When Were CDs Invented, and How Do They Work?

By: Marshall Brain  | 
Several compact disc cds layered on top of each other.
Have you ever wonder how compact discs work? William Whitehurst / Getty Images

CDs and DVDs seem like they've been around forever. 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, physical package.

But when were CDs invented, and how do they actually work? In this article, we will explore the origins of the compact disc. We will also look at how CDs and CD drives work, while investigating what the future holds for this technology.

Advertisement

When Were CDs Invented?

The invention of the standard compact disc revolutionized the way music, data, and later video were stored and distributed, marking a significant leap from analog to digital storage media. Developed jointly by Philips and Sony, the CD format was finalized in 1982 and famously documented in the Red Book standard. This collaboration not only unified the technical specifications for the digital audio disc but also ensured its widespread acceptance across different manufacturers and industries.

The CD's innovative use of a 120 mm optical disc to store data digitally allowed for higher quality audio recordings compared to the analog formats of the time, such as vinyl records and cassette tapes. It employed a laser to read information encoded in tiny pits etched into the surface of the disc, a method that was revolutionary, providing users with unprecedented sound clarity and fidelity.

Advertisement

Though originally released in 1978, Billy Joel's "52nd Street" was the first compact disc produced for commercial purposes. The introduction of the CD and its players brought about a transformative impact on the music industry and consumer behavior. It gradually replaced vinyl records and cassettes as the dominant format for music albums, offering users the convenience of skipping tracks, a longer playback duration, and improved durability.

Beyond sound quality, the technology behind CDs paved the way for subsequent optical storage solutions, including the digital versatile disc (DVD) and Blu-ray disc, which expanded the capacity and types of content that could be stored, ranging from high-definition video to complex software and large databases.

The CD's innovative use of a 120 mm optical disc to store data digitally allowed for higher quality audio recordings compared to the analog formats of the time, such as vinyl records and cassette tapes. It employed a laser to read information encoded in tiny pits etched into the surface of the disc, a method that was revolutionary, providing users with unprecedented sound clarity and fidelity.

Though originally released in 1978, Billy Joel's "52nd Street" was the first compact disc produced for commercial purposes. The introduction of the CD and its players brought about a transformative impact on the music industry and consumer behavior. It gradually replaced vinyl records and cassettes as the dominant format for music albums, offering users the convenience of skipping tracks, a longer playback duration, and improved durability.

Beyond sound quality, the technology behind CDs paved the way for subsequent optical storage solutions, including the digital versatile disc (DVD) and Blu-ray disc, which expanded the capacity and types of content that could be stored, ranging from high-definition video to complex software and large databases.

Advertisement

Understanding the CD: Material

Diagram showing the cross-section of a CD
Cross-section of a CD

As discussed in How Analog and Digital Recording Works, an audio CD can store up to 74 minutes of music, so the total amount of digital audio 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

Advertisement

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.

Advertisement

Understanding the CD: The Spiral

Illustration of spiral track data design on a CD. 
A CD has a single spiral track of data, circling from the inside of the disc to the outside.

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...

Advertisement

Understanding the CD: Bumps

An illustration of bumps on a CD surface.
A close-up illustration of the data bumps on the surface of a compact disc.

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.

Advertisement

The incredibly small dimensions of the bumps make the spiral track on a compact disc 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.

Advertisement

CD Player Components

View of disc drive motor and laser lens inside a CD player.
Inside a CD player

The CD player has the job of finding and reading the audio signals 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.

Advertisement

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 audio recordings) 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.

Advertisement

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.

Advertisement

CD Encoding Issues

A single recordable CD.
Recordable CD
Photo courtesy Ernest von Rosen, AMGmedia

The compact disc recordable (CD-R) has its own unique history. 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:

Advertisement

  • 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.

CD Data Formats

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). There are also Super Audio CDs (SACDs), which offer an enhanced listening experience.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Advertisement

Compact Disc FAQ

What is the use of a compact disc?
A compact disc or CD is a disc that's made of plastic and used to store data such as video, audio and text files. Compact discs replaced floppy discs because they are more efficient to use and can store more data.
When did CDs replace cassettes?
CDs replaced cassettes starting in 1991. By the late 2010s, CDs were on their way out, replaced by streaming music and digital downloads.
What does DVD stand for?
DVD stands for digital versatile disc or digital video disc.
Are DVDs being discontinued?
Streaming services have already pretty much replaced discs, including DVDs. The growth of the streaming industry is going to make DVDs even more obsolete in the future.
How can I play CDs without a CD player?
You can copy your CD to an MP3 file and move that to a USB drive. Or connect the USB cable to your external USB device and the other end to the USB port in your car. You can also use your personal computer or music systems that also feature a CD player.

Advertisement

Loading...