DVD Discs Explained
There are a number of different types of DVD video discs, and DVD data discs - the disc type, which has no bearing whether the disc contains video or data . . . is differentiated by a number (DVD-5, DVD-9, etc.). The most common disc type is DVD-5, which stores the data on a single side of the disc, and uses only one "layer". In general, most video (movie) DVD's offer the following:
Categories of DVD Discs
DVD movies (stamped discs with video) - these discs are made
DVD-ROM (stamped discs with data)
DVD-recordable (burned discs with either video or data) - If they have movies on them they will have two folders, DVD_AUDIO and DVD_VIDEO where DVD_AUDIO is empty and DVD_VIDEO has all the movie files in it. Movie DVD discs ca be either stamped (from the movie companies) or recordable (burned by users on their PC's). Data DVD discs can be either DVD-ROM (made by the software vendor) or recordable (files have been burned onto the
The following DVD discs can be formatted for movies or computer files.
DVD-1, 2, 3, and 4 - the standard exists but these are not used
DVD-5 4.7GB, single sided, single layer (just over 2 hours - approx 133 minutes)
DVD-9 8.5GB, single sided, dual layers (gold color on data side – just over 4 hours)
DVD-10 9.4GB, double-sided, single layer (each side stores 4.7GB – approx 4.5 hours)
DVD-18 17 GB. Double-sided, dual layers (each side stores 8.5 GB – over 8 hours)
NOTE: the play times lists are just averages – it can vary greatly depending on how much compression is used, how much space is devoted to menus and extras, which Audio format is used, etc.
StampingThis is the process used to create commercially available DVD's - not recordables. Just like CD's - DVDs are 5.25" in diameter, and are about 1.2 mm thick. Just like CD's they use one long spiral of bumps (the stamping process embeds "pits" in the substrate from below, so from the top of the surface, they are bumps). The manufacturing process for stamped discs goes something like this (using the single-sided, dual layer process as an example):
NOTE: when describing discs, there is always confusion about top vs bottom. For these pages we use the same orientation of a disc placed into the player . . . where the label is on top, and the laser shines the beam onto the disc surface from below.
Stamper Discs - Master Discs (metal)
The manufacturers realize how critical quality control is. Therefore they create a hierarchy of discs. The final discs used for stamping DVD's - the sons - used in the production line can have problems, or simply go bad over time. Therefore several mother discs are created and locked away in a vault. If a mother discs goes bad - then it is recreated from the father disc.
Stamped Discs (plastic)
There are two methods, the first being more common, so we will detail only the first method
Method 1 - created by Matsushita - each layer is stamped
Method 2 - created by 3M - one layer is stamped twice - once directly onto the plastic, then a resin coating is applied, and the second stamping is applied to it
For single-sided discs, the label is silk-screened onto the nonreadable side. Double-sided discs are printed only on the non-readable area near the hole in the middle.
The following images show cross sections of DVD's and a magnification of a tiny portion of the spiral data track showing two side-by-side portions of the track. Keep in mind that the actual spiral is microscopic, and stretched out would be over 7 miles long !!!
BCA (Burst Cutting Area)
This is a thin ring near the inner hub, that contains a series of barcode that is visible to the eye, but can also be read by the laser. The stripes contain up to 188 bytes of data, and are used to identify the manufacturer, supply an encryption key for copy protection. The DVD forum created a standard for the BCA data:
Bytes 1 and 2
length of data stored in the BCA
free-form data from manufacturer
Their basic makeup is very similar to stamped discs, and the physical dimensions are the same. They use a polycarbonate (plastic) substrate to form the disc. In addition, they then have a metallic substance sprayed into the plastic surface to supply reflectivity, and a laquer is applied on top of the metal. However, the physical pits/bumps do not exist since there is no stamping. Instead, the final step in manufacture is to add either an organic dye (DVD-R, +R) or a metallic phase-change material (DVD-RW, +RW) on top of the laquer. This final surface changes reflectivity characteristics when a laser beam is applied to it. Therefore the disc can be written to !!
NOTE: Recordable discs have an extremely complex lead-in area on the outer portion which defines many parameters required for proper reading. Due to the complexity it will not be discussed here.
DVD-R comes in two flavors - version 1.0 and version 2.0 (version 1.0 is the most compatible format with older players). These discs will have one of several colors of organic dye (gold/green/blue) applied - it is a polymer (typically cyanine or azo) that is applied on top of the polycarbonate substrate. This is where the data is stored. The image below show the disc reversed (normally the label is on top).
Wobbled Groove - the top layer has a physical groove with wobbles stamped into it during the manufacturing process. The groove defines the track, and the wobbles are used to aid with synchronization. The frequency of the wobbles is 8 times that of a synch frame. The dye is applied to the surface, and melds into the groove. The sectors and addressing information are also applied to the groove - so DVD-R discs come pre-formatted.
The blank disc dye is semi-transparent, and when read, the laser uses a low-power setting, passes through and is reflected back by the underlying metallic layer.
During the burning process, the laser is set to high-power (6-12 mW) and uses a series of pulses for each burned bit. Pulses are used so that the dye does not overheat. The heating of the dye causes a chemical change that makes that area dark - and it absorbs light - which means no light hits the metallic layer, and no light is reflected. The metal layer is underneath the polymer dye. Its function is to provide something for the reading laser to bounce off of, and be picked up by the optical sensors in the reader assembly. It does not itself hold data.
Once the organic polymer dye is burned - it's burned forever !!! No more changes.
Same as the -R and +R discs, except instead of a polymer dye, a special "phase-change" material is applied to the substrate. There are two different power levels used for writing and erasing data on a phase-change disc. The highest power level causes the surface of the material to become less reflective, and a lower power level causes the material to become more reflective. Therefore there is no need for a metallic layer underneath - the phase-change layer itself is metallic.
Phase-change material is also used for CD-RW discs. When the alloy is heated with a low-power laser, it melts and cools into a crystalline form, causing high reflectivity. When heated by a high-power beam, it melts and cools into a smooth metal, causing low reflectivity. The beam writes marks with high-power pulses and erases between the marks with low-power pulses.
These discs also come pre-formatted with a wobbled groove stamped into the surface.
Once the phase-change material is burned - it merely changes electrical "phase", but similar to the metallic particles in tapes - this metal can be changed over and over again.
t would seem that the physical nature of stamping pits onto the surface of a disc would result in a much more rugged disc, than burning a thin layer of dye or metallic spray . . . but you don't have to worry about that - the life expectancy of DRD recordable discs is 100 years !!!
DVD discs used by DVD Burners
So far, computer DVD burners only support DVD-5. Nevertheless, even DVD-5, with just one side and a single layer holds 7 times the amount of data as a CD does !! The vast majority of movies come in either DVD-5 (Single Layer) or DVD-9 (Dual Layer).
Playing 2-sided Discs
DVD players were not made for 2-sided discs, sometimes called “flippers” – they can only read one side (the top side). Therefore you have to turn the disc over to play the other side !!
Reduction of Capacity with Dual-Layer Discs
The capacity of a dual-layer disc is slightly less than double that of a single-layer disc. The laser has to read "through" the outer layer to the inner layer (a distance of 20 to 70 microns). To reduce inter-layer crosstalk, the minimum pit length of both layers is increased from 0.4 um to 0.44 um.
Although the pit lengths are longer, the read data rate is the same because for dual layer discs – the scan speed increases from 3.49 m/s (single-layer) to 3.84 m/s (dual-layer).
Obviously, having two layers on one side of a disc result in read difficulties. So longer pits are used, spaced farther apart, and they are easier to read correctly and are less susceptible to jitter. But the increased length means fewer pits per revolution, which results in reduced capacity per layer.
DVD-9 has two “layers” on the same side – hard to grasp how this is possible, but there are two, and they each have different reflectivity and characteristics so that the laser can tell them apart.
You would assume that this would double the capacity to 9.4 GB – however, DVD-9 actually only holds 7.95 GB or 8.54 billion bytes, according to the DVD FAQ. DVD-10 holds 8.75 GB or 9.40 billion bytes, so it's got 10% higher capacity than a DVD-9. I believe the second layer on a dual-layer disc has to be written with slightly longer pits to ensure readability - see section 3.3 of the DVD FAQ: (http://www.dvddemystified.com/dvdfaq.html#3.3
These double-sided discs are typically used with short movies. They store a wide-screen version on one side, and a letterbox pan-and-scan version on the other.
DVD-10 holds about 10% more capacity than DVD-9, because
NOTE: they can also be used to store a long movie by splitting it across both sides. However, DVD-9 is a much better choice for long movies because you do not have to be interrupted in the middle of the movie
These super-size discs are used for data but are not commonly used for movies. The first commercial DVD-18 title, The Stand, was released in October 1999. A DVD-18 requires a completely different way of creating two layers. A single-sided, dual-layer disc (DVD-9) is produced by putting one data layer on each substrate and gluing the halves together with transparent adhesive so that the pickup laser can read both layers from one side. But in order to get four layers, each substrate needs to hold two. This requires stamping a second data layer on top of the first, a much more complicated prospect. Only a few replicators can make DVD-18s, and the low yield (number of usable discs in a batch) makes it more difficult and expensive than making DVD-9s.
Why two DVD-9 Discs are preferred over one DVD-18 (so far)
For video over 4 hours (miniseries, boxed stes, etc), you will need more than the 8.5 GB, 4-hour playback that a single DVD-9 disc offers. So it would seem that DVD-18 would be perfect for this, since it can store the entire movie on one disc. BUT, although it holds the same amount of video as two DVD-9’s, you still have to get up in the middle of the movie to turn it over !! Remember, DVD players can only read one side !! Many prefer using two DVD-9 discs for long movies, for the following reasons:
DVD-18 is expensive
DVD-18 only has a tiny inner rim to identify each side. The text on the rim is very tiny and hard to read. DVD-10’s also have this problem.
DVD-9’s have one blank side, which allows nice DVD labels to be attached
This will change soon, as DVD-18 media prices come down, and dual-sided players become the norm !!!