CDs and DVDs consist of the same basic materials and layers but are manufactured differently. A DVD is actually like two thin CDs glued together. A CD is read from and written to (by laser) on one side only; a DVD can be read from or written to on one or both sides, depending on how the disc was manufactured. Recordable DVDs (DVD-R, DVD-RW, DVD-RAM) can be manufactured with one recording layer on each side. Prerecorded DVDs (DVD-ROM) can be manufactured with one or two recorded layers on each side.
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The polycarbonate substrate makes up most of the disc, including the area that is read by the laser (opposite the label side on CDs). It is present on both sides of a DVD, even a 'single-sided' disc with a label on one side. This substrate provides the disc depth necessary to maintain laser focus on the metal and data layers. It also gives the disc enough strength to remain flat. Anything in or on the polycarbonate layer that interferes with the ability of the laser to focus on the data layer will result in the misreading of data. Accordingly, fingerprints, smudges, or scratches, as well as such substances as dirt, dust, solvents, and excessive moisture (which polycarbonate will absorb), can interfere with the ability of the laser to read the data. Contact of any foreign material with the polycarbonate substrate layer should be avoided.
As its name implies, the data layer of CDs and DVDs is the layer that contains the data. The data appear as marks or pits that either absorb light from the laser beam, or transmit the light back to the laser photosensor by way of the metal reflective layer. In CDs, the data and metal layers are very close to the top of the disc (label side); in DVDs, they are in the middle of the disc (see Figures 1Ð6). The types of data and metal layers used depend on the type of disc-read-only (ROM), write-once (R), or rewritable (RW, RAM). Table 1 shows the relationship between the data and metal layers and the disc type.
Table 1: Disc type, read/record type, data layer, and metal layer
The dye-based (R discs) and the phase-changing film layers (RW discs) both hold data by allowing or blocking light transfer through the data layer. The laser-affected ('written') areas of the data layer absorb the 'reading' laser beam as it is emitted from the laser to the metal layer and reflected back to the laser photosensor. The light and dark areas give reflectivity effects that are similar to the interference effect of the 'pressed' and molded data in the metal/substrate layer in ROM discs. The reflection, whether the result of dye, film, or pressed effects, is represented digitally as ones and zeros by the firmware in the disc drive as the laser reads the disc.
ROM discs are commercially available or made-to-order prerecorded discs, also called 'replicated' discs. Examples of CD-ROMs include the Audio-CD, Video-CD, CD-i, and CD+G, as well as any number of CDs used in computer applications. Among DVD-ROMs are the DVD-Video, DVD-Audio, and any of various DVDs used in games and computer applications.
The data in CD-ROM or DVD-ROM discs are not actually in a separate layer. A molding machine uses a stamper to impress the pits (depressions) and lands (surface), which form the data, into the polycarbonate substrate surface. Metal is then sputtered or condensed onto the molded substrate to form a 'reflective data layer.' The reflective metal layer in ROM discs can also be considered the data layer because the metal is integrated with the pits and lands in the polycarbonate (see Figures 2 and 3). The metal layer in ROM discs is usually aluminum. For double-sided DVD-ROM discs, the semi-reflective layer is gold, silver alloy, or silicon.
Figures 2, 3: Layers that make up ROM discs
The recordable, write-once optical disc (CD-R, DVD-R, DVD+R) has its data-recording layer sandwiched between the polycarbonate substrate and the metal layer (see Figures 4 and 5). This layer is an organic dye. The dyes used in CDs and DVDs are the same basic types; those used in DVDs, however, are patented by the manufacturer, and the disc color does not indicate the type of dye used. The dyes in both CDs and DVDs are photosensitive. Bits (marks) are written to the dye by a chemical change caused by the laser light beam. This dye degrades over time, eventually making the disc unreadable.
Figures 4, 5: Layers that make up R discs
The data layer in CD-R discs consists of one of three basic dye types, each yielding a different disc color depending on the type of dye and the type of reflective metal used in the disc. Even on a plain, unlabelled disc, the label side can be a different color from the reading side. If the label side of a recordable disc does not have a printable surface, a label attached, or some other protective layer, it will have the color of the metal used (silver or gold). As for the laser reading side, the color will be as indicated in Table 2.
Table 2: Dye type and color appearance'CD-R discs (recordable discs)
The data-recording layer of the rewritable optical disc (CD-RW, DVD-RW, DVD+RW, DVD-RAM) also lies between the polycarbonate substrate and the metal reflective layer (see Figures 6 and 7). This is a phase-changing metal alloy film. A laser beam writes bits (marks) to the film by heating the film above the melting temperature in the areas selected for bits. The rapid cooling enabled by the dielectric layers on both sides of the phase-changing film causes these bit or mark areas to remain in the amorphous state caused by melting. By heating the phase-changing film to a specific temperature above the crystalline temperature but below the melting temperature, the film can revert back to the crystalline state, thereby erasing previous bits. The writing and erasing processes can be done together in a single pass when rewriting a disc.
Figures 6, 7: Layers that make up RW and RAM discs
The metal layer in optical discs reflects the laser beam back to the laser photosensor in the laser head. Three types of reflective metals are typically used for this layer: aluminum, gold, and silver or silver alloy. In 'double-layer' DVDs, silicon is sometimes used as one of the semireflective layers.
RW, ROM, and RAM discs (CD-RW, CD-ROM, DVD-RW, DVD+RW, DVD-ROM, DVD-RAM) use aluminum for the reflective layer, mainly because it is inexpensive and easy to apply. Aluminum oxidizes on exposure to oxygen from the environment or to moisture that has penetrated the disc. In some earlier CDs, poor sealing allowed oxygen to come into contact with the aluminum metal layer, causing the aluminum to oxidize. Oxidation of the aluminum diminishes its reflectivity, making the disc unreadable by the laser, and is sometimes referred to as disc 'rot.' It is the primary cause of ROM disc degradation from environmental influences. Not so, however, for RW and RAM disc degradation; the phase-changing film in these discs normally degrades at a faster rate than the aluminum in the disc oxidizes.
In R discs (CD-R, DVD-R, DVD+R), gold, silver, or a silver alloy is used for the reflective layer. Silver is slightly more reflective than gold but can lose reflectivity with corrosion on exposure to adverse environmental conditions. Silver corrodes through reaction with sulfur dioxide, an environmental pollutant that can migrate through the disc with moisture. Gold is noncorrosive, very stable, and longer lasting, but it is also expensive. Either metal should outlast the dye. Aluminum is not used with these discs because it can react with the dye in the recording (data) layer.
DVD-ROMs can be manufactured with two reflective metal layers that allow the laser to read data from both layers using one side of the disc. These 'double layered' DVDs provide up to four times the capacity for content (video, audio, computer applications) as do 'single-layered' DVDs. The laser beam must pass through a semi-reflective metal layer to read data from a fully reflective layer. The outer metal layer (silicon, gold, or silver alloy) is semi-reflective; that is, it reflects back some of the laser beam and allows some of it to pass through to a fully reflective layer (aluminum) and then reflect back. Both parts are thus reflected to, and detected by, the photosensor in the laser head, which focuses on one layer at a time. Figure 8 shows the most common construction for single-sided, double-layered DVDs, while Figure 9 shows an alternative construction.
Figures 8, 9: Two types of double-layer, single-sided DVD-ROM construction
The difference between these two types is that one (Figure 8) has the metal data layers on separate sides (halves) of the disc. In addition to passing through a semireflective metal layer, the laser beam must also pass through a special adhesive that binds the two disc halves together and does not hinder the laser beam. In the example shown as Figure 9, the two metal data layers are on the same half of the disc.
DVD-ROMs can also be double-sided. Figure 10 shows a double-sided DVD that may typically be a DVD-Video providing the video in a full-screen TV version on one side of the disc and a wide-screen version on the other side. When double-sided DVDs also have double layers on both sides, they may have almost quadruple the capacity of single-sided/single-layered DVDs (Figure 11).
Figures 10, 11: Two types of double-sided DVD-ROM construction
A very thin lacquer layer is applied to the label side of CDs to protect the metal from exposure to the environment. (DVDs have no such protective lacquer coating.) That layer also gives some limited protection from writing on or labeling the disc. However, the CD is more sensitive to damage on this side than on the polycarbonate side. Since the metal layer is so close to the surface of the label side, pointed objects can easily damage the CD by deforming the metal or exposing it to the environment. Some solvents can also affect lacquer coatings and expose or react with the metal. Once the metal is damaged, the laser cannot read data in the damaged areas.
Sometimes a manufacturer will add an additional layer designed specifically to provide more resistance to fingerprints and scratches on the label side of CDs. One particularly effective modification has been the application of lacquer completely around the edges of the disc. In earlier CDs, moisture had been allowed to penetrate to the metal through unprotected areas of the disc edge. Clearly, it is as important to protect the edges of CDs as it is their surfaces.
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An optional layer may also be added to a CD or DVD to provide a labeling surface (see 'Disk Surface Printing,' page 24). Such surfaces are of four types:
These layers are applied over the lacquer layer on CDs or over the polycarbonate substrate on a single-sided DVD. Some discs have an extra coating on which text or logos are printed. In many cases, the lettering appears to be stenciled, but it is not part of the coating; what one sees is the reflected surface of the metal rather than imprinted text or logos. Typically, one can see through this lettered area-and even through the metal-by holding the disc up to light.
Because these lettered areas are particularly susceptible to damage, it is most important to avoid writing on or scratching in these areas. The only disc surface area that is completely safe from writing or scratching is the clear inner hub or the 'mirror band,' since no data are recorded in these areas.
Hello all,
Once again I am seeking your help and advice in the craft of
metalwork. I have just been given two disk cutters, one cutting small
disks the other large. I have been making enamel beads and cutting the
disks with a saw and then doming them. However, when it comes to the
disk cutters I plead ignorance and would rather not use them until I
have had some type of instruction (I guess this is what happens when
one drifts into a craft instead of being formally trained). Does one
use a hammer or vice to drive the rod? Yup, I'm that ignorant :-{
I work with copper and fine and sterling silver, but mostly copper.
So if anyone out there can help I would appreciate it once again.
David in Victoria
PS the list of demonstrations offered by the Creative Jewellers Guild
of British Columbia seems to have something for everybody.
Note From Ganoksin Staff:
Looking for a disc cutter for your jewelry projects? We recommend:
- Reading our guide to the Best Disc Cutters Review for jewelry work here
- Searching for more discussions about Jeweler's Torches here on the Orchid forum
David,
This is a good question which I will answer from experience. I too
had the problem of cutting a circle with a disc cutter. I took a deep
breath and thought, 'ok, this way'. I chose 'poorly'.
If you notice on your disc cutter there is a straight, slightly sharp
edge and a beveled edge. The straight edge is the cutting part and
the beveled edge is the one you can hammer against. Use your
utilitarian hammer, not the nice polished ones that my students always
try to grab.
A hammer works fine, follow that with a wood dowel and out pops your
cutter.
-k
G'day; Thought about making your own disc cutter? First
buy/steal/bludge some drill rod (we call it silver steel - it is high
carbon steel) of the diameter(s) you need. Broken drills work
perfectly. Square off the end nicely Get a piece of steel about an
inch (2.5 cms) thick; a piece of inch rod or even 3/4 inch is OK.
Now drill a hole right through, about 1/8" (3 mm) from am edge, of
the same diameter as the drill rod so the rod and the bit of steel
make a slide fit. Now use a hacksaw to cut a slot at the side of
the 1" rod, just past the hole you cut for the drill rod. Heat the
(only) the very end of the drill rod to cherry red and quench it in
water, which makes that end dead hard. And there you are. Simply
insert the piece of sheet metal into the slot, making sure you have it
resting against the far depth, insert the drill rod, hardened end
into the hole, give it a good bash with a heavy hammer - and there
you go. How to get the disc out? Well, if you had used a slightly
bigger drill to open one end of the hole, the disc will fall out
easily. Want the disc domed a bit? Simply grind the drill rod to the
sort of dome you want, go through the papers (grits) until you have a
semi polish, then polish it on your buff using tripoli. You may have
to use a bit harder bash to cut the disc. (Yes, it does work - I often
do it) Now. If you were to make a pattern on the business end of your
rod (flower, etc) using burrs in a flexishaft before you do the
hardening, and you stamped out the silver/gold/copper disc a bit
harder over an anvil - why, you'd get patterned discs! And you can
use those for all sorts of things. Drill a hole in the centre of one
and you have a pretty rivet washer' Solder tiny jump rings
opposite each other - you have the beginnings of a necklace' You
think of the rest - I'm tired! Cheers, ' John Burgess;
@John_Burgess2 of Mapua Nelson NZ
I bought my circle cutter years ago from Allcraft. The directions
suggested that 'gentle tapping produces the best results.' I found
that 'gentle tapping' produced NO results. In my set-up, I work on
top of an old stump, so that the metal circle and the sharp-edged
cutter come out onto wood, not something that will scar either one of
them. I used a vibrating tool to mark the top of each of the cutters
so that I wouldn't accidentally put them in upside down and then
hammer on the cutting edge! I use an old claw hammer and really slug
away at the cutters, once the sheet metal is in place. I store the
cutters in a plastic bag, so they don't get dust or grit on them,
which could affect the ease with which the cutters slide in their
respective holes. When the circle is cut, and if the cutter doesn't
slide out easily, I move the whole thing over to the edge of the
stump, place something large and padded on the floor beneath the
hole, and hammer out the cutter, using (as someone has already
suggested) an old wooden dowel. I don't know if my cutter wasn't
that great to start with, but the circles I produce often have a
flange on the edge, which has to be cut off, and then the edges need
abrading. It's too time-consuming. Nowadays, I rarely use my circle
cutter but instead buy metal circles already cut, from places like
Indian Jewelers Supply, Metalliferous, etc. HTH, Judy Bjorkman
@JLBjorkman
I'd just add a couple of small tips to what others have said. I also
place my cutter on a thick piece of wood, but I place a thin sheet of
soft copper (brass OK too) between the cutter and the wood. This is so
when I get really overzealous about whacking those punches, I don't
drive the punched disc straight into the wood (where it then needs to
be pried out). (Periodically I anneal the copper and hammer it flat,
because it tends to get deformed and hardened. Don't worry, the soft
copper won't mar the metal you're punching.) I use a 2 lb hammer (if
you're metric, I suppose your hammer would weight about a kilogram) -
sort of looks like a small sledge hammer with a short handle. The
large hammer head also means I don't have to be so fussy about a
precise aim. I place the cutter and wood block where I can really come
down on it with the hammer - usually on a cement floor. I hold the
hammer with both hands and give the punch one good strong blow. No
problem with 'ghost cuts' this way. Rene Roberts
Thank you Orchidians for helping me become just a wee bit more
knowledgeable about the ways of working with metals.
In general you all mentioned that one should strike the cutter very
hard with a hammer. Well, being a timid soul and not very strong at
the best of times, I applied a simple physical law F=MA; force equals
mass (the hammer) times Acceleration and hit the cutter with a four
pound mallet (used for splitting wood) with a moderate, and controlled
force. I also sat the whole stuff on a tree stump which worked very
well as a support. I used annealed 20 gauge copper and had to hit the
cutter twice when using the one inch disc cutter, and once with
cutters of smaller diameters. I was lucky enough not to get the ghost
marks Karen talks about.
Finally it was very easy with my disc cutter to tell which end was
which; the end receiving the blow was bevelled and just slight larger
that the cutting steel.
Thank you all again, for giving me the confidence to get with it.
David in Victoria.
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