Compact audio discs were developed by Philips and Sony in the 1980s. The disc consists of a clear layer of polycarbonate plastic laid on top of a reflective aluminium surface. Data is magnetically recorded and stored on the disc digitally in binary code. The '1s' are represented by small pits in the plastic, while the '0s' are the remaining smooth plastic surface. When the disc is played, a laser scans its surface with the effect that only the '0' areas reflect the beam back. These reflected pulses are picked up by a photodetector, which converts them into a digital electrical signal. By the early 1990s CDs had become the prevalent medium for audio recordings and digital storage.
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