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February 20, 2001

By Karen Kenworthy


Welcome back! Did you have a good week? I sure did. A couple of clients kept me real busy. And I still found time to work on a fun project of my own. Best of all, my father turned 72 last week. He and my mom live just a few miles from the secluded Power Tools workshop, in a nearby town.

Tomorrow I'll be there for dinner and a party. My brother Bill and his family will be there too, and even my dad's sister Phyllis has driven up from Texas to get in on the fun. If you're in the neighborhood, you can drop by too. I'll save a slice of birthday cake for you.

Digital Memories

Have you ever said or written something, while in the back of your mind you're thinking "I'm going to regret this"? Well I have. Just last week, right here where we are now, I answered an innocent question from reader John Taylor. Here's what I had to say:

"Although all CDs look pretty much alike, there are two very different types of CDs that can be read by most computer CD drives. ...One, properly called a CD- ROM (Compact Disc-Read Only Memory), can contain computer files ... The other type, called an Audio CD ... cannot contain disk files.

Looks innocent enough, doesn't it? But I knew as I wrote it that nothing in the world of computers is that simple. And in no time, dozens of readers wrote to remind me. Audio CDs can contain data files. And data CDs can contain audio.

It all started back in 1982, when Philips Electronics and Sony designed the first Compact Disc. The CD was created for one purpose: To store music. It was seen as the eventual replacement for audio cassettes and eight-track tape cartridges, offering better quality sound and more durability.

Sound is stored on a CD as a series of numbers. Each number represents the amplitude, or height, of the original sound wave at a particular moment in time. Samples are taken 44,000 times each second. And each is stored as a 16-bit binary number, ranging in value from 0 (total silence) to 65,535 (maximum volume). The result is a very accurate picture of the original sound wave.

Today, it's obvious why CDs have become a popular way to store and distribute large amounts of data. A thin disc, just over 5 inches in diameter, can hold more than 350 million 16-bit binary numbers. That's more than 700MB of computer data.

But for some reason, original designers of the CD didn't seriously consider this. For one thing, the data on an audio CD is written in one long spiral, like a phonograph record, rather than the concentric circles of data found on hard disk drives and diskettes. As a result, no two sound samples are the same distance from the disc's edge or center, forcing the read heads to move constantly. This makes it harder to precisely jump from one location to another, something computer data storage devices must do often.

Audio CDs also rely on a technique called "Constant Linear Velocity" or CLV. To understand CLV, let's take a little walk along a CD's spiral data track. Like the data stored on the CD, we'll start our trip near the CD's central hub.

Our first lap around the CD doesn't take long. That's because, near the hub, our spiral's circumference is small. But as our journey progresses, we'll gradually move outward. By the time we reach the disc's outer edge, our laps will be more than three times longer than when we began.

Because we walked at a steady pace, our "linear velocity", or speed along the track, has been constant. That's CLV. With each step we covered the same amount of ground, err data.

But our radial velocity, the rate at which we completed our laps, varied continuously. When we started, our radial velocity was high. As we neared the outer edge our radial velocity slowed dramatically.

Now let's turn this metaphor inside out. Because of CLV, a CD drive must spin its disc faster when data is being read near the hub. The disc's rate of rotation must be reduced dramatically as date nearer the edge is read. This keeps data flowing from the disc at a constant rate. But the acceleration and deceleration of the disc takes a noticeable amount of time, dramatically slowing random access like those performed by computers reading data.

But the audio CD's biggest problem, from a computer's point of view, is its unreliability. Read the data from an audio CD, and you'll never get the same result twice. Errors are common, and there's no provision to correct them. When playing music, a few errors are acceptable. After all, each 16-bit number only represents 1/44,100th of a second of sound. An error that brief isn't audible. But *any* error in reading computer data can be disastrous.

CD Flavors

With all these deficiencies, you might think no one would try to store computer data on a CD. But never underestimate the determination of a computer user looking for more storage space.

In 1984, just two years after the birth of the audio CD, a data CD specification was released. The biggest change from the original CD specification was the addition of Error Correcting Codes (ECC) and Error Detection Codes (EDC).

These codes are extra bits added to the data being stored. They allow small- scale errors to be automatically corrected, replacing the misread bits with their correct values. Larger errors, those involving the loss of more data, cannot be automatically repaired. But they can be detected, preventing corrupted data from being mistaken for good.

It wasn't long after data CDs appeared, that some bright folks tried to combine audio and data on a single CD. The first approach was simple. Just record data, in place of sound, on an audio CD's first track. The remaining tracks would contain sound as usual.

As long as these CDs are read by computers, which understood how to read data and audio information, all was well. The computer can read data, or play music, as it pleases. But insert one of these "mixed mode" CDs in a regular audio CD player, and the results could be less satisfying. Trying to play the wildly varying data in the first track, as if it were sound, the CD player could damage both speakers and eardrums.

It wasn't long after this first experiment that a new approach was tried. Someone noticed that most CDs aren't filled. After the sound or data they hold, there is usually some blank space. Why not, they asked, use this left-over space to hold another CD?

At the beginning of any CD, you'll find a small table of contents, followed by either sound or data. After that is a marker, indicating the end of the CD's contents. Space permitting, why not immediately follow this by another table of contents, additional sound or data, and another end marker? This cycle could be repeated until the CD author ran out of space or information.

Each of these CDs within a CD is called a session. Each session looks and acts like a different CD. So one session can contain pure audio, while another session contains pure data. The two aren't mixed within a session, so the problem encountered with mixed-mode CDs is avoided.

Normal CD players, like those in your car or on your shelf at home, only see the first session of a multi-session CD. That's why this session usually contains music or other audio tracks.

Computer data is usually stored in the second, and subsequent, sessions. Computer CD drives see the last session on a multi-session CD, allowing them to view CD's data. This session can contain picture files, movies, and even programs. If this session contains a file named Autorun.inf in its root directory, any program it specifies will be run automatically when the CD is inserted.

Older CD drives can *only* see this last session. But newer, multi-session compatible drives can view any session they chose, giving them access to everything a CD contains. Still, by default, they'll only view the last session on a disc. To view other session, the drive must be order to do so by software such as a session selector.

Coloring Book

If you'd like to learn more about CD formats, you might want to check out the official CD specifications. These are used by drive manufacturers and CD duplicators, to design their hardware and software. Each volume of these specifications is know by the color of its cover:

Red Book -- CD media, and the format of audio CDs.
Yellow Book -- Data CDs, a.k.a. CD-ROMs.
Orange Book -- Multi-session CDs, among other things.
Blue Book -- CD-Extra (also known as Enhanced Music CD).
Green Book -- CD-I (CD-Interactive).
White Book - Video CDs.

The complete set is available from Philips Electronics for "just" $500. Or you can order separate volumes for prices ranging from $100 to $150 each. To order online, visit http://www.licensing.philips.com/

I've got go now, and get ready for my dad's birthday party. But if you have time this week, visit my home page at:


You won't find copies of the CD specifications. But you will find a lot of free programs, newsletter archives, and other cool stuff. Best of all, everything is free!

And if you see me there, or anywhere on the 'net, be sure to wave and say "Hi!"