Why is a CD 74 minutes long? Answer: Beethoven.
Sony and Philips worked under the following constraints:
- The bits to be etched the CD had to be 0.83 microns long and 1.6 microns apart. Hence about 750,000 bits fit in 1 sq. mm
- CD uses 16-bit audio with 2 (stereo) channels.
- Sound is sampled at at 44.1 KHz for historical reasons. So 1 sq mm translates to about 750,000 bits / 44,100 Hz / 32 = 0.53 seconds of music.
Philips came up with a 11.5cm CD that could store about 65 minutes of music. Sony made that 12 cm because 11.5cm can store around 65 minutes of music, while a CD that’s 12cm can store about 74 minutes. One of the supposed reasons is that a slow rendering of Beethoven’s 9th symphony (a popular piece in Japan) would take 74 minutes.