Digital Data on a Compact Disc

Binary data (0's and 1's) are encoded on a compact disc in the form of pits in the plastic substrate which are then coated with an aluminum film to make them reflective.

The data is detected by a laser beam which tracks the concentric circular lines of pits. The pits are 0.8 to 3 micrometers long and the rows are separated by 1.6 micrometers.

Compact Disc Drive Details

In a compact disc player, a laser beam must track a spiral row of pits which are 0.5 micrometers wide with track spacing 1.6 micrometers. Tracking is aided by a three-beam laser arrangement. In addition to staying on the track, which is much narrower than the 100 micrometer groove separation on a vinyl record, the rotation speed must be adjusted as the beam tracks inward or outward. A linear speed of 1.25 m/s is maintained by increasing the rotation speed from 3.5 to 8 revolutions per second as the beam tracks inward toward the center of the disc.

Laser Tracking on the CD

The laser beam in the compact disc player must precisely track a row of pits which encode the binary data on the disc. In the "three-beam" system, a grating is used to produce the first order diffraction maximum to each side of the main beam. Those diffracted beams overlap the track, and the reflected light from the two side beams should be equal, on the average, if the main beam is centered on the track. If they are unequal, then their difference can be used to generate an error voltage to correct the tracking. The illustration of the side beam positions is not to scale; they deviate about 20 micrometers from the main beam. 激光在紧凑盘播放器中必须精确跟踪一排坑，这些坑编码着光盘上的二进制数据。在“三束光”系统中，利用光栅产生主光束两侧的第一级衍射极大值。这些衍射光束会覆盖轨道，如果主光束居中于轨道上，那么来自两侧光束的反射光应大致相等。如果它们不相等，其差异可以用来生成误差电压以校正跟踪。图中所示的侧光束位置并非按比例绘制；它们偏离主光束约20微米。

Scaled Views of a Compact Disc

Data on a compact disc is stored in the form of pits in the plastic substrate. 紧凑盘上的数据以塑料基材上的凹坑形式存储。

A reflective layer of aluminium is applied to reflect the laser beam. A protective coating is then applied to the top. The laser system reads the data from below.

Detection of CD Pits

The tracking laser beam sees the pits as raised areas which are about a quarter-wavelength high for the laser light. 跟踪激光束看到坑作为隆起区域，这些区域的高度约为激光光的四分之一波长。

The reflected light from the pit is then 180° out of phase with the reflection from the flat area, so the reflected light intensity drops as the beam moves over a pit. The threshold of the photodiode detector can be adjusted to switch on this light level change.