How do CDs work?

All the packages have been unwrapped and Junior's pretty happy -- he finally got that new CD stereo he's been nagging his parents about. Unfortunately, they forgot to buy him headphones. The music's been so loud, dad hasn't heard any commentary for the last eight bowl games, and he's had enough. He bounds the stairs two at a time and is about to threaten his son within an inch of his life when he reflects, "When I was a teen, we'd throw a big, black record on the phonograph, then flip it to hear the other side. I wonder how those CD's work?" While dad attempts to fine-tune Junior, let's get an SwRI Whizard to answer his question.

"Many of us use CDs (compact disks) every day, and because they are so reliable, we don't question how they are produced or the technology used to play them. The LP (long play) records of an older generation were made of vinyl that was etched into grooves and tracks, then read by a needle on the record player. However, unlike the records of old, a compact disk uses a laser instead of a needle to read the surface of the disk. The CD is also much more practical than its predecessor: a CD weighs almost nothing and is difficult to scratch or break.

"A CD is made of polycarbonate plastic that is coated with a very thin layer of aluminum to add reflectivity. To make a CD, the information, whether for an audio CD (also known as CD-DA, for digital audio) or for a CD-ROM (CD-read only memory, used by computers), is burned onto the surface of the CD. (CDs, by the way are extremely inexpensive to mass-produce: Each costs less than a dollar. Remember that next time you purchase one for $19.95.)

"The surface on the non-label side of a CD consists of pits (acting like bumps to the laser) and lands, which are the normal surface area between pits. When the laser beam strikes a land, it is reflected into a photosensor, representing an "on" state. When the laser beam hits a pit, the beam is scattered so that it is not reflected in the photosensor, creating an "off" state. The transitions between the "on" and "off" or pits and lands, and the associated timing represent channel bits. Fourteen channel bits make up a data symbol that translates to a traditional 8-bit data value; channel bits on CDs are grouped together and called frames. Each frame can contain either 24 bytes of computer data or twelve 16-bit audio samples (six stereo samples). Groups of 98 frames are called blocks (or sectors on CD-ROMs); a block (or sector) is the smallest addressable unit in CD technology. There are 75 blocks for every one second of CD rotation, yielding a total of 7,350 frames per second. A CD holds 74 minutes of audio play-back and a CD-ROM holds 650 MB of data. CDs have a better sound quality than records or cassettes because CDs contain more information than tapes or records. A CD is able to hold enough information to produce a better dynamic range, which is the variation between the softest and loudest amplitude levels produced on a player. The human ear can hear anything from silence, which is 0 decibels (dB) to about 90 dB. The dynamic range on a cassette is 0 to 48 dB and the dynamic range on a CD is 0 to 96 dB, which means that CDs actually play more sound than most people can hear -- even Junior."

Thanks to this month's Whizard, Morgan Orceyre, an analyst in the Space Science and Engineering Division. Orceyre specializes in multimedia systems and computer programming.

The Lighter Side SwRI Home

March 25, 2013