Compact Disc

A Compact Disc (see also CDDA in our WFTD archives), is a small optical disk on which data such as music, text, or graphic images is digitally encoded. It is essentially a digital medium that is read by a laser. Some of the original standards of the audio CD, such as the 44.1kHz sampling rate, 16-bit depth, size and finished minutes (74 minutes at that time), were decided upon as early as 1979 by Sony and Phillips. While prototypes were shown as early as 1979, the Compact Disc technology was introduced by in large in Europe and Japan in the fall of 1982. 30,000 CD players were sold in the US in 1983, and by the end of the decade CDs were outselling vinyl as a popular music delivery format. An audio CD is specified by Red Book standards, which define information such as lead-in, lead-out, table of contents and more. While there are other standards used by other forms of data stored on CDs, they all are derived from Red Book in one way or another. The compact disc is read by a laser that receives a series of digital pulses from a track of bumps as it follows a spiral track from the middle of the disc outwards. The track width is tiny at just 0.5 microns wide, with 1.6 microns separating the tracks. The CD initially rotates in the player at 500 rpm as the laser reads the inner tracks; the speed slowly decreases to 200 rpm as the outer tracks are read. This keeps the data rate constant as the laser traverses the disc and is known as CLV (Constant Linear Velocity).