8
OPTICAL STORAGE MEDIA
Media that can be recorded to, removed, and then transported to another device has long been a dominant force in most of the entertainment industries, long before the emergence of the Internet, microwave communications, etc. Metallic, shellac—paper—wax-based composites, and polyvinyl chloride records were long considered the only durable (primarily consumer) means to exchange recorded music and voice among users. Analog audio tape became the next formidable method for the common exchange of media, and it was one of the earlier forms for easily recording, playing back, and rerecording data for computers.
Removable form factors used for storing computer data have been around for as long as data storage itself has. Punch cards and paper tape were two of the more prominent forms of media storage that began around the mid-1960s and remained in use in some places through the mid-1980s.
Today there are many more reliable, durable, and higherdensity formats in use for the storage of computer data. We find those form factors to be in the shape of solid state memory, transportable spinning hard disk drives, data tape, videotape, holographic devices, and optical mediums. This chapter focuses primarily on the evolution of optical storage, which includes familiar recent developments such as Blu-ray Disc and less familiar forms, such as the early laser disc and holographic recording.
KEY CHAPTER POINTS
• Defining removable optical media along with the development of the compact disc (CD), DVD, laser disc, and Blu-ray Disc followed by the death of HD DVD
• Holographic media for data storage, its development and history, and how holograms are made for moving images and other data using various innovative technologies
• Guidelines for the care, storage, and handling of optical media
Defining Removable Media
Removable media that contains data may be divided into two broad categories: (1) those which are essentially self-contained storage systems (e.g., a USB stick, SSD memory card, a transportable hard disk, or a solid state drive) and (2) those which depend upon a secondary mechanical device that performs the read and/ or write functions to and from physical media (e.g., a CD-ROM, DVD, or laser disc). These kinds of media are intended to be easily interchanged and can be stored in an off-line environment usually for indefinite periods of time.
Removable Hard Drives
Although it is certainly possible to remove the spinning hard drive from your personal computer by opening the chassis and disconnecting it from the rest of the computer, this is not a practical approach for most of the common applications for the physical exchange of media. Drive products that can be removed from a chassis and plugged into another chassis, which are actually hard drives and which differ in physical architecture from transportable drives, are still available and employed when highvolume data sets are needed. Such drives usually have no power supplies and cannot be directly connected with USB or eSATA connections. These drives are set into a transportable “sled” configuration and are often found in video or film-transfer production facilities that are not equipped with high-speed data links or in those that require the security of a lockable device that can be removed and then stored off-line in a vault or other secure location.
In this chapter, the storage methods discussed will define “removability” to apply to media that is routinely extracted from the record/playout device with a simple user action that does not involve technician level actions to achieve transportability. These fall principally into the categories of optical media, and potentially the recent offerings in holographic media captured in an optical fashion.
Optical Media Definitions
We are accustomed to using CDs and DVDs for a multitude of purposes, including media storage, gaming, music, and entertainment. They have been the replacement for linear tape for viewing, recording, or listening to music, home movies, or theatrical motion pictures. Optical storage has evolved over the course of the last few decades to become the quintessential formats of choice for the storage of physical data on media that is durable and low cost.
Most users were only familiar with a relatively finite set of optical media types until about the mid-to-late 1990s when significant changes in optical formats allowed for a new generation of recording features and storage options. To begin, we will first look at defining and categorizing these forms of optical media by their technical names and properties.
Terminologies with Misconceptions
DVD, discs, and disks: these are three common terms in the physical media storage universe that are misused or confused on occasion. To set the record straight, as represented by industry accepted terminology, we will now hopefully clarify these terms according to ISO/IEC definitions instead of industry rhetoric.
Disc
Starting with the most common mistake, the usage and spelling of the words “disk” and “disc” are still confusing to many. Essentially, the spelling “disc” (with the ending “c”) grew from reference to the CD, a spin-off of Laserdisc technology.
A disc refers to optical media, such as an audio CD, CD-ROM, DVD-ROM, DVD-RAM, or DVD-Video disc. Some discs are readonly (ROM), others allow you to burn content (write files) to the disc once (such as a CD-R or DVD-R, unless you do a multisession burn), and some can be erased and rewritten over many times (such as CD-RW, DVD-RW, and DVD-RAM discs).
All discs are removable, meaning that when you unmount or eject the disc from your desktop or Finder, it physically comes out of your computer. The media can then be shelved, moved to another device, placed into a jukebox, or kept in an archive or library environment for backup or data protection purposes.
Disk
When spelled with the ending letter “k”, disk refers to magnetic media, including the outdated floppy disk, a computer’s hard disk drive, external hard drives, the components of disk arrays, and the ever familiar term RAID (Redundant Array of Independent Disks). Disks will always be rewritable unless intentionally write-protected, locked, or damaged. Disks may also be partitioned in two or more volumes and may be virtualized for mass storage and collaborative functions.
To protect the fragile nature of the disk’s recording surfaces and heads, most disks will be sealed inside a metal or plastic casing that houses most of the mechanisms, electronics, and firmware, collectively making up what is best known as a “hard (disk) drive” (HDD).
CD Technology
Sony demonstrated its first optical digital audio disc inSeptember 1976. By 1979, Sony and Philips Consumer Electronics had established a joint task force to develop and design a new digital audio disc. A year later, the Red Book would be produced, which would become the Compact Disc standard, officially called Compact Disc-Digital Audio (CD-DA) for audio.
Audio CDs have been in commercial or consumer use since October 1982. Standard CDs, whose diameter is 120 mm, can store up to 799 Mbytes of data, essentially up to 80 min of uncompressed audio. A smaller version referred to as the Mini CD will have diameters ranging from 60 to 80 mm. The Mini CD was popular for singles, advertisements, and device drivers, with an audio capacity of up to 24 min. An even smaller business card-sized rectangular CD with sizes ranging from 85 × 54 mm to 86 × 64 mm holds about 10–65 Mbytes of CD-ROM data for around 6 min of audio storage.
This disc technology expanded to encompass data storage (CD-ROM), write-once audio and data storage (CD-R), rewritable media (CD-RW), video compact discs (VCDs), super video compact discs (SVCDs), photo CD, picture CD, CD-i, and enhanced CD.
Audio is almost always compressed before recording to a CD, often in formats such as AAC or MP3.
CD Components
The physical media is an almost pure polycarbonate plastic 1.2 mm in thickness and 15 to 20 g in weight. Moving from the disc center outward, the components include the following: the center spindle hole, the first-transition area (i.e., the clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim (see Fig. 8.1).
Figure 8.1 Optical compact disc component layout.
A thin layer of aluminum (a silvery white member of the boron group of chemical elements, with symbol Al and atomic number 13) or, more rarely, gold (symbol Au) is applied to the disc surface, making it reflective. The aluminum layer is protected by a film of lacquer that is normally spin-coated directly onto the reflective layer. A label may be printed onto this lacquer layer.
The data storage makeup for the CD is comprised of a series of tiny indentations known as “pits,” which are encoded in a spiral track molded into the top of the polycarbonate layer. Each pit is approximately 100 nm deep by 500 nm wide and will vary between 850 nm and 3.5 µm in length. Lands separate the areas between the pits.
The distance between the tracks is called the pitch, which for the CD is 1.6 µm. CDs are read by focusing a 780 nm wavelength (near infrared) from a semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits appears as ridges by the laser, and the lands produce an intensity difference in the reflected laser light. A photodiode pickup sensor measures the intensity changes, producing “read” data from the disc.
NRZI Encoding: Non-return-to-zero inverted (NRZI) encoding is used to produce the binary data representation. The reflection intensity changes that are measured from pit to land or land to pit will indicate a binary one. No change indicates one or more zeros. At least two, but no more than ten, zeros between each one is defined by the length of the pit. In turn, the data is decoded by reversing the eight-to-fourteen modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, which reveals the raw data stored on the disc.
Recordable Compact Disc (CD-R)
This format, which stands for Compact Disc-Recordable, is an extension of the CD format that allows data to be recorded only once on a disc. The process used is called dye sublimation (explained shortly). CD-R is defined in Part II of the Orange Book standard, the set of specifications created by Philips and Sony that define the optical signal characteristics, physical arrangement, writing methods, and testing conditions for CD-R and CD-RW (Orange Book Part III) discs.
Digital Versatile Disc
Digital Versatile Disc is the proper name for the familiar DVD nomenclature, contrary to the belief that the disc was used primarily for video storage and often mistakenly called “digital video disk (disc).” The development of the recordable DVD has progressed to include both single layer (SL) and dual layer (DL) discs. Table 8.1 describes DVD capacities, data rates and rotational speed parameters.
Physical versus Application Formats
One of the things to understand in this technology is the differences between the physical formats (such as DVD-ROM and DVD-R) and the application formats (such as DVD-Video and DVD-Audio). The DVD-ROM is a base format that holds data, whereas DVD-Video (more often simply a DVD) defines how video programs (usually theatrical releases of motion pictures) are stored on disc and played back using a DVD-Video player or a DVD drive in a personal computer.
The difference is similar to that between a CD-ROM and an audio CD. The DVD-ROM (DVD-read only memory) family includes these recordable variations:
•DVD-R/RW
•DVD-RAM
•DVD+R/RW
with the application formats including the following:
•DVD-Video
•DVD-Video Recording (DVD-VR)
•DVD+RW Video Recording (DVD+VR)
•DVD-Audio (DVD-A)
•Super Audio CD (SACD).
Other special application formats for gaming have also been created for console devices such as the Sony PlayStation 2 and Microsoft’s Xbox.
DVD-R
This is the familiar short name for Digital Versatile Disc-Recordable. A DVD-R allows data to be written to the media once and then read many times. DVD-R uses the dye-sublimation recording technology and conforms to ISO/IEC DIS 23912:2005: Information Technology that specifies the quality of the prerecorded, unrecorded, and recorded signals, the format of the data, the format of the information zone, the format of the unrecorded zone, and the recording method. The 80-mm nominal diameter disc has a capacity of 1.46 Gbytes per side, and the 120-mm version holds 4.71 Gbytes per side. The disc may be manufactured as either single or double sided. A 9.4-Gbyte dual layer (DL) version (DVD-R DL) was introduced in 2005.
The full ISO/IEC 23912:2005 specification for DVD-R provides information about the conditions for conformance, the environments in which the disc is to be operated and stored, the mechanical and physical characteristics of the disc, and the definitions and requirements for the mechanical interchange between data processing systems. The document further defines the physical disposition of the tracks and sectors, the error correcting codes, and the coding method used.
For interchange, ISO/IEC 23912:2005 further defines the characteristics of the signals recorded on the disc, which enables data processing systems to read the data from the disc, the interchange of discs between disc drives, the standard for volume, and file structure. It also provides the specifications for full data interchange between data processing systems.
Dye Sublimation and Manufacturing
The DVD manufacturing and duplication processes use a printing technique called dye sublimation, which employs heat to transfer dye onto a medium such as a plastic card, paper, or fabric. When mass duplicating the DVD, an original master disc is turned into an etched glass master through the use of a laser beam recorder (LBR). The master is used to create molded, stamped, and later lacquered discs. The steps of the manufacturing process include the following: injection molding, dye coating, edge cleaning, drying, sputtering, bonding glue, online scanning, prewriting (for DVD-R only), printing, and packaging.
DVD+R
The short name for Digital Versatile Disc+Recordable, DVD+R is a format for a recordable DVD, which also uses dye-sublimation recording technology. DVD+R conforms to ISO/IEC 17344:2005: Information Technology—Data interchange on 120-mm and 80-mm optical disc using the “+R” format. Data capacities are the same as DVD-R at 4.7 Gbytes (120 mm) and 1.46 Gbytes (80 mm) per side.
The DVD+R format allows data to be written once to the disc, but can be read from many times. The disc is suitable to applications such as non-volatile data storage, audio, or video. A DVD+R disc is incompatible with the older DVD-R standard and can be found in both single layer (SL) and dual layer (DL) variants.
DVD-RW
The rewritable (RW) format of the DVD family, formally called the Digital Versatile Disc-ReWritable. This format uses phase change recording technology conforming to ISO/IEC 17342:2004: Information technology—80 mm (1.46 Gbytes per side) and 120 mm (4.70 Gbytes per side). DVD-RW media allows for data to be written to the disk several hundred times and read many times.
DVD+RW
The RW in the suffix stands for DVD+ReWritable, a format of a rewritable DVD disc that uses phase change recording technology and conforms with ISO/IEC 17341:2005. +RW format has a capacity of 4.70 Gbytes (120 mm) and 1.46 Gbytes (80 mm) per side.
DVD+RW is an erasable format that is based on CD-RW technology. It is not supported by the DVD Forum despite some of the member companies still backing it. The format uses a high-frequency wobbled groove that provides for highly accurate sector alignment during the recording process. There is no embossed addressing information like that found in other recording mediums. For this DVD+RW format, data can be written to and read many times from the disc. Most DVD+RW drives will also write to CD-R and CD-RW media.
Differences between “- or +” RW
The DVD-RW and DVD+RW technologies are similar in nature with the -RW using the wobbled groove and the +RW using the high-frequency wobbled groove. The +RW Version 2.0 will format as either CAV or CLV, whereas the -RW is limited to CLV.
The -RW format is specifically suited for audio and video recording (but not for computer data storage or backup), whereas +RW can be used for both data and audio/video.
“Plus” Formats
The former DVD+RW Alliance was brought together as a group of electronic hardware, optical storage, and software manufacturers who in 1997 created and promoted a format standard of recordable and rewritable DVDs, known as the “plus” format. As of 2004, plus format DVDs were available in three forms: DVD+R, DVD+RW, and DVD+R DL. In late 2005, DVD+RW DL (dual layer) was released. With that and the emergence of Blu-ray Disc and HD DVD, the development of the DVD as an essentially sustainable media has ceased.
The Alliance had two major working groups. The DVD+RW Product Promotions Group was responsible for the promotion work of the Alliance for the plus format products. The DVD+RW Compatibility and Convergence Group was responsible for working on those technical issues related to the compatibility between the various hardware products using the plus format.
At the time of its formation, the alliance members included the following:
Dell Computer Corporation
Hewlett-Packard (HP) Company
Mitsubishi Chemical Corporation
Royal Philips Electronics N.V.
Ricoh Company, Ltd.
Sony Corporation
Thomson SA (RCA)
Yamaha Corporation
The unofficial site that represented this Alliance, http://www.dvdplusrw.org states that it dedicated to the DVD+RW and DVD+R format. Furthermore, the organization states it is “by far the largest independent online source for DVD+RW information.”
Blu-ray Disc
Blu-ray Disc (BD) is a high-density physical media format with a capacity of 25 Gbytes per layer, which uses a blue-ultraviolet laser and a 0.1-mm data depth. The much thinner cover layer enables higher bit densities, but its manufacturing processes required that significant changes to production equipment be made.
Initially, Blu-ray was intended for consumer/home recording, professional recording, and data storage. The first release of a Blu-ray disc recorder by Sony (in Japan, April 2003) was designed for home recording only and worked only with Japan’s digital HD broadcast system. Later on, the development of a read-only format, called BD-ROM, enabled and promoted the mass market distribution of prerecorded movies.
The principal Blu-ray backers were Dell, Hitachi, HP, LG, Panasonic, Philips, Pioneer, Mitsubishi, Samsung, Sharp, Sony, and Thomson (see Table 8.2 for the various Blu-ray disc formats and drive speeds).
Technical Details for Blu-Ray Disc
The base Blu-ray Disc format has a capacity of 25 Gbytes (per layer) and employs a 0.1-mm recording depth as a method to reduce aberration from disc tilt. The laser is a 405-nm blue-violet semiconductor with 0.85 NA (numerical aperture) lens design to provide 0.32-µm track pitch (which is half of what the pitch of a DVD is) and as little as 0.138-µm pit length. Figure 8.2 compares additional technical format differences (such as wavelength, capacity, and ratio of capacity) between the DVD and the Blu-ray disc formats.
Variations include 23.3-Gbyte capacity with 0.160-µm minimum pit length (used by Sony’s Professional Disc system) and 25-Gbyte capacity with 0.149-µm minimum pit length. The physical discs use phase-change groove recording on a 12-cm diameter, 1.2-mm thick disc, similar to DVD-RW and DVD+RW. The data transfer rate is 36 Mbits/second. The recording capacity for a single layer is about 2 hours of HD video (at 28 Mbits/second) or about 10 hours of standard definition video (at 4.5 Mbits/second).
The two base formats for Blu-ray are called Blu-ray Disc Rewritable (BD-RE) and Blu-ray Disc Recordable (BD-R).
Increased Capacity Specifications
Sharp promoted a Blu-ray disc type with a recording capacity based on the BDXL specification created by the Blu-ray Disc Association’s member companies, which allows for discs with capacities as high as 128 Gbytes.
The current Blu-ray Discs offer a dual-layer format for capacities up to 50 Gbytes. This new technology allows for triple (100 Gbytes) and quadruple (128 Gbytes) layer discs. In an optical disc recording, the storage capacity of the disc is principally determined by the spot size, which is proportional to the lightsource wavelength λ and inversely proportional to the numerical aperture (NA) of the objective lens. Thus, capacity is in inverse proportion to the square of spot size.
Figure 8.2 Comparison of DVD (= 650 nm) to Blu-ray (= 650 nm) Disc in terms of wavelength, capacity, and ratio of capacity (information provided by the Blu-ray Disc Association).
The latest technology has discs with 100-Gbyte recording capacities (launched only in Japan) as of July 2010 and with a player (from Sharp) that will handle the format also introduced in Japan only. These versions had no other scheduled delivery plans outside of Japan as of October 2010.
Development is expected to continue, with progress made in additional formats and multilayered technologies.
Higher Density Multilayer
In response to meeting the promises of the Blu-ray Disc made when the format was first released in 2003, the Blu-ray Disc Association announced the BDXL and IH-BD formats. Both BDXL and IH-BD are specially designed formats with specific market segments in mind. Newly designed hardware is required to playback or record BDXL or IH-BD media. However, because the new media specifications are extensions of current Blu-ray Disc technology, future BDXL and IH-BD devices can be designed to support existing 25-Gbyte and 50-Gbyte Blu-ray Discs.
Note that for PlayStation users, there is no hope that a firmware update will save the Playstation 3 (PS3) this time.
BDXL
In April 2010, the Blu-ray Disc Association announced a new BDXL (High Capacity Recordable and Rewritable disc) format capable of storing up to 128 Gbytes (write-once) or 100 Gbytes (rewritable). The BDXL format is known also as BD-RE Version 3.0; it also carries the tag “TL” (triple layer). The wavelength of the LD remains 405 nm, with the first two individual cover layer thicknesses (L0 = 100 µm and L1 = 75 µm) also the same as SL and DL BD-RE Version 2.1. The third layer, L2, has a cover layer thickness of 57 µm.
The minimum mark length for BD-RE is 149 nm, and for BDXL it is 112 nm. The data transfer rate is the same as the higher end of BD-RE, at 72 Mbits/second, with the write speed for media at 2×.
The format requires a new player to access these discs since the format goes three or four layers deep and will likely require a more powerful laser. Aimed at mass storage for corporate data uses, who are currently still using other mediums for archiving (e.g., LTO or DLT tape), this disc hopes to be a new permanent archive option, along with the new Intra-Hybrid Blu-ray Discs (IH-BD), designed with one 25-Gbytes read-only layer and one 25-Gbytes rewritable layer on the same platter.
IH-BD
The Intra-Hybrid Blu-ray Disc (IH-BD) incorporates a single BD-ROM layer and a single BD-RE layer so as to enable the user to view, but not overwrite, critical published data while providing the flexibility to include relevant personal data on the same physical disc. This lets consumer specific applications, in which combining published content with related user data on a convenient, single volume is desirable, move forward with these new features.
Both the ROM and the RE layers on IH-BD discs provide 25-Gbytes of capacity.
HD DVD
Blu-ray Disc’s format competition was the now defunct HD DVD (High-Definition/Density DVD), developed by Toshiba. HD DVD’s first consumer-based release was in Japan on March 31, 2006, with the United States joining on April 28, 2006. Although their product introduction beat Blu-ray by nearly three months, it was not long before HD DVD was found to be rejected by consumers and eventually the studios and others stopped supporting it.
Before 2003, when the DVD Forum had selected Toshiba’s Advanced Optical Disc (AOD) format as the successor to DVD, the company promised it would be finished sooner than Sony’s Blu-ray. The DVD Forum subsequently renamed the Toshiba format “HD DVD” to associate it with the DVD product. Meanwhile, Sony continued its work on Blu-ray, even though their efforts looked significantly behind. The Blu-ray technologies, which included the blue-ultraviolet laser change, would require extensive retooling of the manufacturing lines. Thus, it was believed that HD DVD would reach the market faster and cheaper in essence because of its similarities to the DVD player assembly, as well as the support that came from Microsoft on the PC desktop, which also added the format in its Xbox 360 game console.
The Walt Disney Company’s then CEO Michael Eisner sold HDi (once called “iHD”) and the Windows Media Digital Rights Management (DRM), hoping again to gain studio acceptance. On the surface, it looked as though Microsoft now had the time, technology, and studio deals on its side, whereas the Blu-ray technology was falling further behind and was complicated even more by both its new manufacturing requirements for the disc and the player and its floundering support from the studios.
The entire competitive landscape was frequently compared with the Betamax/VHS format battles of the 1980s that created a format war that confused and irritated consumers.
Figure 8.3 Format for Selected Optical Recording Media Globally
It became obvious by the end of 2004 that Toshiba would not be delivering their HD DVD a year ahead of Blu-ray as promised. The industry began to view Blu-ray as more credible, despite the DVD Forum’s continued commitment to HD DVD. Sony had successfully developed more hardware partnerships than Toshiba, which remained the only significant manufacturer of HD DVD players. The struggle by Toshiba continued, and by 2005, there was speculation that the company was ready to drop HD DVD and join the Blu-ray consortium, but Microsoft pushed to continue with the plan.
The first HD DVD players still were not ready until early 2006. Blu-ray players would debut just a few short weeks after the HD DVD players were launched. However, by year end 2006, Microsoft had begun selling an external HD DVD player (which cost approximately $200) for the Xbox 360 at about the same time Sony introduced its PlayStation 3 with an integrated Blu-ray player.
The Obsoleteness of HD DVD
The industry had heavily backed Blu-ray; but in fairness, Microsoft had worked equally to create the perception that HD DVD was a viable product. The reason, many believe, is that the HD DVD platform would be Microsoft’s last effort to gain adoption of its VC-1 (by now a SMPTE Standard) and HDi (formerly iHD) standards, and its implementation of the Advanced Content interactivity layer in the HD DVD, which they used in the Xbox 360 HD DVD add-on, as well as stand-alone HD DVD players.
Microsoft was further unsuccessful in getting WinCE into other embedded markets (ranging from music players to handheld computers to smart phones) and had similarly failed to establish Windows Media as a download format against ISO’s AAC and H.264, popularized by Apple’s iTunes.
In a final act of desperation, HD DVD managed to sign up Paramount and DreamWorks as new exclusive movie studios for HD DVD, pitting roughly half of the motion picture studios behind each of the two rival formats. Warner Bros. was unique in that it offered titles in both formats. Right or wrong, Microsoft’s efforts to prolong the format war had nothing to do with the players or the media; it was driven by the promotion of its proprietary software for platforms that were already segregated by the marketplace.
Confused by the format uncertainty, there were slow sales across the board. With Microsoft’s refusal to cooperate, Warner Bros. then announced a pullout of HD DVD just prior to the Consumer Electronics Show (CES), which destroyed the momentum of Microsoft’s HD DVD marketing push at what is the largest annual consumer electronics showcase for new products. Not long after this, retailers including Wal-Mart withdrew support for HD DVD, putting their support behind Blu-ray.
With the obsoleteness of HD DVD, the strategy behind Microsoft’s aspiration to inject its hand strongly into media development was deflated, plus it caused a huge hole in the VC-1 standard despite the fact that it had already been written into the Blu-ray standard along with H.264. Blu-Ray developers were moving toward H.264, which allowed them to master progressive scan HD discs; in addition, in the high stakes of consumer electronics, H.264 would ultimately drive the higher performance 1080p screens, dubbed “true HD” (for marketing purposes only), and change the course of movie watching all over again.
H.264 would become the replacement for MPEG-2 as a compression technology that would allow delivery to mobile devices, as well as downloadable versions using the same codec for playback on devices such as the PSP and iPod. Bluray Disc would enable the much desired digital rights management (DRM) needed to satisfy the studios, which along with High-Definition Multimedia Interface (HDMI Version 1.3 and beyond) would add insurance to the copy protection the studios demanded through High-bandwidth Digital Content Protection (HDCP).
Holographic Storage
Holographic storage is truly a form of solid state storage that uses optical technologies and a polycarbonate plastic-like media for containing the data. Employing the same principles as those in producing holograms, laser beams are used to record and read back computer-generated data that is stored in three dimensions. The target for this technology is to store enormous amounts of data in the tiniest amount of space.
Holography breaks through the density limits of conventional storage by recording through the full depth of the storage medium. Unlike other technologies that record one data bit at a time, holography records and reads over a million bits of data with a single flash of laser light. This produces transfer rates significantly higher than current optical storage devices. Holographic storage combines high storage densities and fast transfer rates, with a durable low-cost media at a much lower price point than comparable physical storage materials.
The flexibility of the technology allows for the development of a wide variety of holographic storage products that range from consumer handheld devices to storage products for the enterprise. Holographic storage could offer 50 hours of high-definition video on a single disk, storage of 50,000 songs on media that is the size of a postage stamp, or half a million medical X-rays on a credit card.
Technology for Holographic Data Recording
Light from a single laser beam is split into two beams, the signal beam (which carries the data) and the reference beam. The hologram is formed where these two beams intersect in the recording medium. The diagram in Fig. 8.4 shows the schematic view of both the write and read functions for holographic recording and reproduction.
Figure 8.4 Schematic of holographic storage showing write and read functions.
WORM and Rewritable
Holographic media can be “write once-read many” (WORM), where the storage medium undergoes some irreversible change, and “rewritable” where the change is reversible. Rewritable holographic storage is achieved via the photorefractive effect typically found in crystals. Data that is written to a holographic device occurs with a single flash of the laser, effectively in parallel, which is unlike current storage technologies that must record and read one data bit at a time. The system works through the principles of mutually coherent light that comes from two sources that are blended to create an interference pattern in the media, that is, areas that are light and areas that are dark.
Holograph Recording Physics
In the areas where there is constructive interference, the light is bright. In this region, electrons are promoted from the valence band to the conduction band of the material, given that the light has relinquished the electrons’ energy to cross the energy gap. Positively charged vacancies are called holes. The holes must be immobile in rewritable holographic materials.
Where there is destructive interference, the amount of light is far less, thus much fewer electrons are promoted.
Electrons in the conduction band are free to move in the material. Their movements are determined by two opposing forces presented to the electrons. The first force is the Coulomb force between the electrons and the positive holes that they have been promoted from. The force either drives the electrons to stay where they are or it moves them back to their original location. The second force is a pseudo force or diffusion that encourages movement to areas where electrons are less dense. When the Coulomb forces are not too strong, electrons will move into the dark areas.
The strength of the hologram is impacted by the recombination factors deciding whether or not a hole moves back to a valance band. Immediately after being promoted (i.e., a result of the Coulomb forces), there is a possibility that a given electron will be found to recombine with a hole and thus move back into the valence band. The faster the rate of recombination the fewer the number of electrons that have this opportunity to move into the dark areas. After some electrons have moved into the dark areas and recombined with those holes, a permanent space-charge field between the electrons that moved to the dark spots and the holes in the bright spots is left. This electro-optical effect leads to a change in the index of refraction.
Only a reference beam is necessary for the information to be retrieved, which is “read out” from the hologram. The reference beam is sent into the material in a way precisely identical to when the hologram was first written. As a result of the index changes in the material that were created during writing, the beam splits into two parts. One beam part recreates the signal beam where the information is stored, which is not unlike the way a CCD imager converts its information into a more usable form.
Theoretically, holograms are capable of storing one bit per cubic block, equal to the size of the wavelength of light used in writing its data. For example, 632.8-nm wavelength light from a red-laser could store 4 Gbits/mm3. However, in actuality, the data density would be far lower, due in part to the need for adding error-correction, imperfections in the optical systems, costs of producing a “perfect” medium, and other technology and manufacturing limitations.
Write Functions
The process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator (SLM). The SLM translates the electronic data of 0s and 1s into an optical “checkerboard” pattern of light and dark pixels. The data are arranged in an array or page of over one million bits. The exact number of bits is determined by the pixel count of the SLM.
At the point where the reference beam and the data carrying signal beam intersect, the hologram is recorded in the light sensitive storage medium. A chemical reaction occurs causing the hologram to be stored into the media. By varying the reference beam angle or media position with minute slight offsets, layers of hundreds of unique holograms are recorded in the same volume of material (see Fig. 8.5 for the write function in pictorial form).
Figure 8.5 Hologram of write function with reference beam and signal beam combining to form the hologram.
Reading Data
In order to read back the data, the reference beam deflects off the hologram, in turn reconstructing the stored information. This hologram is projected onto a detector that reads the entire data page of over one million bits all at once. This distinguishing principle of parallel read-out provides holography with its fast transfer rates (see Fig. 8.6 for the read function in pictorial form).
Figure 8.6 Hologram data recovery uses only the single reference beam and the detector array.
Holographic Innovations
Although no one has successfully mass marketed this technology, vendors have been working on the commercialization of the technologies for well over a decade. The best known, most publicized development and line of products came from InPhase Technologies. Originally founded in December 2000 as a Lucent Technologies venture that spun out of Bell Labs research, InPhase worked on a holographic storage technology capable of storing in excess of 200 Gbytes of data, which could be written four times faster than the speed of current DVD drives.
Demonstration
Partners in developing, using, and testing the technology have included Turner Broadcasting System (TBS), who in conjunction with InPhase and the Hitachi Maxell prototyped recording medium, provided for one of the more formidable tests in October 2005 when engineers from InPhase and TBS introduced a promotional advertisement into InPhase’s Tapestry holographic disk as a data file.
The content was recorded using InPhase’s holographic prototype drive onto a holographic disk manufactured by Hitachi Maxell (an InPhase partner and investor). The file was then played back from the drive, electronically migrated to a videoserver and played back to air.
TBS was investigating the feasibility of using holographic storage for broadcasting television content, especially given the need to store high-quality, high-definition (HD) movies with their extremely large data requirements. The capacity of holographic disks enabled TBS to store broadcast programming as files on media other than spinning magnetic disks (which consume massive amounts of space and power) or physical data tape (e.g., LTO-4). The real value, according to TBS, is that the high data transfer rate allowed TBS to migrate files on and off the videoserver disks and the holographic media quickly.
The InPhase Tapestry holographic system was capable of storing more than 26 hours of HD video on a single 300-Gbyte holographic disk, which was recorded at a 160-Mbit/second data rate. Maxell’s holographic removable drive housed one 13-cm optical disk with storage capacity for up to 150 million pages, more than 63 times the capacity of a DVD.
The line of 300-Gbyte InPhase Tapestry products was to commence in 2006, which was to represent the initial offering in the family of InPhase holographic drives and media. According to predictions from Maxell, the product road map would have put capacities ranging up to 1.6 Tbytes and with data rates of 960 Mbits/second by year end 2010.
In a joint demonstration with Ikegami at the IBC 2007 in Amsterdam, an initial product was shown based on an Ikegamibranded 300 Gbytes external holographic drive associated with a PC. The demonstration intended to show a cost-effective, tapeless solution for archiving large video files finished on nonlinear editing systems and acquired with Ikegami Editcam and Editcam HD tapeless camcorders. The external holographic drive enabled users of Ikegami’s camcorders to transfer edited or camera-original video content via FireWire or FTP interfaces to highly stable 300-Gbyte holographic cartridges for archiving and retrieval.
The versions shown were not rewritable, although that was expected to change as development continued.
Marketplace
These products were not intended for the consumer, as DVD and Blu-ray Discs were. Due to the high costs of the recording platforms, this marketplace was aimed at broadcasters, among other users such as medical data or other permanent high data concentrations.
Holographic Hiatus
Since these tests, InPhase had ceased business and then reemerged only a month later on March 18, 2010 when Signal Lake, an investor in next-generation technology and a computer communication company, announced that it had acquired the majority stake in InPhase Technologies, Inc. Signal Lake was the original founding lead investor in InPhase that helped it spin out of Bell Laboratories in conjunction with the then Lucent New Ventures Group in December 2000. As of late October 2010, the two principle venture capital investors (Acadia Woods Partners and Signal Lake) were at odds on how to restart the InPhase organization (more information on InPhase and holographic storage for archive purposes is included in Chapter 12).
Collinear Dual Laser Approach
Traditionally, holography will employ two beams, a separate signal (data) and a second reference beam, to create the interference pattern that is then stored onto the holographic material. The methodology requires a precise alignment and is complicated even further when the light must be focused onto rotating media that would be compatible with CD and DVD-like drive designs. An alternative approach uses two laser beams that are concentrated into a single beam of coaxial light. The concept creates a three-dimensional hologram composed of data fringes.
The player system is housed in a ruggedized platform similar to an industrial DVD player. By employing a server system and a single objective pickup system, tracking and focus are better controlled. With a media rotation speed of 300 rpm, such a system will establish 23,000 pulses per second, as the lens floats above the revolving disc. The lens system has an integral microadjustment servolike system that compensates for flutter or vibration.
Two beams of differing wavelengths are used. For data recording and reading, a 532-nm green solid laser is employed. For the servo tracking control and focus, a red semiconductor laser is used to read a built-in reference track embedded into the media in a pitted aluminum substrate. The red laser is selected so that it will not photosensitize the holographic recording material (refer to Fig. 8.7).
Figure 8.7 Collinear holography with its two laser beam formats.
A dichroic mirror is employed to converge the two beams onto the same optical path before entering the objective lens. The disc media is coated with a reflective film, and the objective lens provides for the correct focus of the beam on to the film. Functionally, the recording laser beam is divided by beam splitters into the information beam and the reference beam. The information beam is converted into 2D page data through the employment of a digital micro-mirror device (DMD), similar to that used in DLP displays for television or digital cinema projection. The two beams are then merged back into the same optical axis through beam splitters incident to the objective lens. The holographic medium then captures the data in the form of interference patterns.
To reconstruct the data stored in the hologram, the reference beam is channeled incident to the objective lens. This “reconstruction beam” passes through the objective lens and is returned to a CMOS sensor and decoded using a Fast Fourier Transform (FFT).
Data Security Opportunity
This technology offers new possibilities for data protection. Since the 3D hologram is recorded using the 2D page data and is surrounded by a reference pattern formed collectively through the mirrors of the DMD device, the page data cannot be read back except by reproducing the exact reference pattern that was used to record the data. This essentially becomes a form of encryption, with over a million key combinations for each 2D page. Each page could employ a different reference key. The Holographic Versatile Disc (HVD) system is capable of writing 23,000 pages/second, which could yield a fully encrypted disc that would require 22 gigakeys/second to unlock.
Holographic Versatile Disc
Optware Corporation, in Yokohama, Japan, was established in 1999 as a development venture for holographic recording technology developed from a team of former Sony optical engineers who developed a different method of holographic storage called collinear Holographic Versatile Disc (HVD), which was described previously in this chapter. The technology was designed to enable storage of up to 3.9 Tbytes of data on a CD-sized disc with a data transfer rate exceeding 1 Gbit/second.
In 2005, Optware stated that it planned to release a Holographic Versatile Card (HVC) media product around the end of 2006, with a capacity of around 30 Gbytes. A reader device and a reader/writer device would also be launched as part of their HVC-related products to coincide with the standardization of the technology, which was expected in December 2006 by Ecma International, the organization promoting standardization of information and communication technologies. The card would be almost the same size as a credit card, with the drive system to be the size of a surface-mounted hard disc drive system.
As of 2010, with no further announcements or development in the limelight, and potentially as a result of the proliferation in high capacity Blu-ray Disc (BD) technologies, it is unclear where the product line will go.
Holographic Forums
The Holography System Development Forum (HSD Forum), which was originally formed from the HVD Alliance and the HVD FORUM, is the coalition of corporations with a purpose to provide an industry forum for testing and technical discussion of all aspects of HVD design and manufacturing. As of August 2009, the HSD Forum consisted of over 20 corporations including Apple, Mitsubishi, Fuji Photo Film Company, Optware, Hitachi, and many more.
Holographic Disc Cartridge (HDC) Standards
At its eighty-eighth General Assembly (December 2004), Ecma International, a private membership-based nonprofit SDO, established its Technical Committee 44 (TC44), which was to be dedicated to standardizing HVD formats based on the Optware technology. On June 11, 2007, TC44 published the first two HVD standards: ECMA-377, which would define a 120-mm diameter, 200-Gbyte HVD recordable cartridge; and ECMA-378, defining a 120-mm diameter, 100-Gbyte HVD-ROM disc. The next stated goals are for the 30-Gbyte HVD cards, followed by the submission of these standards to the International Organization for Standardization (ISO) for approval.
Micro-Holographic Storage
April 2009 saw General Electric (GE) Global Research demonstrating a micro-holographic storage material that could eventually result in the capability of storing as much as 500 Gbytes of data on a standard 120-mm Blu-ray/DVD-sized disc.
Unlike the DVD or Blu-ray Disc that store data only in the surface layers of the disk (for Blu-ray Disc, this is single, dual, triple, and potentially quadruple layers), the holographic storage involves storing data throughout the entire disk in multiple layers. The hardware and format of GE’s holographic storage technology will be similar to that used in current optical storage technology, allowing micro-holographic players to be backward read-compatible with existing CDs, DVDs, and Blu-ray Discs.
Targets and Worries
Long-term archiving of medical information in a nondestructible format is certainly a target for these forms of optical storage. The hardship is that holographic storage technology research and preliminary product development is now exceeding a full decade or more. Every 5 years, the industry is finding a new storage technology; the issues become not in making the technology work, but in dealing with concern over the support of legacy hardware and apprehension over the adoption of newer technologies. Thus, we ask the question, “Will we need to continually migrate to a new technology that barely has any track record of survival?”
Care and Handling
Optical media should be handled on its edges only, as the materials are subject to physical damage that could render some or all of the data unrecoverable. Compact discs may be prone to damage from both normal use and the effects of environmental exposure. On a CD, the pits are much closer to the label side of a disc, thus enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more susceptible to damage occurring on the label side of the disk.
Scratches that appear on the disc’s clear side may be repaired by refilling them with similar refractive plastic or by carefully polishing the surface with an optical cleaner.
Some guidelines to caring for physical media include the following:
•Allocate a cool, dry, dark environment where the air is clean for disc storage.
•Only handle discs by the outer edge or the center hole.
•For marking the label side of a disc, only use a nonsolventbased felt-tip permanent marker; avoid adhesive labels.
•Avoid getting any dirt or other foreign matter to contact the disc; remove any foreign material (including fingerprints, smudges, and liquids) by wiping with a clean cotton fabric in a straight line from the center of the disc toward the outer edge. Never wipe in a direction going around the disc.
•Avoid bending discs, or exposing them to sunlight or extreme temperatures
•Use plastic cases that are specified for CDs and DVDs, and return them to their cases after use, storing them in upright position, that is, “book style.”
•Do not open a recordable disc package until you are ready to record data onto the disc, check the disc before recording, and clean the disc or discard a noncleanable disc.
•Use a CD/DVD cleaning detergent, isopropyl alcohol, or methanol to remove stubborn dirt or material.
Further Readings
Care and Handling of CDs and DVDs—A Guide for Librarians and Archivists. http://www.itl.nist.gov/iad/894.05/papers/CDandDVDCareandHandling-Guide.pdf
Normative References (from ITL/NIST)
ISO 18927:2002 Imaging materials—Recordable compact disc systems—Method for estimating the life expectancy based on the effects of temperature and relative humidity, first edition
IEC 60908 (1999–02): Compact disc digital audio system. This document including amendments approximates the Philips-Sony Red Book
ISO/IEC 10149:1995 Read-Only 120 mm Optical Data Disks (CD-ROM)
EMCA130 2nd Edition—June 1996 Data interchange on read-only120 mm optical data disks (CD-ROM)
Orange Book, part B—Recordable Compact Disc System, November 1990 (SONY and Philips Corp.)
ISO/IEC 16448:2002 Information technology—120 mm DVD Read-only disk
ISO/IEC DIS 23912:2005 Information technology—80 mm (1.46 GBytes per side) and 120 mm (4.70 GBytes per side) DVD Recordable Disk (DVD-R)
ISO/IEC 17344:2005: Information technology—Data interchange on 120 mm and 80 mm optical disk using +R format—Capacity: 4.7 GBytes and 1.46 GBytes per side
ISO/IEC 17342:2004: Information technology—80 mm (1.46 GBytes per side) and 120 mm (4.70 GBytes per side) DVD re-recordable disk (DVD-RW)
ISO/IEC 17341:2005: Information technology—Data interchange on 120 mm and 80 mm optical disk using +RW format—Capacity: 4.7 GBytes and 1.46 GBytes per side
ECMA-267, 2001, 120 mm DVD Read-Only Disk, 3rd edition
ECMA-337, DVD+RW—Rewritable Optical Disks, 4.7 Gbytes
ECMA-338, DVD-RW—Rewritable Optical Disks, 4.7 Gbytes)