Why does a CD sample sound 44,100 times a second, and how does a laser read it?

A CD samples sound 44,100 times a second for two reasons, one scientific and one a matter of convenience. The scientific one is hearing. People can commonly hear from 20 to 20,000 hertz, and the sampling theorem says the rate must be more than twice the highest frequency to record, so more than 40,000 samples a second. The convenient one is video tape. Early digital audio was recorded on analog video cassettes, because they were the only transports with enough capacity, and 44.1 kHz was the highest usable rate compatible with both PAL and NTSC video. That rate, from the PCM adaptors that were the most affordable way to move data from the studio to the factory, was inherited by the CD.

The format, launched in Japan in October 1982, is a two-channel 16-bit PCM encoding at 44.1 kHz. Philips had proposed alternatives such as about 44 kHz, but Sony prevailed on the sample rate and on 16 bits per sample rather than 14. The numbers are stored as tiny pits in a spiral track with flat lands between them, about 5.38 kilometres of spiral. A laser shines on the disc, the height change between pits and lands alters how light is reflected, and a photodiode reads the changes in intensity as a signal. Layers of coding, including cross-interleaved Reed-Solomon coding, are then undone to reveal the raw data.

By 1991 the CD had overtaken records and cassettes in US sales. Its 74 minutes are the subject of a legend: Philips' history says the capacity was set to hold Beethoven's Ninth, though Philips engineer Kees Immink calls that a myth.