A compact disc (CD) stores digital data using microscopic pits and lands arranged in a spiral track that a laser reads to decode information. This optical storage technology relies on light reflection to retrieve audio, video, or computer files.
CDs use a 780-nanometer laser diode to read data through a 1.2mm-thick polycarbonate layer. The laser beam reflects off the aluminum layer beneath, and the CD player's photodetector measures these reflections. Pits (indentations) scatter light, while lands (flat areas) reflect it uniformly, creating the binary code that represents digital information.
The spiral track on a standard 12cm CD spans about 5.6 kilometers. Data is encoded using eight-to-fourteen modulation (EFM), which converts 8-bit bytes into 14-bit symbols to improve error correction. Each pit or land represents either a 0 or 1 in this binary system.
CDs store approximately 700MB of data. Audio CDs sample sound at 44.1kHz with 16-bit resolution, capturing 44,100 samples per second for each stereo channel. This high sampling rate preserves audio quality by capturing more points in each sound wave.
The CD player's motor spins the disc at constant linear velocity (CLV), maintaining consistent data flow. As the laser reads the spiral track, the motor slows rotation to maintain constant data throughput. The player's tracking mechanism keeps the laser focused on the correct track despite disc imperfections.
Contrary to popular belief, CDs don't store data magnetically like hard drives. They use optical technology, reading data through light reflection rather than magnetic fields. This makes CDs more resistant to electromagnetic interference but vulnerable to physical scratches that disrupt the laser's path.
Another misconception is that CDs store analog audio. While vinyl records capture sound waves directly, CDs digitize audio by measuring and recording discrete points along each wave. This digital process enables precise reproduction but requires sufficient sampling rates to avoid distortion.
Introduced in 1982, CDs revolutionized music distribution by offering better sound quality and durability than vinyl records or cassette tapes. Their digital format enabled exact replication, eliminating generation loss during copying. Today, CDs remain relevant in professional audio mastering and archival storage.
While streaming services have reduced consumer CD use, their technology influenced later formats like DVDs and Blu-ray discs. The principles behind CD operation continue shaping modern optical storage solutions.