CDs andDVDs 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.

Butwhen 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.

1. When Were CDs Invented?
2. Understanding the CD: Material
3. Understanding the CD: The Spiral
4. Understanding the CD: Bumps
5. CD Player Components
6. What the CD Player Does: Laser Focus
7. What the CD Player Does: Tracking
8. CD Encoding Issues
9. CD Data Formats

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.

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.

As discussed inHow 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

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 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...

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 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 almost3.5 miles (5 km) long!

To read something this small you need an incredibly precise disc-reading mechanism. Let's take a look at that.

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:

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 aCD-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 thebits that make up thebytes.

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.

The compact disc recordable (CD-R) has its ownunique 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:

There are severaldifferent 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.

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