Audio Compact Disk - An Introduction

EE 498

Professor Kelin J. Kuhn



Two lectures of material

The conventional audio compact disk is a high density media for storing digitally sampled audio. A CD audio disk holds approximately 74 minutes of stereo music recorded with 16-bit resolution -- and incorporates a number of error reduction, detection and correction techniques.


I. The disk itself

A. Size and overall construction

The CD disk is a 120 mm diameter disk of polycarbonate. The center contains a hole 15 mm in diameter. The innermost part of the disk does not hold data. The active data area starts at the 46 mm diameter location and ends at the 117 mm diameter location. The 46-50 mm range is the lead in area and the 116-117 range is the lead out area[1]. Disks are written from the center to the outside (this increases manufacturing yield, and also allows for changes in disk size).

A CD disk contains a long string of pits written helically on the disk. The edges of the pits correspond to binary "1"s.

Each pit is approximately 0.5 microns wide and 0.83 microns to 3.56 microns long. (Remember that the wavelength of green light is approximately 0.5 micron) Each track is separated from the next track by 1.6 microns.

The area between the pits is termed "land". So, a highly magnified section of track might look something like:

Pits are formed in the polycarbonate disk by an injection molding process. As such, they represent some of the smallest mechanically fabricated objects made by humans. The width of a CD pit is approximately the wavelength of green light. The tracks are separated by approximately three times the wavelength of green light. Diffraction from these features (so very close to the wavelength of light) is what gives CD disks their beautiful colors.

A thin layer (50-100 nm) of metal (aluminum, gold or silver) covers the pits. An additional thin layer (10-30 microns) of polymer covers the metal. Finally, a label is silk-screened on the top. Notice that the pits are far closer to the silk screened side of the disk (20 microns) than they are to the read-side of the disk (1.55 mm). Thus, it is easier to permanently damage a disk by scratching the top -- than the bottom!

B. Making the disk

The fabrication of a CD disk is a fascinating process. This process is discussed in some detail in The Compact Disk Handbook, Chapter 7 and only the high points are summarized here[2].

The process begins by making the "glass master". To do this, a glass plate about 300 mm in diameter is lapped flat and polished. The plate is coated with photoresist.

A mastering tape is made containing the information to be written on the disk. A laser then writes the pattern from the master tape into the photoresist.

The photoresist is developed. A layer of metal (typically silver over a nickel flash) is evaporated over the photoresist. The master is then checked for accuracy by playing the disk.

The master is then subject to an electroforming process. In this electrochemical process, additional metal is deposited on the silver layer.

When the metal is thick enough (typically a few mm's) the metal layer is separated from the glass master. This results in a metal negative impression of the disk -- called a father.

The electroplating process is then repeated on the father. This typically generates 3-6 positive metal impressions from the father before the quality of the father degrades unacceptably. These impressions are called "mothers".

The electroplating process is repeated again on the mothers. Each mother typically makes 3-6 negative metal impressions called sons or stampers. The sons are suitable as molds for injection molding.

Polycarbonate is used to injection mold the CD disks.

Once the disks are molded, a metal layer is used to coat the disks. Aluminum, gold, copper and silver are all reflective enough to be optically acceptable. Gold is typically too expensive and copper has a peculiar appearance. Thus, aluminum and silver are the most commonly used metals.

Following metal deposition, a thin plastic layer (1-30 microns) is spin-coated on over the metal. This can be a nitrocellulose layer suitable for air drying, or an acrylic plastic that is cured in UV.

Finally, the logo and other information is silk screened on the top.

C. Reading the pits

The CD disk is actually read from the bottom. Thus, from the viewpoint of the laser beam reading the disk, the "pit" in the CD is actually a "bump".

The polycarbonate itself is part of the optical system for reading the pits. The index of refraction of air is 1.0 while the index of refraction of the polycarbonate is 1.55. Laser light incident on the polycarbonate surface will be refracted at a greater angle into the surface. Thus, the original incident spot of around 800 microns (entering the polycarbonate) will be focused down to about 1.7 microns (at the metal surface). This is a major win, as it minimizes the effects of dust and scratches on the surface.

The laser used for the CD player is typically an AlGaAs laser diode with a wavelength in air of 780 nm. (Near infrared -- your vision cuts out at about 720 nm). The wavelength inside the polycarbonate is a factor of n=1.55 smaller -- or about 500 nm.

The pit/bump is carefully fabricated so that it is a quarter of a wavelength (notice a wavelength INSIDE the polycarbonate) high. The idea here is that light striking the land travels 1/4 + 1/4 = 1/2 of a wavelength further than light striking the top of the pit. The light reflected from the land is then delayed by 1/2 a wavelength -- and so is exactly out of phase with the light reflected from the pit. These two waves will interfere destructively -- so effectively no light has been reflected.

The spacing between pits is equally carefully selected. Recall from basic optics that the image of a beam passing through a round aperture will form a characteristic pattern called an Airy disk. The FWHM (full-width half-maximum) center of the Airy disk pattern is a spot about 1.7 um wide and falls neatly on top of the pit track. The nulls in the Airy pattern are carefully situated to fall on the neighboring pit tracks. This minimizes crosstalk from neighboring pits[3].

D. The optical train -- three beam pick-up

The most common optical train in modern CD players is the three beam pick-up, depicted below[4].

The light is emitted by the laser diode and enters a diffraction grating. The grating converts the light into a central peak plus side peaks. The main central peak and two side peaks are important in the tracking mechanism.

The three beams go through a polarizing beam splitter. This only transmits polarizations parallel to the page. The emerging light (now polarized parallel to the page) is then collimated.

The collimated light goes through a 1/4 wave plate. This converts it into circularly polarized light.

The circularly polarized light is then focused down onto the disk. If the light strikes "land" it is reflected back into the objective lens. (If the light strikes the pit, now a bump, it is not reflected.)

The light then passes through the 1/4 wave plate again. Since it is going the reverse direction, it will be polarized perpendicular to the original beam (in other words, the light polarization is now vertical with respect to the paper).

When the vertically polarized light hits the polarizing beam splitter this time, it will be reflected (not transmitted as before). Thus, it will reflect though the focusing lens and then the cylindrical lens and be imaged on the photodetector array. The cylindrical lens is important in the auto-focusing mechanism.

E. Three beam autofocus

If the objective lens is closer to the compact disk than the focal length of the object lens, then the cylindrical lens creates an elliptical image on the photodetector array.

If the objective lens is further away from the compact disk than the focal length of the object lens, then the cylindrical lens again creates an elliptical image on the photodetector array. However, this elliptical image is perpendicular to first image.

Of course, if the disk is right at the focal length of the objective lens, then the cylindrical lens does not affect the image and it is perfectly circular.

So, if the disk is too far away -- then quadrants D and B will get more light than quadrants A and C. Similarly, if the disk is too close -- then quadrants A and C will get more light than D and B. A simple circuit generates an autofocus signal based upon the output of the photodetector[5].

The output of this correction signal can be used to drive a simple auto-focus servo. A typical example of such a servo is illustrated below[6].

F. Three beam tracking

When the laser beam goes through the diffraction grating, it is split up into a central bright beam plus a number of side beams. The central beam and one beam on each side are used by the CD for the tracking system.

Consider a segment of the CD player containing several tracks.

If the optical head is on track, then the primary beam will be centered on a track (with pits and bumps) and the two secondary beams will be centered on land. The three spots are deliberately offset approximately 20 microns with respect to each other.

Two additional detectors are placed alongside the main quadrant detector in order to pick up these subsidiary beams. If the three beams are on track, then the two subsidiary photodetectors have equal amounts of light and will be quite bright because they are only tracking on land. The central beam will be reduced in brightness because it is tracking on both land and pits.

However, if the optical head is off track, then the center spot gets more light (because there are fewer pits off track) and the side detectors will be misbalanced.


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