The compact disc's centre hole matches a small Dutch coin
The hole in the middle of every compact disc is 15 mm wide because a Philips engineer based it on a Dutch ten-cent coin. That detail hides inside a format sold more than 200 billion times by 2007, whose single data spiral, unwound, would stretch more than five kilometres.
Philips and Sony developed the CD together to hold uncompressed stereo audio, and it went on sale in Japan in October 1982, the second optical disc to reach the market after the bulkier LaserDisc. A standard disc is 120 mm across and 1.2 mm thick, and originally held 74 minutes of music or about 650 MiB of data. Squeezing the track spacing later raised that to 80 minutes, with unofficial discs stretching to 99. Joop Sinjou chose the hole size using the dubbeltje coin.
Physically it is a polycarbonate disc moulded with microscopic pits, each about 100 nm deep and 500 nm wide, arranged along one spiral that starts near the centre and winds outward, with 1.6 micrometres between turns. A thin aluminium coating, occasionally gold, reflects light, and lacquer and the printed label protect it. A near-infrared laser reads through the clear underside, where the pits appear as bumps. Transitions between pit and flat surface mark ones; unchanged stretches mark zeros. Because the pits sit just under the label, that side is actually the more fragile one.
Playback runs at a constant surface speed of 1.2 to 1.4 metres per second, so the disc spins at around 500 revolutions per minute near the centre and slows to about 200 at the edge. The laser rides on a sled that tracks the spiral, driven by a worm gear or linear motor, though some early Philips players used a swinging arm like a gramophone's.
Commercially the CD swept everything before it. By 1991 it outsold vinyl records and cassettes in the United States, and by 2000 it held 92.3 percent of the American music market. It also spawned CD-ROM, which briefly offered more room than PC hard drives, and recordable versions, before MP3s, downloads and streaming eroded it from the mid-2000s.
Source: Compact disc
