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*note: the CDROM glossary has complete details on the various BOOK STANDARDS used in mastering.
Like gramophone records, the information on optical discs is recorded on a spiral track. The laser starts 'reading' the disc from the inside and ends at the outside. When played back, a laser beam shines on the ridges and lands. When the beam strikes a 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. A D/A-converter (digital to analogue converter: DAC) converts the series of pulses back to binary coding, and then to decimal values. Now the original analogue signal can be rebuilt. Cross Interleaved Reed Solomon Code (CIRC)
Figure 2 (left): 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,
however often they are played. However, they must to 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. Scanning the disc (part II) Bit streams are pressed into the silver colored disc in the form of a spiral-shaped track of pits. These pits are scanned from the reverse side of the disc (this makes them to appear ridges to the laser) by a microscopically fine laser beam during playback. The scanning begins at the inside of the back of the disc, and proceeds outwards. 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 hits a land, all of its light is reflected and the cell gives off current. When the laser beam shines on a ridge, only half of the light hits the surface. The other half goes into the high part of the ridge. 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. The disc data is converted into electrical pulses (the bit stream) by reflections of the laser beam off of a photoelectric cell. 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. 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. Glass Mastering - getting your data onto our master stamper CD Glass Mastering comprises a number of stages needed to create a metallized glass master from which the stampers are produced. The processes are carried out in a class 1,000 clean room. Operators wear special clothing including facemasks and footwear to minimize any particles.
 The active surface
(called the "father" -
containing pits) of the developed glass
master is then metallized either with
silver by evaporation or a nickel or
nickel alloy created by sputtering. The
glass master is then played on a disc
master player (DMP) to check for any
errors. Audio masters are actually
listened to at this stage. The final stage is then making the reverse image stamper or "mother". The mother is then form pressed onto the extruded "children" membranes which ultimately contains all the binary information used to play the disc.
There are 20,000
tracks like this one on one compact disc. The scanning must be very accurate because the track of ridges is 30 times narrower than a single human hair. There are 20,000 tracks like the one shown to the left on one compact disc. The lens which focuses the laser beam on the disc has a depth of field of about 1 µm (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 µm 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. The disc
The CD has several layers. Notice how the ridges contain binary information. 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.
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! -Courtesy
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