Audio
Audio Compact
Disc (CD) - Introduction
The audio compact disc or CD is a media for storing
digitally sampled audio. A CD audio disc holds approximately 74 minutes of
stereo music recorded with 16-bit resolution -- and incorporates a number of
error reduction, detection and correction techniques.
The compact disc was
launched in 1982. The first recording released on CD was Billy
Joel's 52nd Street. The CD offered the home consumer for
the first time, high quality digital audio in a robust and
convenient format. The main attractions for the CD was the ability
to play your favorite 'record' more than just a few times without
leaving scratches, noise or damage, as the CD was very durable. The
consumer also enjoyed instant access between tracks, the remote
control facility and the small size.
The disadvantages at
launch was that few titles were available and the cost of the CD's
was much higher than that of both vinyl and cassette tape. CD
player prices were also much higher than turntables and cassette
decks back in 1982. But the catalog of music through the 1980s
quickly grew and player costs came down. By 1993 just 11 years after
launch, the CD dominated the market in music sales.
Technical information:
The compact disc is read by a laser which receives a series of
digital pulses from a track of bumps (125 nanometer high) as it
follows a spiral track from the middle of the disc outwards. The
track width is tiny at just 0.5 microns wide, with 1.6 microns
separating the tracks. The data storage on a 74 minute CD is so vast
(650MB) that the spiral track if it could be unwrapped would be
almost 5 miles long. The CD initially rotates in the player at 500
rpm as the laser reads the inner tracks, the speed slowly decreases
to 200 rpm as the outer tracks are read. This keeps the data
rate constant as the laser traverses the disc. The digital sample
rate is 44.1 kHz which is more than twice that of the highest
audible frequency ensuring high resolution digital to analog
conversion. There is no contact between the laser reader and
the disc during playback ensuring no physical wear of the disc. With
careful handling and storage, CDs will last more than a lifetime.
The CD disc is a 120 mm
(12 cm) diameter disc of mostly polycarbonate. The center contains a
hole 15 mm in diameter. The innermost part of the disc 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.
Discs are written from the center to the outside (this increases manufacturing
yield, and also allows for changes in disc size). A CD disc contains a long string of pits written helically on the
disc. 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. As a size comparison, a human hair is 150 times wider at 75 microns in
diameter.
Each track is separated from the next track by 1.6
microns. The area between the pits is termed "land".
Pits are formed in the polycarbonate disc 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 discs 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 disc (20 microns) than they are to the read-side of
the disc (1.55 mm). Thus, it is easier to permanently damage a disc by
scratching the top -- than the bottom!
The fabrication of a CD:
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 glass plate is coated
on top with photoresist.
A mastering tape is made containing the information to be written on the
disc. 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 disc.
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 disc -- 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 discs.
Once the discs are molded, a metal layer is used to coat the
discs. 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.
Reading the pits:
The CD disc is actually read from the bottom. Thus, from the viewpoint of the
laser beam reading the disc, 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 disc. The FWHM (full-width half-maximum)
center of the Airy disc 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.
The optical train -- three beam pick-up The most common optical train in modern CD players is the three beam
pick-up.
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
disc. 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
through the focusing lens and then the cylindrical lens and be imaged on the
photo-detector array. The cylindrical lens is important in the auto-focusing
mechanism.
Three beam autofocus
If the objective lens is closer to the compact disc
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
disc 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 disc 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 disc is too far away -- then quadrants D and B will get more light
than quadrants A and C. Similarly, if the disc 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 photo-detector.
The output of this correction signal can be used to drive a simple auto-focus
servo.
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 photo-detectors 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.
Consumer Recordable CD
Basically CD-R's have an extra
layer made of dye between the plastic bottom protective layer and
the reflective aluminum or gold layer, which can be modified by a
laser to create non-reflective areas or dots on the disc. The laser
writes these dots onto the disc by heating up the dye which causes
it to change and so it no longer allows the light to be reflected.
These areas that do not reflect the light is what both CD and CD-R
machine read to get the information from the CD.
When you write data to a CD-R, the
writing laser (which is much more powerful than the reading laser)
heats up the dye layer and changes its transparency. The change in
the dye creates the equivalent of a non-reflective area. This is a
permanent change and both CD and CD-R drives can read the modified
dye as a bump later on.
This dye is fairly sensitive to
light - it has to be in order for a laser to modify it very quickly.
Therefore you want to avoid exposing CD-R discs to sunlight.
The CD-R is a write-once disc
that cannot be written over, whereas CD-RW (CD - ReWriteable) can be
written over multiple times. CD-RW discs look the same as CD-R's,
but have different structure - three bottom layers, where a CD-R has
one. Basically, the middle layer is heated by a laser to change its
structure, after which it can be erased and written again. Very
similar to the CD-R but it is the structure change that makes the
big difference.
This structure change is actually
achieved by using materials in the recording layer that, when
irradiated by a laser beam, can change phase (be written - like the
CD-R) and back again (be erased - not like the CD-R).
The recorded areas are irradiated
by a high-powered laser beam (higher power than the read laser
beam), this causes the recording material to rise to a temperature
approaching the melting point. It then cools rapidly. Through this,
the molecules are frozen in a random form, causing it to become
amorphous or non-crystalline and not reflect light so well. This
produces a dots which appears the same as ones written on a CD-R. To
erase the dots the temperature is raised again and then cooled
slowly so that the molecules have got time to organize themselves
into crystals and so they reflect the light. This means that the
dots disappears and has been erased. This is how the amorphous phase
is changed to the crystalline phase and enables repeated overwriting
of data 1,000 or more times.
In the recorded area, the recording
material is in an amorphous (non-crystalline) phase, which has a
comparatively low reflectance ratio. Conversely, the erased areas or
non-recorded areas are in a crystalline phase, with a comparatively
high reflectance ratio. Playback is performed by reading the
differences in the reflectance ratio of these two phases while
tracking the groove. |
