How Does A Compact Disc (CD) Work?

A few years ago, when USB sticks and cloud computing were not as popular as they are today, data was primarily stored and retrieved with the help of CDs. Today, shiny circular discs are known for storing data ranging from a few hundred megabytes to a few gigabytes.

What is a Compact Disc?

Compact disc CD is a digital storage format for optical media jointly developed by Philips and Sony. The CD format was originally developed for storing and playing sound recordings but was later adapted for storing data.

Data on a CD is encoded with the help of a laser beam that etches tiny indentations (or bumps, if you will) on its surface. A bump, in CD terminology, is known as a pit and represents the number 0. Similarly, the lack of a bump (known as land) represents the number 1.

, How Does A Compact Disc (CD) Work?, Science ABC, Science ABC

(Photo Credit: Flickr)

However, one of the biggest challenges with CDs is their delicate, smooth surface, easily scratched if not treated with the utmost care. Cracks and scratches on the surface of a disc impede its ability to store data and make it difficult for a CD player to retrieve the data stored in it.

But why exactly does this happen? Why are scratched CDs harder to read / access?

Before we come to that, we first have to understand how a CD works.

How does a CD work?

A CD is usually around 12 centimeters (4.5 inches) in diameter and consists of a couple of thin circular layers attached one on top of another.

CD layers

Various layers of a CD (Photo Credit: Pbroks13 / Wikimedia Commons)

Most of a CD is composed of a plastic called polycarbonate. The bottom layer is a polycarbonate layer where data is encoded by using tiny bumps on the surface. Above this layer is a reflective layer typically made of aluminum (gold is also used, although quite rarely).

Above the reflective layer is a protective layer of lacquer and plastic, which shields the layers below. The artwork or label is printed on the lacquer layer (i.e., on top of the CD) via offset printing or screen printing.

CDs store information digitally, i.e., with the help of millions of 1s and 0s. Data on a CD is encoded with the help of a laser beam that etches tiny indentations (or bumps, if you will) on its surface. A bump, in CD terminology, is known as a pit and represents the number 0. Similarly, the lack of a bump (known as land) represents the number 1. Hence, a laser beam can encode the required data into a compact disc using pits and lands (0 and 1, respectively).

, How Does A Compact Disc (CD) Work?, Science ABC, Science ABC

Now that you know how a CD is encoded with data let’s look at how a CD player actually reads this stored data.

How does a CD player work?

There are two main components in a CD player that help read a CD: a tiny laser beam known as a semiconductor diode laser and an electronic light detector, basically a tiny photoelectric cell. When you turn on the CD player, an electric motor in the player rotates the CD at a very high speed while reading the outer edge at 200 RPM, and when reading the inner edge, it rotates at 500 rpm.

, How Does A Compact Disc (CD) Work?, Science ABC, Science ABC

The laser beam source inside the player switches on and scans along a track from the center of the disc to the outer rim. It focuses a 780 nm wavelength (near-infrared) beam through the underside of the compact disc. When the beam falls on land (1), it reflects straight back, but it scatters when the beam falls on a pit (0).

When the photocell detects the reflected light, it recognizes that the laser must have hit land and, in turn, sends a signal to a circuit that generates the number 1. Likewise, when it does not detect light, it correctly determines a pit at this point so that the circuit generates the number 0. Thus, the photocell uses the intensity changes of the reflected beam to determine whether there is a 1 or a 0 on the disk.

, How Does A Compact Disc (CD) Work?, Science ABC, Science ABC

All kinds of digital information can be stored with the help of 0s and 1s. (Photo Credit: Pixabay)

Why is it difficult for a CD player to read the contents of a scratched CD?

The data from a CD / DVD / Blu-ray disc is not on the glossy surface but the polycarbonate layer at the bottom of the disc. As already mentioned, a CD player has a laser beam that reflects/scatters from the underside depending on whether it falls on a land/pit. Indentations on the surface of a disc are very, very small, so scratches and cracks mess up the way light bounces off the surface of the CD.

Scratched CD

(Photo Credit: Flickr)

When the laser falls on a scratched spot, it scatters, even if there is no bump at this point. As a result, the photocell transmits incorrect information to the circuit, making it difficult for the CD player to read the data correctly.

Related Articles
Related Articles

However, scratches do not necessarily make a CD unusable. There is an error correction in the way data encoded into a CD, which ensures that minor scratches on the disc’s surface do not make the CD unreadable. CD / DVD players cannot read it unless the scratches are severe or the CD is cracked.

Help us make this article better
About the Author

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

.
Science ABC YouTube Videos

  1. What Are The Different Atomic Models? Dalton, Rutherford, Bohr and Heisenberg Models ExplainedWhat Are The Different Atomic Models? Dalton, Rutherford, Bohr and Heisenberg Models Explained
  2. Why Is Blood Drawn From Veins And Not From Arteries?Why Is Blood Drawn From Veins And Not From Arteries?
  3. Emotions and the Brain: What is the limbic system?Emotions and the Brain: What is the limbic system?
  4. Dark Matter Explained: What Exactly is Dark Matter? | A Beginner’s Guide to Dark MatterDark Matter Explained: What Exactly is Dark Matter? | A Beginner’s Guide to Dark Matter
  5. What Exactly is a Tesseract? (Hint: Not a Superhero Stone)What Exactly is a Tesseract? (Hint: Not a Superhero Stone)
  6. Respiratory System: From Inspiration to Expiration Explained in Simple WordsRespiratory System: From Inspiration to Expiration Explained in Simple Words
  7. What is the Fibonacci Sequence & the Golden Ratio? Simple Explanation and Examples in Everyday LifeWhat is the Fibonacci Sequence & the Golden Ratio? Simple Explanation and Examples in Everyday Life
  8. Digestive System: Ingestion to Egestion Explained in Simple WordsDigestive System: Ingestion to Egestion Explained in Simple Words