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CD Basics:
  • Musical sounds can be turned into numbers and recorded on a CD. A CD stores music using 44,100 samples per second, 16 bits per sample, and two channels (for stereo sound). This means that a CD stores about 10 million bytes (megabytes) of data per minute of music on the CD. Therefore, a three-minute song requires 30 megabytes of data. If you've ever tried downloading files on the Internet, you probably know that 30 megabytes is huge. If you're using a modem to connect to the Internet, 30 megabytes of information would take a few hours to download. The Moving Picture Experts Group (MPEG) has developed systems to compress the data used for videos. DVD movies, HDTV broadcasts and DSS satellite systems use MPEG compression system to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound, called MPEG Audio Layer-3(MP3). MP3 can compress a song by a factor of 10 or 12 and still retain something close to CD quality. So a 30-megabyte sound file from a CD reduces to 3 megabytes or so in MP3. When you download the MP3 file and play it, it sounds almost as good as the original file. If you wanted to, you could download an MP3 file, expand it back to its original size and then record it on a writable CD so you can play it in a CD player. All that you are doing is converting back and forth between different formats to make downloading easier.

Understanding the CD- Material:

  • 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:
  • 44,100 samples/channel/second x 2 bytes/sample x 2 channels x 74 minutes x 60 seconds/minute = 783,216,000 bytes

  • To fit more than 783 megabytes (MB) onto a disc only 4.8 inches in diameter requires that the individual bytes be very small. By examining the physical construction of a CD, you can begin to understand just how small these bytes are.
  • A CD is a fairly simple piece of plastic, about four 4/100 of an inch thick. Most of a 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'll return to the bumps in a moment. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic.

Understanding the CD- The Spiral:

  • A CD has a single spiral track of data, circling from the inside of the disc to the outside. The fact that the spiral track starts at the center means that the CD can be smaller than 4.8 inches 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 MB 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 is how incredibly small the data track is -- it is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A micron is a millionth of a meter.) And the bumps are even more miniscule...

Understanding the CD- Bumps:

  • The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.)
  • You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps.
  • The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long!
  • To read something this small you need an incredibly precise disc-reading mechanism. Let's take a look at that.

CD Player Components:

  • The CD player has the job of finding and reading the data stored as bumps on the CD. Considering how small the bumps are, the CD player is an exceptionally precise piece of equipment. The drive consists of three fundamental components:
  • A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200 and 500 rpm depending on which track is being read.
  • A laser and a lens system focus in on and read the bumps.
  • A tracking mechanism moves 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.

What the CD Player Does- Laser Focus:

  • Inside the CD player, there is a good bit of computer technology involved in forming the data into understandable data blocks and sending 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 hits 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 detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits  that make up the bytes.

What the CD Player Does- Tracking:

The hardest 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 from the center of the disc, the bumps move past the laser faster -- this happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm). Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.

CD Encoding Issues:

Recordable CD

  • If you have a CD-R drive, and want to produce your own audio CDs or CD-ROMs, one of the great things you've got going in your favor is the fact that software can handle 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 this, you don't need to know anything about CD data formatting to create your own CDs. However, CD data formatting is complex and interesting, so let's go into it anyway.
  • To understand how data are 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 cannot be extended gaps where there are no bumps in the data track. To solve this problem, data is encoded using EFM (eight-fourteen modulation). In EFM, 8-bit bytes are converted to 14 bits, and it is guaranteed by EFM that some of those bits will be 1s.
  • Because the laser wants to be able to move between songs, data needs to be encoded into the music telling the drive "where it is" on the disc. 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 such things as song titles.
  • Because the laser may misread a bump, there need to be error-correcting codes to handle single-bit errors. To solve this problem, extra data bits are added that allow the drive to detect single-bit errors and correct them.
  • Because a scratch or a 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 disc, so that it is stored non-sequentially around one of the disc's circuits. The drive actually reads data one revolution at a time, and un-interleaves the data in order to play it.
  • If a few bytes are misread in music, the worst thing 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.