A compact disc is a portable storage medium that can be used to record, store and play back audio, video and other data in digital form.
A standard compact disc measures 4.7 inches, or 120 millimeters (mm), across, is 1.2 mm thick, weighs between 15 grams and 20 grams, and has a capacity of 80 minutes of audio, or 650 megabytes (MB) to 700 MB of data.
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A CD works 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.
CDs are fragile and prone to scratches; they can be repaired, but disc readability may be affected.
History of compact discs
James Russell, an American inventor, envisioned an alternative to vinyl albums to store and play audio recordings. He was the first person to file a patent for a product resembling a combination of laser, digital recording and optical disc technologies in 1966. Philips Electronics and Sony Corp. then purchased licenses of the technology in the 1980s.
The first commercial compact disc, a recording of a series of Chopin waltzes performed by pianist Claudio Arrau, was released in 1982. Prior to that, test recordings were completed in 1979; and in 1981, the BBC demonstrated a CD playing the Bee Gees' album, Living Eyes.
The first CD player, the CDP-101, was released commercially in 1982, and the format began to be used across the globe. Early compact discs were made at just two factories, owned by Philips and Sony.
Before the process became streamlined, individual discs cost $30, but as manufacturers proliferated, prices dropped. Hitachi also released a CD player in 1982, but the majority of sales belonged to Sony, with 20,000 sold in the first year.
CDs joined tape cartridges in generally replacing the phonograph record for playing music. Initially, CDs were read-only, but later technology allowed users to record on them, as well. As the 1980s came to an end, compact discs became the industry standard for audio recordings.
Compact disc formats
With the rise of personal computers (PCs) and other commercial technologies, various compact disc formats branched off to store data. Sony and Philips created specifications for these CD versions -- called Rainbow Books, due to the various colors on the book bindings -- to define each product format. The Red Book outlined the specifications for a standard CD.
Compact disc variations include:
- CD-Read-Only Memory. In 1985, the CD-ROM entered the market and went beyond audio to record optical data storage. CD-ROMs are readable by any computer with a CD-ROM drive. The CD-ROM follows the Yellow Book standard.
- CD-interactive. Released in 1993, CD-i could be played on CD players, but not in a CD-ROM drive. The format was later modified to be read by both. The CD-i follows the Green Book standard of specifications.
- CD-ReWritable. The CD-RW used a metallic alloy that reflected differently than regular compact discs. This change in reflectivity made a CD-RW unreadable to many early CD players. The CD-RW follows the Orange Book standard.
- CD-Recordable. The CD-R is a compact disc that can be written to once and read many times. Like the CD-RW, it follows the Orange Book, but unlike the CD-RW, the CD-R can be read on CD players released prior to its own introduction.
- CD-ROM eXtended Architecture. The CD-ROM XA is an extension of the standard CD-ROM that allows audio, video and computer data to be accessed simultaneously. It follows the Yellow Book standard and was created as a bridge between the CD-ROM and CD-i.
- Photo CD. Designed by Kodak, the photo CD was created for the express purpose of storing photographs in a digital format that could be accessed and edited on a computer. It launched in 1992, and was originally designed to hold 100 high-quality images. It followed the Beige Book standard.
- Video CD. The video CD, or VCD, was created in 1993 and followed the White Book standard. VCD quality was intended to have comparable quality to VHS recordings, but has a much lower resolution than a modern digital video disk (DVD).
The future of compact discs
As other technologies flourish, the CD has seen a steady decline in use, particularly during the early 2010s.
Digital formats have overtaken CDs in the music world, which has seen a huge shift away from physical mediums with the rise of streaming audio and digital downloads. While compact disc sales were more profitable for those in the music industry, convenience and low costs have fewer consumers turning to the physical medium.
When compact discs were initially on the rise, PCs could only store approximately 10 MB of data, which had many turning to the CD for storage. That is no longer the case. With higher capacity hard drives and online storage options consistently entering the market, compact discs and tape cartridges are no longer the top choice for many consumers.
In 1995, Panasonic, Philips, Sony and Toshiba created the DVD format as a possible media replacement for compact discs. A DVD has the same dimensions as a CD, but a much higher storage capacity of 4.7 gigabytes (GB). The format is probably most known for video entertainment storage, but is also used for software and other digital data. DVDs can be played on a DVD player, as well as in a DVD-ROM in a computer.