April 10, 2000

List of Tables and Figures ……………………………………………………………… 3

An Introduction to Digital Versatile Discs (DVDs) ……………………………...…….. 4

How a DVD Works ……………………………………………………………………. 5

DVD Disc Formats …………………………………………………………….. 5

DVD File System ………………………………………………………………. 8

Copy Protection for DVD ……………………………………………………… 9

Polycarbonate (PC) for DVDs ……………………………………………………….. 10

Polycarbonate Chemistry/Preparation ………………………………………………... 11

Production of DVD Discs from Polycarbonate ………………………………………. 12

Glass Mastering ………………………………………………………………. 13

Bonding ………………………………………………………………………. 14

DVD Finishing ……………………………………………………………….. 16

Quality Assurance ……………………………………………………………. 17

Alternative Polymers for DVD Production …………………………………………… 17

Conclusion ……………………………………………………………………………. 19

References …………………………………………………………………………….. 20

Appendices ……………………………………………………………………………. 21

Appendix A – Polycarbonate MSDS

Appendix B – Datasheet for Lexan® from GE Plastics

Appendix C – Datasheet for Calibre® 1080 DVD from Dow

Figure 1 – Exemplary DVD ……………………………………………………………. 4

Figure 2 – Pit Density of DVD Versus CD …………………………………………….. 5

Table 1 – DVD Formats ………………………………………………………………... 6

Figure 3 – Schematic of DVD-5 ……………………………………………………….. 6

Figure 4 – Schematic of DVD-9 ……………………………………………………….. 7

Figure 5 – Schematic of DVD-10 ……………………………………………………… 7

Figure 6 – Schematic of DVD-18 ……………………………………………………… 7

Figure 7 – Schematic of Root Directory (DVD File System) ………………………….. 8

Table 2 – Properties of Polycarbonate ………………………………………………... 10

Figure 8 – Synthesis of Bisphenol-A Polycarbonate ………………………………… 11

Figure 9 – General Structure of Polycarbonates from Tetrasubstituted Bisphenols ….. 11

Figure 10 – Flow Diagram of DVD-5 Replication ………………………………...…. 12

Figure 11 – Flow Diagram of DVD-9 Replication …………………………………… 13

Figure 12 – Flow Diagram of Glass Mastering Process ……………………………… 13

Table 3 – Label Printing for DVDs …………………………………………………… 16

Figure 13 – Printable Areas for Different DVD discs …………………………...…… 17

DVDs are used as a storage media for digital and digitized information. Presently, they are available in six formats: DVD-5 (5 Gbyte, prerecorded); DVD-9 (9 Gbyte, prerecorded; DVD-10 (10 Gbyte, prerecorded); DVD-18 (18 Gbyte prerecorded); DVD-R (3.8 Gbyte, recordable); and DVD-RAM (2.6 Gbyte read/write). Although, they are similar in appearance to better known compact discs or "CDs," DVDs are mastered in a different manner than ordinary CDs. Further, while CDs are made from a single substrate that is approximately 1.2 mm thick, DVDs are made from two component discs or "halves" made from optical grade polycarbonate. Each half is approximately 0.6 mm thick and 120 mm in diameter and the two halves are bonded together to form a DVD. An exemplary DVD is shown in Figure 1. Among the advantages of DVDs over CDs is that each of the two halves may contain more than one information-carrying layer, thereby increasing storage capacity. In contrast, a CD has a single information-carrying layer.

Figure 1 – Exemplary DVDs

Each type of DVD has its own advantages and capabilities. Some disc formats, such as DVD-10 and DVD-9, have similar data storage capabilities. However, the informational layers in a DVD-10 disc are read from two sides of the disc. Thus, to make a DVD-10 disc comparable to a DVD-9 disc requires a dual laser beam pick-up DVD player. Such disc players are relatively expensive. On the other hand, the DVD-9 format offers the ability to have both informational layers read from one side of the disc, using a relatively inexpensive, single laser beam pick-up disc player (Schwartz, Vladimer 4). Thus, the DVD-9 format is one of the more attractive media for applications where an inexpensive disc player is used, but relatively large storage capacity is needed, such as, for example, interactive applications.

How is DVD different from CD? For greater data density, there are smaller pits, a more closely spaced track and a shorter- wavelength red laser. The error correction is more robust and the modulation scheme is more efficient. All this means that a standard DVD can hold 4.7 gigabytes of data, which is seven times the data capacity of a current Compact Disc. With a DVD, you have higher-resolution pictures, more channels of digital sound, richer graphics, and far more multimedia. To go even further, Dual-layer DVDs can hold more than twelve times the information of a CD on a single side. So there's no need to turn the disc over. DVD software can be replicated using existing CD production facilities with some modifications. Non-contact laser optics mean playback without wear and tear. A disc-based format means the kind of split-second random access that no tape format can match ("Chronology of DVD"). As with Compact Disc, DVD is durable, and tolerant of dust, dirt and fingerprints. The DVD standard defines a disc that maintains the overall dimensions, look and feel of the current Compact Disc

Due to digital video decoding technology, DVD delivers far and away the best color, sharpness and clarity in home video. A DVD picture approaches the "D-1" TV studio production standard. Because of variable bit-rate MPEG2 compression, it all fits easily onto a single side of a 4-3/4-inch disc. In fact, a single-layer DVD can hold a 2 hour 13 minute movie with extra space for Dolby® Digital discrete 5.1-channel digital soundtracks. Dual-layer, single-sided discs can hold movies more than four hours long. Since DVD is an optical disc, you get instant access, it can be played repeatedly without wear and tear, and never has to be rewound ("About DVD Features" 5-6).

Under the surface, of course, DVD reveals some substantial differences. Compared to CD, DVD uses smaller pits and a more closely spaced track as shown in Figure 2. The result is a significant increase in data density. The higher Numerical Aperture (NA) lens of DVD helps the laser focus on the smaller pits. Almost every aspect of DVD was developed, refined or reinvented to achieve the seven-fold increase in data capacity and data density. Refinements include smaller pit dimensions, a more closely spaced track (finer "track pitch"), and a shorter-wavelength laser ("About DVD Features" 5-6).

Conventional CD Players and CD-ROM drives use a laser that emits invisible, infrared light at the wavelength of 780 nanometers. DVD Players and DVD-ROM drives use a laser that emits red light at 650 and 635 nm. The shorter wavelengths are better suited to reading the smaller, more densely packed pits. The laser assembly was also refined with a higher Numerical Aperture (NA) lens, resulting in a narrower, more tightly focused laser beam ("About DVD Features" 5-6).

All DVD discs comprise a sandwich of two 0.6mm thick substrates. There are four possible read-only formats plus recordable and rewritable formats:

*DVD-18 is extremely difficult to manufacture and there is virtually no replication capacity for this format.

DVD-5 (4.7GB) Single Sided/Single Layer:

Figure 3 – Schematic of DVD-5

This is the simplest of the family of DVD discs, comprising a single layer with a capacity of 4.7GB. Only one of the two 0.6mm substrates contains data, the other being a blank disc. The two substrates are bonded together to form a 1.2mm thick disc. Single sided discs can be printed on by any conventional method e.g. screen-printing. Alternatively, the blank substrate can be molded with an image in its surface and then metalised to make it visible.

This dual-layer, single sided version has a capacity of 8.5GB, which is slightly less than twice the single layer version, to make it easier for the second layer to be read. Pits on both layers are 10 per cent longer than on a DVD-5 or DVD-10 disc. Each layer is molded in one substrate, the two substrates being joined with an optically transparent bonding layer. These discs can be printed after bonding in the conventional way.

DVD-10 (9.4GB) Double Sided/Single Layer:

This disc comprises two sides each single layer. It differs from the DVD-5 version in that both substrates contain data. To read both sides the disc will need to be turned over for most DVD players/readers. The capacity is 9.4GB, twice the single side/single layer version. Double-sided discs cannot be printed except on the hub inside the lead-in area. Labeling is therefore a problem with such discs.

This version comprises two sides each with a dual layer format. Both layers of each side must be manufactured on a single polycarbonate substrate using a 2P (photopolymer) process. It has the largest capacity of the family but is the most difficult and complex to manufacture. Four stampers are needed, two fitted to standard DVD presses, and the other two used to create pits in the photopolymer (Sharpless, Twynham "DVD Disc Formats").

DVD File System

A new file system was chosen for DVD, which would suit both read-only and writable versions. This file system is a subset of UDF (Universal Disk Format) called micro UDF (M-UDF). The main characteristics of UDF are:

• Robust file exchange
• System & vendor independent
• Writable & read-only media
• Based on ISO 13346

UDF has been extended to provide the necessary features for both write-once and rewritable discs. A combination of UDF and ISO 9660 (known as UDF Bridge) is used on some DVD discs to provide compatibility with existing operating systems, including Windows95. Applications can access the data files using either ISO 9660 or UDF file structures, but the use of UDF is recommended.

DVD-Video discs use only UDF (not ISO 9660) with all required data specified by UDF and ISO 13346 to allow playing in computer systems. The DVD-Video files must be no larger than 1 GB in size and be recorded as a single extent (i.e. in one continuous sequence). The first directory on the disc must be the VIDEO_TS directory containing all the files. All filenames are 8.3 formats. DVD-Video players will ignore all other files not included in the DVD-Video specification. Figure 7 shows a schematic of how files are stored on a DVD.

DVD-Audio discs also only use UDF and uses the AUDIO_TS directory.

DVD-ROM discs use UDF (plus ISO 9660). However Windows95 was not designed to read UDF but can read ISO 9660. The UDF Bridge specification does not explicitly include the Joliet extensions for ISO 9660, which is needed for long filenames. Most current Premastering tools do not include the Joliet extensions but it is expected that this feature will be added in due course. Windows98 does read UDF so these systems have no problem with UDF or long filenames (Sharpless, Twynham "DVD File System").

Figure 7 – Schematic of Root Directory (DVD File System)

The CCIR-601 digital video standard specifies a video rate of 167 megabits per second. At this bit rate, the 4.7-gigabyte capacity of a standard DVD could only store roughly 4 minutes of digital video. Thus, some form of data compression is required. DVD takes advantage of a sophisticated compression technology called MPEG2. Its a set of flexible compression standards, the second to emerge from the Moving Picture Experts Group (MPEG). MPEG2 works by analyzing the video picture for repetition, called redundancy. Over 97% of the digital data that represent a video signal is redundant, and can be compressed without visibly harming the picture quality ("About DVD Features"). By eliminating redundancy, MPEG2 achieves superb pictures at far lower bit rates.

As implemented for DVD, MPEG2 encoding is a two-stage process, where the signal is first evaluated for complexity. Then, higher bit rates are assigned to complex pictures and lower bit rates to simple pictures, using an "adaptive," variable bit-rate process. The DVD format uses 4:2:0 component digital video compressed to bit rates with a range of up to 10 megabits per second. Although the "average" bit rate for digital video is often quoted as 3.5 megabits per second, the actual figure will vary according to movie length, picture complexity and the number of audio channels required.

Due to MPEG2 compression, a single-layer, single-sided DVD has enough capacity to hold two hours and 13 minutes of video on a 4-3/4-inch disc. At the nominal average data rate of 3.5 megabits per second, this still leaves enough capacity for discrete 5.1-channel digital sound in three languages, plus subtitles in four additional languages. Including video, audio and subtitles, the total average data rate is 4.962 megabits per second ("About DVD Features").

Copy Protection for DVD

Copy protection comprises both digital and analogue techniques.

The Content Scrambling System (CSS) is used to scramble the audio/video data on a DVD-Video disc. Each video title set (VTS) can be selectively scrambled using a unique key. The Disc key and Title keys (one per VTS) are stored on the disc in encrypted form. In the decoder, the original keys are obtained by decryption and used to descramble the data. Data other than audio/video is not encrypted. For DVD-ROM drives, the MPEG-2 decoder challenges the drive and receives the necessary keys for decryption. This ensures that only approved hardware/software can be used.

The keys used are unique for every disc title and are encrypted by the CSS Licensing Authority and, usually, the scrambling is carried out during glass mastering. Security is vital and the keys used plus the encryption algorithms must be kept secret. Only those companies involved in designing hardware and software for CSS encoding and/or decoding needs information on the algorithms and systems used.

The Macrovision Analogue Protection System (APS) is based on Macrovision version 7.0 and is used to distort the composite video output to prevent recording and playback on VHS. This does not extend to RGB or YUV outputs for which new methods are required and are currently being investigated. Adding APS to a DVD-Video disc requires the content owner to become licensed by Macrovision and the authoring studio to set a flag to enable APS in the player (Sharpless, Twynham "Copy Protection For DVD").

The DVD Forum Copy Protection Working Group (CPTWG) is currently studying new methods to prevent copying and piracy. Digital Watermarking proposals from various companies are also being evaluated for DVD-Video and DVD-Audio discs.

The polycarbonate obtained from bisphenol-A is the second largest sales volume engineering thermoplastic. Polycarbonate (PC) is an amorphous polymer with attractive engineering properties (see Table 2), including high impact strength, low moisture absorption, low combustibility, good dimensional stability, and high light transmittance (up to 88%). For these reasons, PC is a perfect material for molding DVD substrates. Worldwide consumption of PC in 1992 was estimated to be 60,000 tons. Among the disadvantages of PC are limited chemical and scratch resistance and a tendency to yellow with long-term ultraviolet exposure. These problems have been addressed by the introduction of silicone-coated and free radical stabilized polycarbonate resins (Fried 343). Major producers of polycarbonate include: GE Plastics, Mobay, Fiberite, Dow, Bayer, and Dupont. Depending on quantity purchased and grade, polycarbonates used for DVD production range from $3.50-4.50 per pound.

Table 2 – Properties of Polycarbonate

Polycarbonate can be synthesized by the polycondensation of bisphenol-A and phosgene, as shown in Figure 8. For each repeating unit that is formed, two molecules of hydrogen chloride are liberated. Alternately, if the sodium salt of bishpenol-A is used in the polymerization, the by-product becomes sodium chloride rather than hydrogen chloride. This is an obvious advantage because the salt will precipitate out of the organic solvent used in the polymerization and, therefore, can be easily and safely removed. In contrast, the production of strongly acidic hydrogen chloride requires special consideration for disposal and in the selection of construction material used in the polymerization reactor.

Figure 8 - Synthesis of bisphenol-A polycarbonate by the polycondensation of bisphenol-A and phosgen

Other polycarbonates can be polymerized by modified interfacial condensation or by melt transesterification of tetrasubstituted bisphenols (Fried 344). These polycarbonates have the general structure

Figure 9 – General Structure of Polycarbonates from Tetrasubstituted Bisphenols

Where X represents a halogen, especially bromine, or methyl group. One example is tetramethylbisphenol-A polycarbonate (TMPC), X = CH_3, which has a higher heat-distortion temperature (HDT) and better hydrolytic stability than PC. The HDT or T_g is a result of the greater rigidity of the TMPC changing due to the steric hindrance of the substituent methyl groups. One disadvantage of TMPC its low impact resistance; however, this may be improved through blending with impact-resistant resins such as HIPS, ABS, and MBS. The styrene component of these impact modifiers forms a homogeneous phase with TMPC. Tetrabromobisphenol-A polycarbonate (TMBPC), X = Br, can be blended with PC to increase HDT. Copolymers of bisphenol-A and tetrabromobisphenol-A or tetrachlorobisphenol-A provide better flame retardancy. The polycarbonate obtained from cyclohexanonebisphenol can be blended with PC to increase the HDT (Fried 344).

All DVD discs comprise two substrates each 0.6mm thick and molded separately. The replication process varies somewhat for the different formats. DVD-5 and DVD-10 were the first to be manufactured. DVD-9 has proved to be considerably more difficult due to the different metallisation and bonding requirements (Kirkland 1).

DVD molding is similar to CD molding but with some important differences.

• Two pressings are needed for each final DVD disc
• Each half disc (substrate) is 0.6mm thick instead of 1.2mm
• The thinner disc also requires different molding parameters, such as a shorter injection time and higher mould temperature.
• Molding machines are needed with injection compression where the mould is kept slightly apart until most of the polycarbonate has been injected.

The quality of the final disc, including tilt and jitter, is critically dependent on the molding process. For DVD-5 discs, the active substrate is metalised and then bonded with the blank, non-metalised substrate (Sharpless, Twynham "DVD Manufacturing").

Figure 10 – Flow Diagram of DVD 5 Replication

For DVD-10, both substrates are metalised.

For DVD-9 discs two metalisation layers are required, one being semi-reflective, using gold or silicon. Parameters such as tilt, bonding layer transparency etc are more severe for DVD-9. Also the aluminum layer must be uniform in thickness to avoid jitter.

Figure 11 – Flow Diagram of DVD 9 Replication

The differences between DVD and CD means that much of the mastering process for DVD needs new equipment including improved glass master preparation, laser beam recording and developing (Sharpless, Twynham "DVD Manufacturing").

Figure 12 – Flow Diagram of Glass Mastering Process

• The photo-resist layer is about 120 nm in thickness (instead of 140 nm for CD) but successful mastering using the same thickness as for CDs is possible. Any defects or variations in thickness of this layer must be kept very small.
• Laser beam recording requires a smaller spot size, higher numerical aperture and tighter tolerances than for CDs. Many LBRs designed for DVD mastering use a UV laser (instead of the blue or violet laser used for CDs). To handle CD and DVD mastering, it is necessary to change the numerical aperture from 0.6 for CD to 0.9 for DVD mastering.
• DVD data is formatted differently from CDs and requires separate formatting hardware/software to handle the RSPC error correction, 8 to 16 modulations and the higher channel data rate.
• Stamper finishing requires more care than for CDs, since tilt (variations in flatness of the final disc) is critical for DVD.
• DVD-9 (dual layer) discs require the upper layer (layer 1) to be mastered with the turntable rotating in the reverse direction. Also, the direction of writing will be either from the inside to outside (parallel track) or outside to inside (opposite track), depending on the application requirements.
• CSS (Content Scrambling System) copy protection is carried out at the mastering stage. The data on DLT is combined with the encrypted keys and the audio and video data scrambled using these keys, which are hidden on the DVD disc.

Bonding is one of the most difficult parts of the process. There are a number of possible solutions. Hot melt bonding is the method used for Laserdiscs where the two substrates just need to be glued together. It is also suitable for single layer (single or double sided) DVDs. The process is simple and relatively inexpensive, but tends now to be replaced by UV bonding.

Radical UV Cured bonding is suitable for dual layer discs because it is transparent. It involves coating one or both of the substrates with a UV cured resin similar to normal lacquer, but with suitable optical and mechanical characteristics.

Cationic UV Bonding involves screen printing the resin over both substrates, curing each with UV light and then pushing the discs together. This method is not suitable for dual layer discs as the resin used is opaque (Sharpless, Twynham "DVD Manufacturing").

Many companies use Radical UV Cured bonding which is compatible with all DVD formats. DVD-9 bonding is particularly difficult, as the bonding layer must

• Be of uniform thickness within close tolerances
• Be optically transparent with no defects such as bubbles
• Not introduce tilt outside the DVD specification

One of the advantages of a DVD-9 disc is that its two informational layers are readable by a disc player from one side of the disc. However, in order for the disc player to read both informational layers, the laser beam from the pick-up must be able to travel through the intermediate layer of adhesive. Thus, the layer of adhesive must be optically clear and, specifically, it should be substantially transparent to radiation having a wavelength from about 635 to about 650 nanometers (nm). Further, its refractive index (n) should be 1.5 to 1.6 and its single path bi-refringence should be between 30 nm to 50 nm (to be compatible with the optical grade polycarbonate of the two DVD halves) (Schwartz, Vladimir 5).

As can be appreciated, air is often entrapped between the two halves of a DVD when it is manufactured. Yet, air bubble inclusions in the adhesive layer are not desirable and at a certain size are problematic. Air bubbles cause diversion of the disc player laser beam and even slight beam diversion can render a disc unreadable. Laser beam diversion is caused by the substantially different refractive index of air, n=1.003, versus the refractive index of polycarbonate, n=1.586 (Schwartz, Vladimir 6). Air bubbles can also contribute to the delamination of a DVD's halves.

As noted, discs in the DVD-9 format have a relatively large capacity and can be used in inexpensive disc players. However, present DVD manufacturing methods are not satisfactory for manufacturing discs in the DVD-9 format. One method of forming a DVD disc is to use a hot-melt adhesive. The hot-melt disc bonding method involves rubber-roller deposition of thermally liquefied, hot-melt adhesive to DVD halves. Adhesive is applied at 120° C. to 150° C. over the entire contact surface of both disc halves. In the next step, the DVD halves are pressed together to allow for uniform adhesion between the two halves (via a polymerization process). Since the disc halves are pressed together (against a flat surface), the desired disc flatness or tilt is easily achieved. However, one problem associated with DVDs formed using hot-melt adhesives is "droop."

DVDs can be exposed to high temperatures (for example, when they are on the dashboard of a car during the summer) and structural instability of the hot-melt adhesive bond can occur under such conditions. The structural instability manifests itself as a dimensional change or droop in the adhesive. Droop may cause the two halves of a DVD to become misaligned or may cause the spacing between them to become non-uniform. Droop of the hot-melt adhesive occurs at the glass transition temperature (or Tg point) of the material. The Tg point for typical hot-melt polymers is about 90° C. Sometimes structural welds are used in DVDs in order to diminish drooping. However, since the entire contact surfaces of the DVD halves are coated with hot-melt polymer, structural welding of discs to prevent disc droop via ultrasonic welding cannot be achieved, because the adhesive bond prevents the necessary vibration. Structural welding using other welding methods may be available.

The hot-melt adhesive bonding method suffers from an additional problem; entrapment of air. Entrapment of air bubbles can occur at the three interface layers of a hot-melt, adhesive-bonded DVD. Among other problems caused by the existence of such air bubbles, the entrapped air affects the cosmetic look of a DVD. In order to hide air-bubble-cosmetic defects that occur in the adhesive layer, translucent and even colored hot-melt adhesives are used. While some attempts to remove the air bubbles rather than hide them have been made, they have not been successful. Vacuum bonding (used to eliminate air bubbles during the bonding process) can not be applied when using hot-melt adhesives because liquefied thermopolymers actively "outgas" (transition from a liquid to a gaseous state), thus affecting the bond between the two halves. The outgased chemical substances also contaminate the vacuum pumping system. Accordingly, the hot-melt adhesive method does not satisfactorily meet the requirements of DVD-9 discs (Schwartz, Vladimir 6). That is, using this method, it is not feasible to produce a DVD with an optically clear adhesive layer that is free from entrapped air.

The other presently used method for bonding DVDs employs UV-curable adhesives. There are two ways of achieving a UV adhesive bond: radical UV and cationic UV. The radical UV method involves simultaneous spin coating and capillary dispersion of a UV-curable polymer between the DVD halves. The cationic method involves application of UV adhesive via a screen-printing process. The cationic or screen printing method has limitations similar to those in hot-melt bonding (although, since cured UV fluid is hard and dimensionally stable even at temperatures above 100° C. there is no droop problem) (Schwartz, Vladimir 6).

The radical UV method requires extremely high quality, expensive DVD substrates (disc halves that have superior flatness, disc surface parallelity, and bonding surface wet-ability). Injection molding of substantially flat 0.6 mm polycarbonate substrates (including near perfect replication of data pits) is a complex and relatively "slow" process (6 to 7 seconds cycle speed). This makes the process relatively expensive and time-consuming rendering the radical UV method much less viable as a real option for most DVD formats.

Another difficulty with the radical UV method involves applying the UV-curable fluid polymer without entrapping air within it. Uniform capillary dispersion of fluid polymer with good "surface wiping" action to displace all air (bulk and surface level) while maintaining bonding layer thickness between the DVD halves is very difficult to achieve with present techniques. Efforts to improve dispersion of the fluid, including increasing the bonding surface energy of the DVD halves by an oxygen plasma pre-clean of the polycarbonate, maintaining fluid and disc temperature (to control constant viscosity of the polymer at approximately 30 to 40 centipoises), and maintaining low water content in the polymer require exacting control procedures and equipment. These complexities in combination with common UV-overcuring problems make the UV adhesive method very expensive and time consuming. However, the radical UV process is the only presently available method of manufacturing DVD-9 format discs (Schwartz, Vladimir 6).

Finishing comprises label printing, for which there are a number of options, and adding the Burst Cutting Area.

Table 3 – Label Printing for DVDs

In the diagram below, the printable areas for DVD-5, DVD-10 and DVD-9 discs are shown in blue.

Figure 13 - Printable Areas (shown in blue) for Different DVD Discs

Burst Cutting Area (BCA) is an annular area within the disc hub where a bar code can be written for additional information such as serial numbers (Sharpless, Twynham "DVD Manufacturing").

DVD inspection and testing requires the use of some different techniques, new parameters to be tested and new readers. DVD glass mastering must be checked using a DVD stamper player to check the stamper prior to replication. DVD inspection is similar to CD inspection but includes tilt. Discs must be inspected after bonding as this stage can introduce tilt and other defects. DVD-10 and DVD-9 discs need inspection of both top and bottom of each disc. DVD-9 needs inspection of the semi-reflective layer and the bonding gap. DVD bit verification requires equipment to read the data (Sharpless, Twynham "Quality Assurance").

New materials and processing programs continue to be at the center of CD and DVD manufacturing. As formats become more sophisticated, processing must stay in stride to answer new challenges. In response to the new, high-density formats, Bayer, Dow, and GE Plastics have all developed new materials, which promise high-quality alternatives with improved performance, low birefringence, low moisture absorption, and even a weapon against piracy.

New for 1999 from GE Plastics is Lexan OQ1050 polycarbonate, which the company touts as being "a new global platform for CD and DVD applications as will as for advanced recordable/rewritable formats." Initially available in Europe and Asia, the resin is scheduled for global rollout in 2000. The resin boasts increased flow for low birefringence and fast cycle times, plus improved optical clarity and resin purity. Lexan OQ1040L resin is especially formulated for DVD manufacturers who need a higher-flow optical-quality polycarbonate. And Lexan OQ1030L, first introduced in 1997, has been refined with an improved particulate level and viscosity consistency, making it usable for DVD, CD-ROM, CD-Audio, and CD-Recordable media (Goldsberry 4).

Dow Plastics’ newest material promises to not only offer superior qualities for DVD with respect to molding, but also was developed as an answer to the piracy problems that continue to plague the CD and DVD industry. Polycyclohexylethylene (PCHE) resin was tested unsuccessfully for DVD applications in cooperation with Axxicon. Axxicon’s trials on equipment currently used for molding DVD disks indicated that PCHE resin could be incorporated into existing manufacturing processes. PCHE resins have a distinctly low specific gravity, high dimensional stability and rigidity, and very low inherent moisture, important in maintaining the extreme flatness of the disks (Goldberry 4).

Dow also hopes that by monitoring the supply of PCHE, the industry can prevent unauthorized replicators from obtaining the material, thereby eliminating the illegal production of disks (Tullo 1). The company is proposing an independent legal entity that would track the supply of polymers similar to the way the supply of paper used for currency is monitored.

Bayer Corp. showcased its Makrolon DP1-1265 polycarbonate resin, which was introduced two years ago for the DVD market. This material allows molders to achieve cycle times under 4 seconds. Since molding is the slowest step in the optical memory disk production process, gains in cycle time are reflected throughout the process (Cafaro 1).

Polymethylmethacrylate (PMMA) had its day in the sun in the optical media industry during the days of the laserdisc. It was an ideal material for molding a thin, large diameter disk because of its extremely low birefringence. It is also less expensive that polycarbonate. Because of its tendency to undergo dimensional changes when exposed to moisture, specifically when the moisture is absorbed from only one side as in CDs, PMMA could not take advantage of this market. However, DVDs, like Laserdiscs, have tow surfaces exposed to moisture and are symmetrical, making PMMA a good alternative (Tullo 2).

Two companies have begun promoting PMMA for DVD use. Cyro Industries’ Acrylite DQ501 acrylic molding compound, developed in conjunction with Rohm GmbH, offers high light transmittance, storage capacity, surface hardness, and elastic modulus. It also has low birefringence, optical purity and clarity, and precise mold surface reproduction at a lower material cost than polycarbonate (Goldsberry 4).

Elf Atochem North Americal has also introduced a new acrylic resin into the DVD market. Called Plexiglas VOD-100, the new material offers reduced viscosity for improved replication and superior optical properties. It also has reduced birefringence and increase light transmission. When molding with polycarbonate, replicators must balance three processing parameters: birefringence, disk flatness, and signal quality. Often they can improve one of these, but only at the expense of one of the others. Plexiglas acrylic resin allows replicators to remove birefringence form the processing equation (Goldsberry 5). DVDs made from the Plexiglas acrylic resin are also more scratch resistant that those made of polycarbonate.

Just as the original CD created a revolution in audio, DVD will raise the standard for home video picture quality. In fact, picture quality approaches "D-1," the CCIR-601 TV studio production standard. DVD delivers far and away the best color, sharpness and clarity in home video, far surpassing the Laserdisc standard. DVD also offers high resolution, with exceptional rendering of fine picture detail. Video distortion is extremely low, which reduces unwanted color "noise." Because the recording format is component video, as opposed to NTSC composite video, the pictures are free of the well-known drawbacks of NTSC - artifacts including dot crawl and cross color distortion. And because DVD is an optical format, the picture quality doesn't degrade over time and repeated use. DVD looks like a Compact Disc. But instead of just playing music, it delivers more than two hours of high-quality video - for movies, children's programming or music concerts.

A single-sided, single-layer DVD can contain up to 133 minutes of video enough to handle 95% of all movies, without the interruption of flipping the disc over or changing discs. The optical disc technology of DVD completely outperforms videotape. For example, you can play a DVD hundreds of times, without picture degradation. You can go from scene to scene in a split second without rewinding! Because DVDs can deliver more than 500 lines of horizontal resolution, the picture detail is more than twice as good as a VCR.

DVD offers a high capacity multimedia data storage medium, designed to accommodate a complete movie on a single disc, content rich multimedia or very high quality multi-channel audio. The market for DVD has grown faster than CD or VHS did in their first two years in the USA, Europe and Asia. DVD-Video titles predominate, but DVD-ROM is forecast to grow even faster and DVD-Audio will launch in 2000.

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