. So long as the inner portion of the CD conforms to the standard, and so long as the recorded portion ends with an appropriate margin before any edges, then it should be playable if you can safely insert the CD into your player. I'm not personally interested in oddly shaped media, but I admire the flexibility inherent in a design that permits such liberties.

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.

The diagram below illustrates the different layers of a CD (though the reflective metal layer is really so thin that it should just be represented by a line).

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Diagram from "Recording to CD: More Than Meets the Ear" by Lionel L. Dumond.
http://www.digitalprosound.com/Features/2000/Sept/RecCD.htm
There is a good description how a CD is manufactured in the EE 498 Class Notes by Professor Kelin Kuhn of the University of Washington.

Most of us understand that any CD is read through the clear polycarbonate layer. So, of course it is important to protect that side against fingerprints and scratches. Then, when you put CDs down on your desk, do you turn them so the label side is down toward the desk? I usually do. But that can be very dangerous!

Rather than being the "safe" side, the label side of a CD is perhaps the most vulnerable part of the disc. When we put the label side down on a desk, there is only a very thin coating between gritty desktop and vital reflective metal layer.

In contrast, the other side is protected by the thick (1.2mm) and hard polycarbonate. Fingerprints and even scratches on that layer, have not touched the underlying data at all. It is possible to carefully clean and even to polish that surface to remove such damage. In fact, because the laser beam is relatively diffuse at the surface of the polycarbonate and doesn't focus until it reaches the reflective layer, many flaws on the polycarbonate surface will simply go unnoticed.

The reflective layer itself is very delicate and any physical damage to its recorded portion will immediately destroy data. So the easiest way to scratch the reflective layer and permanently damage a disc is from the label side.

philips-cdlayers-14.gif (5K) kuhn-cdbeam-95x617.gif (1K)
Diagram from Philips' CD ROM General Info
From Kuhn EE498 Notes

Another surprising thing is that information is recorded on a CD-DA or CD-ROM using a series of bumps. It's surprising because these bumps are called "pits" and are usually pictured as, well, pits. And they do start out as pits in the polycarbonate layer (looking down from the top in the diagram above. But the disc is read from the bottom, through the transparent polycarbonate. So the pits appear as bumps to the scanning laser.

The unmarked areas between pits are called "lands" or sometimes just "land". If this nomenclature of "pits" and "lands" still makes no sense to you, try this: Look down onto the top of the polycarbonate and visualize a broad plain. Sing to yourself the famous Woody Guthrie song, "This land is your land, this land is my land..." Got it? That's the land. Then you lease the land out to miners and they dig pits. Voila.

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 the next section we'll talk about how the "pits" and "lands" are read by the laser beam.


Last Updated Saturday April 16, 2005 13:57:19 PDT