Summary of How CDs Work
This article explains the physical construction and data storage mechanics of a Compact Disc (CD). It details how over 783 MB of data fits onto a 4.8-inch disc using microscopic bumps arranged in a single, continuous spiral track. The text describes the CD's layered composition, including polycarbonate plastic, an aluminum reflective layer, and an acrylic protective coating. It also highlights the incredibly small dimensions of these data features, noting that the spiral track stretches nearly 3.5 miles if uncoiled.
Parts used in the CD:
- Injection-molded clear polycarbonate plastic
- Microscopic bumps arranged as a spiral track
- Thin reflective aluminum layer
- Thin acrylic protective layer
- Printed label
Understanding the CD: Material
As discussed in How Analog and 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:
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 (12 cm) 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 one-hundredths (4/100) of an inch (1.2 mm) 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. A cross section of a complete CD (not to scale) looks like this:
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 (12 cm) 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.) Looking through the polycarbonate layer at the bumps, they look something like this:
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!
For more Detail: How CDs Work
- How much digital data can a standard CD store?
A CD can store up to 74 minutes of music, totaling 783,216,000 bytes. - What is the thickness of a CD?
A CD is about four one-hundredths of an inch or 1.2 mm thick. - Does the data track on a CD start from the center?
Yes, the single spiral track of data circles from the inside of the disc to the outside starting at the center. - How wide is the data track on a CD?
The data track is approximately 0.5 microns wide with 1.6 microns separating one track from the next. - What are the dimensions of the bumps on a CD?
The bumps are 0.5 microns wide, a minimum of 0.83 microns long, and 125 nanometers high. - Why do some sources refer to pits instead of bumps?
They appear as pits on the aluminum side but look like bumps on the side the laser reads from. - How long would the data track be if stretched out straight?
If lifted off and stretched into a straight line, the track would be almost 3.5 miles or 5 km long. - Can non-standard shapes like business cards hold CD data?
Yes, plastic baseball cards and business cards can be put in a CD player, though they hold less data due to size constraints.
