A Crisp, Clean Sound From Lasers : CDs Make Music With Light, Sensors in Place of Needles

Today’s compact disc players--and the advanced CD recorder-players promised by Tandy on Thursday--owe their existence to low-power lasers and sophisticated optical sensors and computer chips.

By using a laser beam to pick up encoded information stored on a disc’s groove, CD equipment eliminates one of the biggest drawbacks of conventional phonograph technology: the need to drag a stylus through a record’s groove, subtly damaging the recording with every playing.

As a result, CD technology--developed jointly by N.V. Philips of Holland and Sony of Japan--delivers crisp, clean sound, without the wear that characterizes the technology pioneered before the turn of the century by Thomas Edison.

The disc’s vast storage capacity--a single 4 3/4-inch diameter CD can hold 550 megabytes, or 275,000 pages of information--as well as its ability to combine sound, text and video has also made it a favorite of visionaries in the computer industry. So far, however, the use of the technology has been limited by the inability to repeatedly erase and re-record material on CDs, a hurdle that Tandy now claims to have overcome.

Creates Pits

Here’s how the technology works in the case of audio CDs, the most widespread current application. Music is electronically transformed into digital information--a long series of zeroes and ones that makes up the “bits” that a computer can read.

The CD, made of molded plastic, is impressed with a fine spiral of tiny pits, which in turn represent the digits. A reflective aluminum film is then applied.

When the player’s laser beam is shined on the disc, the light that bounces back contains the data on the disc. This beam is then read by an optical sensor, which takes the string of zeroes and ones and sends it to a microprocessor--a computer “brain”--to be converted to sound.

What Tandy has done is to find a way to make an inexpensive machine to create pits containing digital data on blank discs. “To the player, it looks exactly like a conventional compact disc,” according to Mike Grubbs, director of Thor CD Technology at Tandy.

However, unlike conventional CDs, discs made with the Tandy High-Intensity Optical Recorders can be erased and re-recorded like audio cassettes, videocassettes or computer floppy disks.