PC911 > How-To > Miscellaneous > Once More Into The Fire

- Xin Li -

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Contents

Reading Requirement

Introduction

Welcome back you pyromaniacs! You are not content with just knowing how to write audio and data CDs, are you? No! You want to know how to write bootable CDs, how to write Hybrid CDs that work on both Macs and PCs. You want to know how to squeeze more than 74 minutes of audio onto a single disc. If that's what you want, then you are definitely in the right place. But before we start with the fun stuff, let's take another look at the technical side of the CDR, the power behind the magic.

How It All Works

In the Guide to CD Burning Part 1, I gave the impression that there was a single layer on the CDR disc, and that's where the data is written. That description is, unfortunately, something of an oversimplification. In reality, there are actually four different layers present in a CDR disc:

At this point, allow me to make a slight digression to address a question which many people have asked: Why do CDRs have different colors? The question can be easily answered by looking at the middle two layers, the reflective and the dye layer. The reflective layer is usually one of two colors, either gold, or a silver alloy mixture. The dye layer can also have different colors, depending on what it is made of. One common compound for the dye layer is cyanine, which is bluish in color. Another popular mixture is phthalocyanine, which is colorless, more or less. The resulting color of the CD is thus the color addition of the two layers. For example, green/gold discs combine a gold reflective layer with a cyan-colored dye, resulting in a gold appearance on the label side and a green appearance on the writing side.

Ok, back to the main point of this section. One thing to note about the dye layer is that unlike the other three layers, which are smooth, the dye layer is pre-grooved. In other words, etched on the back of the plastic layer are grooves laid out in a spiral path. The dye then fills those grooves to form the dye layer. So why is it pre-grooved? The purpose is to guide the CDR laser. With this pre-grooved layer, CDR drives no longer have to figure out where it's burning, which simplifies it's design, thus lowering the price. Just think of the CD laser as a dog and the pre-grooved tracks as leash that guides the dog and tells it where to walk. To record data on to the disc, the laser actually melts the dye layer. Once the dye layer is melted, the lower plastic layer (which has also been heated up) actually flows into the hole created in the dye layer, and forms a pit. So then during the actual reading process the same laser reads the disc at lower power. At each pit, laser light reflects off the gold layer. The reflected light enters the laser reader, which detects the varying reflectance as the pits go by.

Rainbow Colors

Whenever you are writing something that is going to be shared among thousands, if not millions, of people, you had better have a standard. CD writing is no exception. Depending on what data you are writing to a CDR, there is always a standard by which the CDR software must comply. Otherwise, the data written by one CDR will be unreadable by another. Below is a list of the most common standards, each of which correspond to a specific data type being written to the CD:

So many standards and so many colors! What is a person to do? Thankfully, for the most part you don't have to know any of the technical details. But here, I will take time out to discuss Red Book, because information from it might help you to understand some of the other concepts discussed later in the article. Since what I am about to discuss is rather peripheral to the main purpose of this article, feel free to skip to the next section if this discussion is not likely to hold your interest.