A compact disc (CD) is a small, round optical disc used to store digital data. Originally developed in the late 1970s and launched commercially in the early 1980s, CDs revolutionized how music, software, and data were distributed. Though largely replaced by digital downloads and streaming today, understanding how a CD works remains a fascinating look into optical and digital technology.
Structure of a CD
A compact disc is made primarily of polycarbonate plastic, about 1.2 mm thick. It is 12 centimetres in diameter and weighs about 15 grams. On one side is a shiny, reflective aluminium layer that stores data, protected by a thin layer of clear plastic and a lacquer coating. A printed label often appears on top.
Despite their simplicity in appearance, CDs use precise engineering. The underside is smooth and reflective because that’s where a laser beam reads the information. Unlike magnetic tapes or vinyl records, CDs have no grooves. Instead, they store information in the form of tiny indentations called pits, separated by lands (the flat areas between pits).
How Data is Stored
Data on a CD is encoded in binary—that is, as a series of 0s and 1s. These binary digits are physically represented by the pits and lands on the disc. A pit does not directly represent a 0 or 1; rather, it's the change between a pit and a land, or vice versa, that represents a 1. No change means a 0. This encoding system is known as Non-Return to Zero Inverted (NRZI).
The pits are microscopic—about 0.5 micrometres wide and 0.83 micrometres long, spaced just 1.6 micrometres apart. These are arranged in a single, long spiral track starting at the centre of the disc and ending near the outer edge, unlike vinyl records which read from the outside in.
Reading the Disc
To read a CD, a CD player or drive uses a laser diode, typically emitting an infrared beam (wavelength around 780 nanometres). The disc spins—at varying speeds depending on the location being read—and the laser focuses onto the reflective layer through the polycarbonate surface.
As the laser passes over the surface, it reflects differently from pits and lands. A photodiode sensor detects these changes in reflection. A land reflects the laser beam directly back, while a pit scatters it slightly due to its depth (about one-quarter of the laser’s wavelength). The resulting pattern of reflected and non-reflected light is translated into the binary code that the CD player’s digital processor can decode.
Error Correction
Since scratches, dust, or fingerprints can interfere with laser reading, CDs use error correction algorithms, notably Cross-Interleaved Reed-Solomon Coding (CIRC). This method adds redundancy to the data, allowing the CD player to detect and correct many types of errors without skipping or stopping playback.
Audio CDs vs. Data CDs
There are different formats of CDs based on their intended use:
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Audio CDs (CD-DA) follow the Red Book standard and can hold up to 74–80 minutes of sound, stored as uncompressed PCM audio (44.1 kHz sample rate, 16-bit, stereo).
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CD-ROMs store computer data and use the Yellow Book standard. These can contain programs, files, or video and are often formatted with file systems like ISO 9660.
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CD-Rs (Recordable) and CD-RWs (Rewritable) use special dye or phase-change materials that can be written to by higher-powered lasers in CD burners.
CD Writing Process
In CD-Rs, a laser alters the dye in the disc to mimic the effect of pits, making them appear darker to the reader. Once written, this change is permanent. In CD-RWs, the material can change state between crystalline and amorphous, enabling multiple rewrite cycles.
CD Lifespan and Limitations
CDs can theoretically last decades if stored properly—away from heat, light, and physical damage. However, they are vulnerable to “CD rot,” where the reflective layer oxidizes or the protective lacquer deteriorates. Also, compared to modern storage media, CDs have limited capacity—typically 700 MB, which is relatively small today.
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