What Is A CD?

Date First Published: 21st June 2023

Topic: Computer Systems

Subtopic: Computer Hardware

Article Type: Computer Terms & Definitions

Difficulty: Easy

Difficulty Level: 2/10

CONTENTS

Storage Capacity
How Do CDs Read Data?
CD Formats
Advantages and Disadvantages Of CDs
Difference Between A CD and A DVD
History

Learn about what a CD in this article.

Stands for a Compact Disc. A CD is a type of portable optical storage media most commonly used to store digital audio recordings and data. A standard CD measures 120 mm (4.7 inches) in diameter, 1.2 mm (0.046 inches) in thickness, and weighs between 15-20 grams. CDs are made from thick, polycarbonate plastic.

Storage Capacity

A standard CD has a storage capacity of 700 MB, which is equivalent to around 80 minutes of digital audio and 60 minutes of video content.

How Do CDs Read Data?

CDs read data by focusing a 780 nanometer wavelength semiconductor laser onto a single track of the disc. As the disc rotates, the laser beam measures differences in the way light is reflected off the polycarbonate layer on the bottom of the disc, converting it to sound. The incoming pulses are decoded by the microprocessor and are sent to the computer to be processed.

CD Formats

Below are the different CD formats.

CD-ROM - A CD used as a read-only optical memory device. This type of CD disc can be read, but not written to.
CD-Recordable - A type of CD format that can only be written to once and read multiple times. The data burned on a CD-Recordable disc is permanent and cannot be erased or modified.
CD-Rewritable - A rewritable CD format that can be written to and erased around 1000 times, allowing users to burn their own CDs to share files, music, and back up files. This CD format requires specialised readers that have sensitive laser optics.
CD-Interactive - A CD format that enables limited interaction with films and games. As of now, this format is only found in training and education locations and was discontinued in 1998.
CD-Musical Instrument Digital Interface (MIDI). A CD format designed to store music performance data. playback is performed by electronic instruments that produce the audio.
Photo CD - A CD format designed for storing photos in a digital format that could be accessed and edited by a computer.
Video CD - A CD format designed for distributing films on CDs. It was designed to have similar quality to VHS recordings, but has a much lower resolution than a modern DVD.

Advantages and Disadvantages Of CDs

The advantages of CDs are:

Nearly all computers and other types of digital devices have optical disk drives and can read CDs. Even if there is no CD drive, they can be read by a DVD drive.
They are a small storage media, which makes them very portable.
Data can be read and accessed quite quickly.

The disadvantages of CDs are:

Compared to other optical storage media, CDs have quite a low storage capacity. This makes them less suitable for storing films and large files.
They are quite fragile and easy to damage. Scratches can cause CDs to skip or stop working. They can also warp when exposed to extreme heat and become unusable.
They only work for a limited number of writes. For example, CD-RW discs can only be written to around 1000 times.

Difference Between A CD and A DVD

CDs and DVDs look very similar in that they are both the same size and have a labelled and unlabelled side where the data is read by a laser, but the main difference between a CD and a DVD is that a DVD is capable of storing much more data than a CD. Compared to a DVD, a CD can hold seven times less data. Due to the smaller storage capacity of CDs, they are most commonly used to store audio content rather than video content. DVDs represent the second generation of CD technology.

History

CDs were first introduced in 1982 by Sony and Philips. In October 1982, CDs were first released in Japan and branded as Digital Audio Compact Disc. The first commercial compact disc, Billy Joel's "52nd Street", was released at that time.

They were then released on 2nd March 1983 in North America and Europe. At the time of the introduction, a CD could store much more than a hard disk drive, which would usually only store 10 MB. By 2010, hard drives often had as much storage space as a thousand CDs.

In the music world, digital formats have become more widely used than physical formats, like CDs due to the increase in streaming audio and internet downloads. Due to convenience and lower costs for the creators, fewer users use CDs today.