How do CD’s work?

SDStaff Ian replies:

My question has always been, how did Fleetwood Mac come together out of Mick, John, Lyndsey, Christine, and Stevie? Some things are just beyond comprehension. But not digital sound.

Digital equipment uses the binary number system, in which there are only two digits, 0 and 1. The advantage of using binary numbers for data storage is that 0’s and 1’s can be represented in simple electronic circuits by the presence of voltage. If the circuit has voltage, it is ‘on,’ and the number being represented is 1. If it’s off, the bit is 0.

Getting sound from a CD is no more mysterious than getting it from any other recording device. A CD is a spinning object with indentations on its surface which, when read, cause a speaker to vibrate at a certain frequency. A phonograph record has the same thing. So did Edison’s foil-covered wax cylinder.

In the latter two cases, the groove on the disk or cylinder was a physical analog of the waveform, and the stylus vibrated as it moved along the groove. If you take that groove, and draw it, you’ll have a continuous wave that does up and down, drawings of which we’ve all seen in science class.

In order to digitize this waveform, the amplitude of the wave is sampled at a certain time interval. Measure the wave at small enough intervals, and you can get a very accurate representation of the wave. Our ears can hear up to about 20 kHz (20,000 vibrations per second), so the sampling has to be at least twice that fast, to represent the peaks and the valleys of the wave. A CD player does this sampling at a rate of 44.1 kHz (44,100 times per second). Each 1/44,100 seconds, the laser samples a value between 0 and 63,535 (actually, it samples a 16-bit length of track consisting of ‘pits’ burned into the CD’s surface which diffuse the laser, and untouched ‘lands’ which reflect the laser back, which we might represent as a 16-bit binary number between 0000000000000000 and 1111111111111111), and then the digital to analog converter takes this number and converts it into an analog electrical pulse of a certain amplitude, which ultimately vibrates the speaker. When we hear a sequence of these pulses in succession at a certain frequency, our ears interpret the vibrations of the air as sound.

As for DVD, and CD-ROM, same principle. The audio CD player has a computer which converts this digital information into sound, but of course a computer can do much more than just that. While pictures have more information to them than sound, a picture on a TV screen is still just a bunch of dots, and these dots are just as easy to quantize into digits as sound is. A computer’s CD-ROM drive, of course, works much the same way, with the distinction that the information has to be encoded so that the computer’s operating system can interpret certain groups of 1’s and 0’s as video, audio, code, or text.

SDStaff Ian, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

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