CD Player
The CD player has the job of finding and reading the
data stored as bumps on the
CD. Considering how small the bumps are, the
CD player is an exceptionally precise piece of equipment. The drive
consists of three fundamental components:
- A drive motor spins the disc. This drive
motor is precisely controlled to rotate between 200 and 500 rpm
depending on which track is being read.
- A laser and a lens system focus in on
and read the bumps.
- A tracking mechanism moves the laser
assembly so that the laser's beam can follow the spiral track. The
tracking system has to be able to move the laser at micron resolutions.
Inside the CD player, there is a good bit
of computer technology involved in forming the data into understandable
data blocks and sending them either to the DAC (in the case of an audio
CD) or to the computer (in the case of a CD-ROM drive).
The fundamental job of the CD player is to
focus the laser on the track of bumps. The laser beam passes through the
polycarbonate layer, reflects off the
aluminum layer and hits an opto-electronic device that detects changes in
light. The bumps reflect light differently than the "lands" (the rest of
the aluminum layer), and the opto-electronic sensor detects that change in
reflectivity. The electronics in the drive interpret the changes in
reflectivity in order to read the bits that make up the bytes.
The hardest part is keeping the laser beam
centered on the data track. This centering is the job of the tracking
system. The tracking system, as it plays the
CD, has to
continually move the laser outward. As the laser moves outward from the
center of the disc, the bumps move past the laser faster -- this happens
because the linear, or tangential, speed of the bumps is equal to the
radius times the speed at which the disc is revolving (rpm). Therefore, as
the laser moves outward, the spindle motor must slow the speed of the
CD. That way,
the bumps travel past the laser at a constant speed, and the data comes
off the disc at a constant rate.
