If all you could see is the recorded surface of 2 discs, could you pick out the Music CD or DVD Movie? The discs are the same diameter, the same thickness, similar shiny surface and weigh the same. So what are the physical differences between these two formats? Are there differences within the CD and DVD families? The CD replaced the vinyl record, and if the DVD replaces the CD, then what can we expect to replace the DVD in the future?

CLOSER LOOK

To see the difference, one has to examine a micrograph of the CD and DVD. In this micrograph, the pits are clearly smaller and the tracks are closer in the DVD image.

The CD pits are larger because they are created with an infrared laser having a wavelength of 780 nm (nanometer). Think of the wavelength as the distance between two ripples in water. For example, consider shore waves. The distance between these wave crests could be measured in feet. Now drop a rock in a quiet pond and these wave crests could be measured in inches. Now set a cup of coffee on the hood of a car with the engine idling and the wave crests could be measured in a fraction of an inch. And so goes it with laser wavelengths, except on a much smaller scale. Now the typical human eye can discern wavelengths between 400 nm - 700 nm. Different wavelengths of light represent different colors to the eye as shown at right: [1]

400 nm Border Ultra-Violet
420 nm Violet
450 nm Blue
514 nm Green
578 nm Yellow
600 nm Orange
647 nm Red
700 nm Border Infra-Red

Most individuals could not see the CD laser at 780 nm. However, the DVD pits are created by a red laser with a wavelength of 650 nm for DVD-R General discs and 635 nm for DVD-R Authoring disc. So the typical human eye could see the light from these two lasers. As a matter of fact, the newer laser pointing devices use the same wavelength as the DVD laser.

CAPACITIES

There are a number of capacity options in the recordable media for both CDs and DVDs. Data listed in Table 1 reflect the majority of recordable media on the market. [2]

NOMENCLATURE

True disc capacity and data thru-put rates were some of the most confusing and time consuming items to resolve because of ambiguous and incorrect information presented on different Internet sites. So for clarity, it must be said that manufacturers of CDs, DVDs and hard drives market the capacity of their media or hardware based on the every-day decimal system. For example, the following comes from the International Standards Institute (SI) and applies to almost everything except for some computer items:

Kilo or KB =
10^3 = 1,000
Mega or MB =
10^6 = 1,000,000
Giga or GB =
10^9 = 1,000,000,000

Now, in the computer world, Fdisk, DOS and the Windows operating system presents disc and hard drives capacities in binary bits and many Internet sites will mistakenly represented these capacities with decimal symbols like KB, MB and GB, which are misleading. Because of this confusion, the IEEE in 1999 adopted a new standard for designation of binary bits as follows: [3]

KiB or kibibyte =
2^10 = 1,024
MiB or mebibyte =
2^20 = 1,048,576
GiB or gigibyte =
2^30 = 1,073,741,824

To consider why CD and DVD capacity nomenclature is important, we must put the information to use. Consider a DVD package that indicates the disc capacity is 4.7 GB. This would equal 4.7 x 1,000,000,000 or 4,700,000,000 bytes. Let's say you spent 6 hours 'CPU time' rendering a home movie and want to put it on a DVD. The operating system or application indicates the size of your movie is 4.5 gigabytes and the DVD is 4.7 GB and you think you have plenty of room to spare. Actually, the size of your movie in binary bytes is 4.5 x 1,073,741,824 = 4,831,838,208 bytes. Consequently, your home movie would not fit on the DVD which means you would have to re-render it at a lower bit rate or edit out footage so it would fit. Again it would take 6 hours of 'CPU time'. So you have lost time, wasted effort and you have to do without your computer for another 6 hours just because the size nomenclature was different between the media and application. Fortunately, the Windows Explorer displays both GB and gigibytes when the Properties option is selected for a file, group of files, folder or drive. [4]

Another point about capacity to consider is if the disc is dual-sided or dual-layered or both. CDs do not have this capability, only DVDs. The common one-sided DVD has 4.7 GB of storage. A two-sided DVD has twice that or 9.4 GB. Now here is where it gets tricky. A DVD can be made with two recording layers sandwiched on the same side and this is called a dual-layer disc. The dyes are transparent enough where the read laser can focus on the second layer. This is kind of like standing at a screen door and focusing on something in the yard, the screen mesh seems to disappear, but the image in the yard is not as clear as looking through just glass. Thus, the capacity of a dual layered disc is only 8.5 GB, which is not the same as twice the capacity of the original single layer. The second layer only adds about 3.8 GB of capacity because of slightly reduced optics. The final option is a dual-sided disc that has dual layers. The capacity of this type of disc is 17 GB, which is twice the capacity of a dual layer disc.

CD disc capacities over 80 min pushes the limit of the specifications. Writing or reading discs with 90 or 99 minutes is considered 'overburning'. This process often violates specifications for track pitch, pit size and writing in areas intended for other data like the lead out zones. Potential damage can occur when trying to read/write close to the outer rim of the disc and many drives and software applications prevent it for safety reasons.

One final note about tracks and pits. If you look at the first micrograph that compares the CD and DVD tracks and pits, you will notice how smoothly the pits are formed in both disc. This is because these discs are manufactured and the pits are stamped into the disc so they are very uniform. However, if you were to burn your own CD or DVD the pits would look more like what you would see in the following micrograph:

The pits in a recordable disc are generally not indentations of the recording media as seen in the first micrograph of manufactured discs; but instead are optically obscure points that greatly reduce the laser reflectivity. For consistency sake and ease of reference, all disc are said to have pits and lands (no pit regions). One of the dyes mentioned later will create a small indention in the dye, but the primary recording mechanism is optical obscurity. Incidentally, as the laser reader tracks over the pits and lands, it records 1s and 0s. But here is the surprising thing, as the laser tracks from a land to a pit it records a 1. Also, as the laser tracks from a pit to a land it records a 1. Where there is no transition between a pit or land, the reader records a 0 or a series of 0s depending on the length of time before the next transition.

DATA TRANSFER RATE

Besides storage capacity, there is a significant difference in the data thru-put rates. Data transfer rates are measured in binary bytes per second. Table 2 lists the data rates at various speeds for CDs and DVDs. [4] [5]

The reason there is a range of RPM for the CD is because the data is read from the CD at a constant linear velocity (CLV). Most people know that the speed of a merry-go-round is faster on the outside than it is closer to the center. So to maintain a constant speed under the CD read head, the reader/writer has to adjust the disc RPM dependent upon where the read head is positioned on the disc. DVD discs utilize both the CLV and CAV (constant angular velocity, i.e. RPM).

Reportedly, discs become unstable when they are rotated above 16x. Eddies from air currents cause the disc to wobble and inconsistency in manufacturing may cause areas on the disc to be denser than other areas, causing resonate vibrations at higher RPM. Both conditions affect the focus point of the read laser causing errors. Higher RPM and higher data transfer rates can be achieved by high quality drives and higher quality discs. Higher data rates can also be achieved by splitting the laser beam to read multiple zones on the disc. Also, later generation DVD players are placing lasers on top and bottom in order to read dual sided discs.

DISC CONSTRUCTION

The construction of a disc has many layers. Starting from the label side, you may or may not see the first 2, but you will always see the last 4. So, first you have an ink-absorbent film for printing, a UV protective layer, a reflective layer, some type of organic polymer dye, and finally an optically clear polycarbonate substrate. Substrates may be tinted, but they remain optically clear to a recording laser. Note a DVD will have a polycarbonate substrate on top as well as on bottom.

The first layer that is present in all discs is the reflective layer. Aluminum reflective layer is used in stamped manufactured CD-ROMs and DVD-ROMs and it is the best reflective material, but is susceptible to the corrosive nature of the organic dyes used in recordable CDs and DVDs. Consequently, gold was first used as a reflective material in CDRs. Because of cost, silver was later used, and as it turns out, silver is the better reflective material anyway. A clear lacquer is used to cover the reflective coatings and should the lacquer break down, then the silver would oxidize and make the disc useless. However, gold won't oxidize and therefore would still be usable. Although, under normal conditions, it would take decades before the lacquer would break down.

Even though a disc spins at different RPM to maintain the same read velocity as mentioned earlier, not all discs will spin at the same read velocity when the read head is located in the same position over the disc. Higher capacity discs, like the 80 min. CD will spin at 1.1 meter/second where the 63 min CD will spin at 1.4 meter/second. So how does a reader know how fast to turn a CD? All writeable discs have a groove stamped into the disc when manufactured and that groove is approximately 3.5 miles long. Interestingly, this groove does not have a smooth curvature, but instead has a sinusoidal wobble of just a few microns that is laid down at 22.05 kHz based on how fast the disc should turn, i.e. 1.1 m/s for 80 min CDs or 1.4 m/s for 63 min CDs. The read head tracks this wobble and the system adjusts the rotation of the disc in order to maintain a 22.05 kHz tone. Once this tone is achieved, the disc is rotating at the manufacturers recommended speed.

These grooves are arranged in a spiral track from the center to the outer rim. The reason the track starts in the center instead of the rim is because there are different CD/DVD diameters as seen in table 1; however, all discs have the same inter-diameter, so by starting in the center, this greatly simplifies the tracking hardware for disc readers/writers. Within this groove is placed an organic polymer dye. [6]

DYES

The dye is the media that records the data. Dyes are applied to the polycarbonate substrate by spin coating the disc and then allowed to cure. It is interesting to note that thinner dye layers facilitates faster writing speeds. Listed below are most of the popular dyes with their good and bad attributes.

Cyanine

Good - This was the first dye developed for CDRs and was optimized for 1x - 4x record speeds but nothing higher. This dye is very flexible to fluctuations of laser power found in cheaper CD writers and requires 6.5 milliwatt at 1x writing speeds.

Bad - This dye provides almost no UV protection. Few CDR manufactures still use this dye. Long laser-write strategy is required for this dye. Laser written areas in this dye may not develop pits, but will appear optically obscure or less reflective.

PhthaloCyanine

Good - This dye is reported to be more stable than Cyanine dye. This dye is very UV protective and is optimized for 1x - 8x record speed. This dye has a longer life expectancy than the other dyes. Laser written areas in this dye will develop pits that appear less reflective. This dye only requires 5.5 millliwatt laser power at 1x writing speeds and short laser-write strategy is required.

Bad - This dye is very unforgiving to lasers with inconsistent output.

Metallized Azo-Cyanine

Good - This dye is optimized for 1x - 8x record speed and is widely used. This dye is respected among the media community.

Bad - This dye offers little UV protection but more than Cyanine. This dye needs steady laser output to optimize for high speed recording.

Advanced PhthaloCyanine

Good - This dye is a blend of PhthaloCyanine and Cyanine, offering the benefits of both dyes in one package. This dye also affords higher UV protection than PhthaloCyanine.

Bad - No information reported.

Formazan (Kodak)

Good - This dye is a blend of PhthaloCyanine and Cyanine offering the benefits of both dyes in one package.

Bad - No information reported.

Super Azo (Verbatim)

Good - This dye is optimized for 24x record speed. High sensitivity for wide range of laser powers. More stable to ultraviolet light.

Bad - Wavelength needs to be optimized to 780 nm. If wavelength is too short, reflectivity is reduced. If wavelength is too long, read errors are created.

TeRL (Verbatim)

This is the dye used for RW disc. No specific data was available. Other RW media includes alloy combinations of silver, indium, antimony and tellurium. These media work with the crystalline form of the metal. High laser power for writing turns the metal surface into an amorphous state (no crystal structure) to make a pit which is non-reflective. Lower laser power erases the pit by heating the metal at a lower temperature which allows a high reflectivity crystalline state to form when cooled.

The following chart lists the base color of different dyes and the apparent disc color when a Gold or Silver reflective layer is used. Certain dyes can be used for both CDs and DVDs; however, dyes for CDs are optimized for 780 nm wavelength lasers while the dye may be chemically altered to be optimized for DVD lasers at 650 nm wavelengths (635 nm for DVD-R authoring).

LASER POWER

Different dyes require different laser power settings; also, the faster the writing speed (higher disc RPM) the more power is required to record a data pit. This is because of the shorter amount of time the laser is exposed to the pit area. To address the first issue, the disc writer will burn a test section in the Power Calibration Area in the lead-in area of the disc when the disc is first tested for writing. This test section consists of 14 different trial burns with different power settings. The hardware reads the newly created pits and determines which of the test produces the best pit or optically obscure spot and that becomes the laser power setting for that disc. Because the dyes are applied with a spin application, some of the dyes may not be uniform from the center to the rim; therefore, some more expensive writers will monitor the full CD writing progress and adjust the power according.

FUTURE MEDIA

HC-R (High Capacity - Recordable)

This is an 830 MB capacity CD. This disc achieves the extra capacity by reducing the track pitch and decreasing the rotational velocity. No information was available for product availability.

DDCD (Double Density CD)

This is a 1.3 GB capacity CD and it has been available in US markets for a short time. This CD sports twice the capacity of CDs employing smaller pit size and tighter track pitch and smaller starting track diameter.

SACD (Super Audio Compact Disc)

As mentioned in part 1 of this series, the SACD is a high performance audio CD that requires a new SACD drive to benefit from the improved features of this disc. This hybrid disc has one layer that is a typical CD-A that is bonded with a translucent DVD layer. The second layer has the capacity of a typical DVD and contains the enhanced sound recordings and has additional provisions for text, graphics and video.

Self-Destructing DVD

When this article was written, there were a couple of news announcements about this technology where the Walt Disney Co. will start renting DVDs that will self-destruct. This is not a Mission Impossible self-destruct in 5 seconds type of disc, but a vacuum packed movie rental. Once the vacuum seal is broken, the customer has 48 hours of viewing time after which a substance in the DVD substrate changes color, optically obscuring the read laser, making it unplayable. News reports indicate the DVDs will be available sometime in August, 2003. There have also been rumors that the audio industry may follow suit by selling discs at a discount rate that will play for a specific period of time longer than 48 hours.

DVD REPLACEMENTS

DVDs are just really getting a strong foothold in the consumer market and it is expected to have a long service life. However, the following technologies are potential replacements for the DVD disc.

HD-DVD (High Density-DVD)

This is the "blue laser" disc. The HD-DVD will utilize a 405 nm blue-violet laser which is really closer to ultra violet on the color scale. This new laser coupled with smaller pits geometry and tighter track pitch will yield a single-sided single-layered disc capacity of 25 GB. Developers hope to stretch this capacity to 30 GB for a single layer and 50 GB for a dual layer. Mpeg2 transfer rates were reported to be 36 Bb/s. [7] [4]

There are two competing formats in the blue laser race. One is Blu-Ray and the other is AOD (Advanced Optical Disc); however, there are a number of blue laser player prototypes found on the Blu-Ray home page. This prototypes may be commercially available this year, but more than likely in 2004.

Fluorescent Multilayer Disc

This is another concept using existing technology to increase bit density. All CD and DVD discs have a reflective layer adjacent to the recording dye. A fluorescent multilayer technique abandons this concept and adds multiple recording layers with no reflective layer to the disc. When laser light hits one of these layers fluorescent light is emitted at a different wavelength. Read heads detect this light and the hardware processes the readings into bits.

Constellation 3D has demonstrated a clear DVD disc that had 20 layers yielding approximately 95 GB of storage with data transfer rates of 1 GB/s. Their research has indicated that 100 layers are possible which would yield a 450 GB disc, and they indicated if blue lasers were utilized, then the capacity could reach 1 TeraByte.

Holographic Storage

This technology is coming of age and we will see it soon in the marketplace. This technology involves splitting a laser light into two beams. One beam is modulated with data and the other beam remains unaltered. Both beams are redirected through a photosensitive material in such a manner that they crisscross. Where the beams crisscross, they will create an interference pattern (amplitude mixing of the two waveforms). This interference pattern alters the optical medium by changing the absorption, refractive index or thickness of the media at a specific spot the size of the laser beam. Additional data can be recorded in the same spot by pointing the laser with a slightly different angle. Many different angles can be used to record data in the same spot. Further multiplexing is accomplished by changing the laser wavelength and repeating the process on the same spot. So one small spot the size of a laser beam cross section can contain a vast amount of data and this process is repeated throughout the total volume of the media.

InPhase Technologies has demonstrated a 100 GB holographic data recorder prototype using a write-once removable disc. They plan to offer future versions with a terabit capacity. Manufacturing and distribution projections indicate limited quantities will be available in 2003 and full distribution in 2004.

Japan-Optware Co. plans to make available holographic recording systems capable of storing one terabyte of data on a 12cm disc with data transfer rates reaching one gigabyte per second. They use a technology called "polarized collinear holography where they split the laser light into one million smaller beams instead of two beams and they also use a separate reference beam (unaltered).

Other reports indicate the Defense Advanced Research Project Agency is creating a holograph system capable of recording 2.8 terabits of data on a 12 cm disc with transfer rates up to 10 gigabits/sec. These are astronomical storage capacities when compared to existing DVDs.

HD-ROM (HD-Rosetta)

Wavelengths were discussed earlier where a 780 nm laser was used to write a CD-R and a 650 nm laser was used to write a DVD-R. Besides the difference in laser wavelength, there is a difference in the width or thickness of the laser beam. A typical CD laser beam is 800 nm wide, where a DVD laser beam is 350 nm wide. The HD-ROM technology utilizes a particle beam of charged gallium ions that is 50 nm wide. With a significantly smaller beam, this technology enables an approximate storage capacity of 165 GB on a surface the size of a DVD disc. This technology is not limited to discs but can use any hardened object. The data does not have to be stored in binary in that a whole image could be stored. This technology is still under development and potential first users would be government agencies, banks, insurance agencies, scientific users and libraries.

MultiLevel Recording Chipset

This is rather interesting logic-based technology programmed into a chipset that would be hardwired into existing CD/DVD product lines. Current CD-RW and DVD-RW discs use phase change recording dyes. In other words, a laser changes the optical properties of the metallic dye by changing the crystalline structure, making the dye translucent or opaque as described in the section on RW dyes. So, current recording dyes have only two states, on or off. MultiLevel recording modulates the laser to achieve 8 levels of disc reflectivity thereby increasing the bit density per square cm. When the disc is played, the hardware, through sophisticated programming, discerns the reflected intensity level of the recorded point and from there decodes a data bit. Current RW dyes would require slight modifications to accommodate this new technology. Calimetrics has demonstrated a 2 GB CD prototype based on current CD technology. They have indicated plans to increase that capacity to 50 GB by incorporating blue laser technology and near-field optics

BEYOND THE DISC

Disc hardware systems are riddled with a number of cumbersome subsystems. For example, the disc handling subsystem has to pick up and rotate the disc at very high RPM while maintaining the disc in near perfect balance to prevent vibrations. The optical reading subsystem projects a laser beam onto the disc in a tightly focused zone and it has to track this zone within micrometer for the entire length, which is nearly 3.5 miles. The electronics subsystem has to register the electrical impulses from the read head and discriminate between pits, lands and other aberrations on the disk, like dirt or scratches. What if all of these subsystems could be abandoned? The following technologies abandon disc technologies in some very interesting ways.

AFM

Atomic Force Microscope technology has been around for some time. Currently, there are a couple of major players, HP, Hitachi and Seagate, that are researching the application of this technology to data storage. Initial protections for devices using this technology range from 0.5 GB to 50 GB. Commercialization of these products range from 2004 to 2010. Nanochip is also developing AFM technology and appear to be further along in the commercialization with the introduction of their MARE.

MARE (MEMS)

Molecular Array REad/write technology is based on nanometer structures etched into silicon to perform mechanical functions like MEMS (Micro Electro Mechanical System). Specifically, this technique etches hundreds of cantilevered arms, in an array, that have points on the end of the arms. When voltage/current is applied, the arm will deflect and form a pit in a silicon-recording medium. Each pit then becomes one bit in an array of bits. Erasing the pit is accomplished by applying a different voltage/current to the point and the pit becomes plastic and returns to the original state.

Nanochip Inc. has achieved a storage density of 258 gigabits per square inch for R/W media and 800 gigabits per square inch for WORM (write-once read-many). Their first release is a 900 megabyte unit with a data transfer rate of 3.1 megabytes per second to be followed with a 1.4 terabyte unit.

SENSI

This is a molecule created by James La Clair, a chemist formerly with Squibbs Institute. The molecule, when illuminated with a laser, will emit a photon when nitrogen is present and shuts off when carbon dioxide is present. This work, still in the research stage, offers great potential. The molecule only measures a few angstroms in size, which raises the possibility of not only terabyte capacities but also pentabyte configurations.

ADVANCED RESEARCH

Data storage research in the 2-dimentional or surface layer media continues with projects like:

NFOR

Near Field Optical Recording produces pits 1/20 the size of the laser wavelength, yielding a dramatic increase in bit density. Roughly, this is accomplished by placing a very small aperture very close to the recording media. The NFR drawback is satisfying high data transfer rates.

SIL

Solid Immersion Lens focuses a light in a highly refractive solid, so that the spot it creates is reduced to below the minimum achievable in air (solids are denser than air, so the refraction is greater).

Other 2-dimensional projects are looking for solutions by considering spatial, spectral or time dimensions. These areas involve work with two-photon phenomenon, persistent spectral hole burning and photon echo memories.

Work in 3-dimensional space continues to promise the best performance and work in these areas involve processes like microholographics, photorefractive crystals, organic photopolymers, photosensitive polymers and ultimately DNA polymers and DNA molecules.

CLOSING NOTE

The field of data storage is rich with research work. Many exciting new products will soon reach the consumer markets. The growth in processing power and multimedia applications has raised the rung in the data storage arena. Storage capacities in megabytes have long fallen to the wayside in preference to gigabyte capacities and yet it won't be long and the GB will fall in preference to terabyte or even pentabyte capacities. The demand for space never seems to end and the baby steps of research continue.

Addendum Note: The Beige book, specifications for photo CDs was inadvertently omitted from part 1 of this series and is noted here for completeness.