---
title: Why a CD is 74 minutes long
date: "2002-07-05T12:00:00Z"
categories:
  - links
wp_id: 1288
description: Discover how technical constraints and a specific recording of Beethoven’s 9th Symphony led Sony and Philips to settle on the 12cm, 74-minute CD format instead of the originally proposed 11.5cm, 60-minute disc.
keywords: [compact disc, sony, philips, beethoven's 9th symphony, digital audio, sampling rate, 16-bit audio]
---

**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](http://pub58.ezboard.com/fmpegplusfrm3.showMessage?topicID=6.topic). 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](http://www.manbottle.com/trivia/74_Minutes.htm_answer.htm) (a popular piece in Japan) would take 74 minutes.

**References**: [1](http://www.howstuffworks.com/cd.htm/printable) [2](http://www.ieee.org/organizations/history_center/sloan/DAR/timeline.html) [3](http://www.ping.be/satcp/realsize01.htm) [4](http://www.candisc.com/04printscreen.html) [5](http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm) [6](http://www.isr.umd.edu/~austin/nsf-crcd/case-study-cd-player.html) [7](http://www.urbanlegends.com/misc/cd/cd_length_skeptical.html)