16. Passage 4
Compact discs (CDs) , which may he found in over 25 million American homes, not
to mention backpacks and automobiles, first entered popular culture in the 1980s. But
their history goes back to the 1960s, when an inventor named James Russel decided to
create an alternative to his scratched and warped phonograph records— a system that
(5) could record, store, and replay music without ever wearing out.
The result was the compact disc (CD). Made from 1.2 mm of polycarbonate
plastic, the disc is coated with a much thinner aluminum layer that is then protected
with a film of lacquer. The lacquer layer can be printed with a label. CDs are typically
120 mm in diameter, and can store about 74 minutes of music. There are also discs that
(10) can store 80, 90, 99 and 100 minutes of music, but they are not as compatible with
various stereos and computers as the 74-minute size.
The information on a standard CD is contained on the polycarbonate layer, as a
single spiral track of pits, starting at the inside of the disk and circling its way to the
outside. This information is read by shining light from a 780 nm wavelength
(15) semiconductor laser through the bottom of the polycarbonate layer. The light from the
laser follows the spiral track of pits, and is then reflected off either the pit or the
aluminum layer. Because the CD is read through the bottom of the disc, each pit looks
like a bump to the laser.
Information is read as the laser moves over the bumps (where no light will be
(20) reflected) and the areas that have no bumps, also known as land (where the laser light
will be reflected off the aluminum). The changes in reflectivity are interpreted by a
part of the compact disc player known as the detector. It is the job of the detector to
convert the information collected by the laser into the music that was originally
recorded onto the disc. This invention brought 22 patents to James Russell, who today
says he is working on an even better system for recording and playing back music.
According to the passage, why did James Russell invent the CD?
The answer may be found in lines 4 and 5, which state that Russell wanted an alternative to his scratched and war ped phonograph records. You may infer that the problem with such records was their poor sound quality.