During playback, a low-powered laser beam, mounted in a movable assembly, reads the digital data through the reflective rear surface of the disk by differentiating between the presence and absence of "pits" beneath the clear, transparent surface of the CD. Changes in reflectivity caused by the pits are translated, via a photo DIODE, into electrical signals that are converted back into an analog signal for reproduction by conventional amplifiers and loudspeakers.
The advantages of the CD over conventional records or tapes include more uniform and accurate frequency response, a complete absence of background noise, wider dynamic range (the difference between the softest and loudest recordable musical sounds), and absence of wear--since nothing mechanical touches the surface of the disc when it is played.
Another application for the CD is called CD-ROM (Compact Disc Read-Only Memory), used for the storage of computer text or programs. Typically, a CD-ROM can store in excess of 500 megabytes (millions of bytes) of data, as compared with the capacity of hard disks for home computers, whose maximum storage capacity is about 30 megabytes.
New uses of CDs include devices that play the discs through a television set. Using a special electronic interface between CD player and TV, the CD-V will play 20 minutes of sound and show 5 minutes of sound-with-picture. Laser videodiscs, with CD-quality sound, can show entire movies on a TV screen. CD-Interactive (CD-I), still in development, will allow the viewer to stop play, request more information, and get it in the form of sound with graphics, a film clip, or a body of text scrolled on the screen.