How does a CD work?

Laser beam

Like gramophone records, the information on optical discs is recorded on a spiral track. However, with a CD the laser starts reading the disc from the inside ring (table of contents) and ends up on the outside. When play back starts, a laser beam shines on the ridges and lands on the data membrane layer. If you look at the image on the right you can see the data layer moving in gray.

During playback, the number of revolutions of the disc decreases from 500 to 200 rpm (revolutions per minute) to maintain a constant scanning speed. The disc data is converted into electrical pulses (the bit stream) by reflections of the laser beam from a photoelectric cell.

When the laser beam strikes "land", the beam is reflected onto a photoelectric cell. When it strikes a "ridge", the photocell will receive only a weak reflection. Thus the photoelectrical cell receives series of light pulses corresponding to the ridges and lands in the disc. These light pulses are the foundation of binary 'digital' data. A simple substitution for the weak signal "0" and the in-focus signal "1" results in a pure digital playback without alteration, every time, without failure or degradation.
In music playback, a D/A-Converter (digital to analogue converter; DAC) converts the series of pulses (binary coding) from a decimal code to a waveform, which can then be processed for amplification. The longer the decimal code, the better the sound. Current standard CD audio is 44,100 pulses per second and 16 bit (decimal places) in digital word length. Thus a 24 bit system sounds all that much better, in fact DVD audio is set to allow 24 bit AND pulse at 97,000 times per second!

The Compact Disc player mechanism

The laser pickup reads the disc from below.
Thanks to this optical scanning system, there is no friction between the laser beam and the disc. As a result, the discs do not wear, no matter how often they are played. However, they must be treated carefully, as scratches, grease stains and dust might intercept or diffract the light, causing whole series of pulses to be skipped or distorted. This problem can be solved, as during the recording the Cross Interleaved Reed Solomon Code (CIRC) is added, which is an error correction system that automatically inserts any lost or damaged information by making a number of mathematical calculations. Without this error correction system optical disc players would not have existed, as even the slightest vibration of the floor would cause sound and image distortions.

When the laser beam hits land, all of its light is reflected and the cell gives off current. When the laser beam shines on a ridge, half of the light hits the upper surface and the other half hits the lower down service. The difference in height between the two places is exactly a quarter of a wavelength of the laser beam light, so the original beam is totally eliminated by the interference between the beam reflected from the surface of the disc and the beam reflected from the ridge. The photocell does not produce current.
It should be noted that the ends of the ridges seen by the laser are "ones" and all lands and ridges are "zeros"; thus turning on and off the reflection is one, steady state is a string of zeroes. As it is not possible to have two ones next to each other, Eight to Fourteen Modulation (EFM) is used to convert 8-bit data bytes to 14 bit units that always have a minimum of 2 and a maximum of 10 zeros between ones. This makes the pits/ridges and lands separating them 3 to 11 bits long, no less, no more. This conversion is done in hardware using a ROM lookup table. To connect these 14 bit units 3 merge bits are used to make sure that there are no "ones" too close to each other. In audio, the third merge bit is used to make sure that the cumulative lengths of the lands and ridges stay equal in the long run, otherwise a low frequency component is created that the processing amplifiers can not handle. Thus 8 data bits are actually 17 channel bits on the disc, but called 16 bit for naming conventions.

The scanning must be very accurate because the track of ridges is 30 times narrower than a single human hair. You can see the "ridge" in the illustration above -it is the DARK ROUND CIRCLE. When the laser light is over top of it, the light 'splits' in two, causing a weak signal. There are 20,000 tracks on one audio compact disc. The lens which focuses the laser beam on the disc has a depth of field of about 1 micron (micron = micrometer = one-millionth of a meter).
It is quite normal for the (compact) disc to move back and forth 1mm during playback. A flexible regulator keeps the lens at a distance of +/- 2 micron from the rotating disc. For the same reason, a perfect tracking system is required. The complex task of following the track is controlled by an electronic servo system. The servo system ensures the track is followed accurately by measuring the signal output. If the output decreases, the system recognizes this as being "off track" and returns the tracking system to its optimum state.
Many CD players use three-beam scanning for correct tracking. The three beams come from one laser. A polarized prism projects three spots of light on the track. It shines the middle one exactly on the track, and the two other "control" beams generate a signal to correct the laser beam immediately, should it deflect from the middle track.

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The disc

The CD is a plastic disc 1.2mm thick and 12cm in diameter, with a silver-colored surface that reflects laser light. The maximum playing time for music recorded on compact disc is 74 minutes. The CD has several layers. First, to protect the 8 trillion microscopically small pits against dirt and damage, the CD has a plastic protective layer. On the top of this layer the label is printed. Then there is the reflecting aluminum coating, which contains the ridges. Finally, the disc has a transparent carrier through which the actual reading of the disc takes place. This plastic forms a part of the optical system. Mechanically, the CD is less vulnerable than the analogue record, but that does not mean that it must not be treated with care.

The protective layer on the label side is very thin: only 0.002mm. Careless treatment or granular dust can cause small scratches or hair cracks, enabling the air to penetrate the evaporated aluminum coating. This coating then starts oxidizing immediately at that spot. If the CD is played extensively, it may be advisable to protect the label side with a special protective foil, which is commonly available in shops.
A CD must never be bent, so care should be taken when removing it from the jewel case. Even slight bending causes stress fractures. The aluminum then becomes deformed, causing some ridges to be blocked. As a consequence, error correction always has to be applied in that area, affecting the final sound.
The reflecting side of the CD is the side that is read. People tend to set the CD down with the reflecting side up. But the more vulnerable side is not the reflecting side but the label side. On the label side, the reflecting layer with its ridges has been evaporated. The sensitive layer on the reflecting side has been protected better than the one on the label side. It is therefore better to store CDs with the reflecting side down. It is best to store the CD back in the jewel case, where it is safely held by its inside edge.
Never write on the label side, even with a felt-tipped pen. The ink may penetrate the thin protective coating and affect the aluminum layer.

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Scratches

CDs are easily scratched, and should never be cleaned with just any cloth. CDs should be cleaned radially: not along the grooves, but at right angles to the direction of the grooves. If a smear, however small, should remain on the CD, running along the direction of the grooves, much information would be lost. It is advisable to use special CD cleaner that operates with a rotating brush at right angles to the direction of the grooves.
Many people think that the digital CD is produced completely digitally, but this is not always the case. Many CDs have an analogue master tape as their source tapes still kept in the library of the record company, used in the past to make records. The quality of a CD made from analogue tape can be surprisingly high. A CD recorded, processed and dubbed digitally does not always sound better than a CD produced with one or two analogue processing stages.
To indicate what stages have been treated in what ways, a useful three-letter code is used on recordings. The letters represent: the recording, the editing/mixing process, and dubbing, respectively. They are printed on the CD and/or on the insert label in a rectangular box. There are three possibilities: DDD (completely digital CD); ADD (analogue recording, digital processing and dubbing); and AAD (analogue recording and processing, digital dubbing). Many CDs carry the ADD or AAD indication. This does not mean that they are inferior to the DDD CDs!

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