Compact Disc Audio

Analog sound data is digitized by sampling at 44.1 kHz and coding as binary numbers in the pits on the compact disc. As the focused laser beam sweeps over the pits, it reproduces the binary numbers in the detection circuitry. The same function as the "pits" can be accomplished by magnetooptical recording. The digital signal is then reconverted to analog form by a D/A converter.

The tracks on a compact disc are nominally spaced by 1.6 micrometers, close enough that they are able to separate reflected light into it's component colors like a diffraction grating.

Laser for Compact Discs

The detection of the binary data stored in the form of pits on the compact disc is done with the use of a semiconductor laser. The laser is focused to a diameter of about 0.8 mm at the bottom of the disc, but is further focused to about 1.7 micrometers as it passes through the clear plastic substrate to strike the reflective layer.

The Philips CQL10 laser has a wavelength of 790 nm in air. The depth of the pits is about a quarter of the wavelength of this laser in the substrate material.

Polarizing Prism

A polarizing prism is made up of two prisms of a birefringent material joined along a diagonal. The angle of cut is such that the plane of polarization parallel to the surface undergoes total internal reflection whereas the plane perpendicular to the surface passes through. Because of the action of the quarter-wave plate, the beam returning from the disc will be polarized parallel to the surface and will be reflected 90°, toward the photodiode detector.

Photodiode Detection

Laser light from the reflective layer of the disc returns through the quarter-wave plate. This causes it to reflect in the beam-splitter so that it reaches the photodiode for detection. However, if the beam strikes one of the pits, which are about a quarter- wavelength in depth, the light is out of phase with the light reflecting from the unaltered plane around it and tends to cancel it. This produces enough change in light level to be detected by the photodiode, and to be coded as the 0's and 1's of binary data.

Laser Beam Positioning

In order to be reliably decoded, the laser beam must be focused within about 0.5 micrometers of the reflective surface, but the location of the bottom of the disc may be uncertain by about 0.5 mm during rotation. To keep the beam focused, a positioning coil drives the focusing lens up or down in response to an error voltage from the detector. One scheme uses a cylindrical lens arrangement to focus light on the detector. When the beam is properly focused, it projects a round beam and a zero error voltage results.