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Cơ bản về Compact Disc (CD)

Ngày nay, CD và DVD hiện diện ở mọi nơi. Whether they are used to hold music, data or computer software, they have become the standard medium for distributing large quantities of information in a reliable package. CDs are so easy and cheap to produce that America Online sends out millions of them

every year to entice new users. Anyone with a computer and

CD-R drive can now create their own CDs and put anything they want on them.

In this version of How Stuff Works we will look

at how CDs and CD drives work. We will also look at all the different forms CDs take,

as well as what the future holds.

Understanding the CD

As shown in How Analog-Digital Recording Works, a CD can store up to 74 minutes of music, so the total amount of digital data that must be stored on a CD is:

To fit over 783 megabytes onto a disk only 12 centimeters in diameter means the individual bytes have to be

physically fairly small. By looking at the physical construction of the CD you can learn how small they are.

A CD is a fairly simple piece of plastic about 1.2 millimeters thick. Most of the CD consists

of an injection-molded piece of clear polycarbonate plastic. During manufacturing this plastic is

impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. We will

return to the bumps in a moment. Once the clear piece of polycarbonate is formed, a thin, reflective

aluminum layer is sputtered onto the disk, covering the bumps. Then a thin acrylic layer is sprayed over

the aluminum to protect it. Then the label is printed onto the acrylic. A cross section of a complete

CD (not to scale) looks like this:

data/knowledge/other/gif/cd-crosssection.gif

data/knowledge/other/gif/cd-spiral.gifA CD has a single spiral track of data circling from the inside of the disk to the outside.

The fact that the spiral track starts at the center means that the CD can be smaller than 12 centimeters if desired,

and in fact there are now plastic baseball cards and business cards that you can put in a CD player. CD business cards

hold about 2 megabytes of data before the size and shape of the card cuts off the spiral.

What the picture on the right does not even begin to impress upon you, however, is how incredibly small

the data track is. The track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next.

The track consists of a series of elongated bumps 0.5 microns wide, a minimum of 0.97 microns long and 125 nanometers

high. Looking through the polycarbonate layer at the bumps, they look something like this:

data/knowledge/other/gif/cd-bumps.gif

[You will often read about "pits" on a CD instead of bumps. They are pits on the aluminum side, but on the

side the laser reads from they are bumps.]

The incredibly small dimensions of the bumps makes the spiral track on a CD extremely long. If you could somehow lift the data track off a CD and stretch it out

into a straight line, it would be 0.5 microns wide and almost 5 miles long!

To read something this small you need an incredibly precise disk-reading mechanism.

How Does a CD Player Really Work?

The CD player has the job of finding and reading the data stored as bumps on the CD. Because the

bumps are so small, the CD player is an exceptionally precise piece of equipment. The drive

consists of 3 fundamental components:

  • A drive motor to spin the disk. This drive motor is precisely controlled to rotate between

    200 and 500 RPMs depending on which track is currently being read.

  • A laser and a lens system to focus in on the bumps and read them
  • A tracking mechanism that can move the laser assembly so that the laser's beam can follow the spiral track.

    The tracking system has to be able to move the laser at micron resolutions.

Inside the CD player there is also a good bit of computer technology to form the data into understandable

data blocks and send them either to the DAC (in the case of an audio CD) or to the computer (in the case of a CD-ROM drive).

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the

polycarbonate layer, reflects off the aluminum layer and returns to an opto-electronic device that detects

changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer),

and the opto-electronic sensor can detect that change in reflectivity. The electronics in the

drive interpret the changes in reflectivity to read the bits that make up the bytes.

[flash=550,150]data/knowledge/other/flash/cd-read.swf[/flash]

The hard part is keeping the laser beam centered on the data track. This centering is the job of the

tracking system. The tracking system, as it plays the CD, has to continually move the laser outward.

As the laser moves outward, the spindle motor slows the speed at which the CD is revolving so that the

data coming off the disk maintains a constant rate.

[flash=400,250]data/knowledge/other/flash/cd-drive.swf[/flash]

What Are the Data Formats on a CD?

If you have a CD-R drive and want to produce your own audio CDs or CD-ROMs, one of the

great things going in your favor is the fact that software handles all the details for you.

You can say to your software, "Please store these songs on this CD" or "please store these

data files on this CD-ROM" and the software will do the rest. Because of that you don't

need to know anything about CD data formatting to create your CDs. However, CD data formatting

is complex and interesting, so here is a bit of detail.

To understand how data is stored on a CD, you need to understand all of the different

conditions the designers of the data encoding methodology were trying to handle. Here is a fairly

complete list:

  • Because the laser is tracking the spiral of data using the bumps, there can not be extended

    gaps in the data track where there are no bumps. To solve this problem data is encoded

    using EFM (eight-fourteen modulation). 8-bit bytes are converted to 14 bits.

  • Because the laser wants to be able to move between songs, there needs to be data encoded

    within the music telling the drive "where it is" on the disk. This problem is solved using what is

    known as "subcode data". Subcode data can encode the absolute and relative position of the

    laser in the track, and can also encode things like song titles.

  • Because the laser may misread a bump, there needs to be error correcting codes

    to handle single-bit errors. To solve this problem, extra data bits allow the drive to detect

    single-bit errors and correct them.

  • Because a scratch or speck on the CD might cause a whole packet of bytes to be misread (known as

    a burst error), the drive needs to be able to recover from such an event. This problem is solved

    by actually interleaving the data on the disk, so that it is stored non-sequentially around one circuit

    of the disk. The drive actually reads data one revolution at a time and un-interleaves the data

    to play it.

  • If a few bytes are misread in music, then the worst that can happen is a little

    fuzz during playback. When data is stored on a CD, however, any data error

    is catastrophic. Therefore additional error correction codes are used when

    storing data on a CD-ROM.

There are several different formats used to store data on a CD, some widely

used and some long-forgotten. The two most common are CD-DA (audio) and CD-ROM (computer data).

If you would like more information on either of these formats, the following links

will help:

Links

If you would like to build a replica of Edison's phonograph, click here for instructions.

Places to buy music CDs:

CDs are incredibly interesting devices. Here are some links that provide more detailed information: