Compact Disc (CD)

CD reading technology is a scaled-down version of LaserDisc. By the early 1980s, inexpensive infrared lasers were available. An infrared laser, having a longer wavelength, cannot be focused to as small a spot as a red laser. However, even with its inferior data density, a compact disc with a 120 mm (4.7 in) diameter could store more than an hour of high-quality stereo music.

The story goes that the CompactDisc's capacity of 74 minutes was chosen because the president of CBS Sony Records, Norio Ohga, wanted the CD to hold Beethoven's 9th Symphony on a single disk.

This is the kind of story that I usually pass off as hogwash. However, if I had been in a position to influence the capacity of the CD,  being a classical music aficionado, I would have said to make the capacity at least 70 minutes without hesitation. This would hold the entire Ormandy recording of Beethoven's 9th Symphony (68 minutes and 14 seconds).^[1] Why Beethoven's 9th? Not only is it perhaps the best-known orchestral work of all time, but there are only three works longer than Beethoven's 9th in the standard repertoire performed regularly. They are Bruckner's 8th Symphony at about 80 minutes, Mahler's 6th Symphony at about 80 minutes, and Mahler's 3rd Symphony at about 95 minutes. Many works in the standard repertoire approach the length of Beethoven's 9th Symphony, but It's hardly worth adding another 25 minutes to the capacity for three works.

According to Sony's website, this is true: https://www.sony.com/en/SonyInfo/News/Press/201104/11-0423E/.

Ohga had an audio engineering background and was a classically trained vocalist. He pushed for a 12 cm disk over an 11.5 cm disk for the CD format to hold a complete recording of Beethoven's 9th Symphony.  

Like the LaserDisc, the CD data is molded into one side of a 120 mm (4.7-inch) diameter clear polycarbonate disk. After a reflective aluminum layer, the CD's data side is coated with lacquer and usually imprinted with a label. This lacquer and label protect the data side of the disk from the environment. The unlacquered, unlabeled side of the disk also has a protruding ring of plastic (the stacking ring) that prevents the optical surface of the disk from easily contacting a flat surface on which the CD is placed.

The laser shines through the unprinted side of the disk to the reflective layer. CD data is recorded as long and short pits representing digital data using a coding where a change from land to pit or pit to land represents a one, and no change represents zero. This provides a built-in clock so the disc can spin at an arbitrary speed, and the data can still be extracted.  

Microscopic image of a CD surface.

CD audio is recorded using 16 bits per channel sampled at 44.1 kbps. This sample rate, being significantly more than twice the highest frequency that needs to be recorded (the Nyquist rate), provides a reasonably faithful reproduction of the original signal.

In 2005, it was discovered that Sony BMG CD deliberately installed malware on computers when played using a CD drive on a Windows computer. This malware modified the operating system with code to prevent copying of the CD. This poorly written code had vulnerabilities that opened the computer to attacks by cybercriminals. After government investigations and class-action lawsuits, Sony BMG removed the code from their CDs in 2007.