CD-ReWritable: The Storage Revolution Continues
CD-ReWritable: The Storage Revolution Continues
By G.A. Marken, President, Marken Communications Inc., marken@cerf.net
In late 1996, an industry consortium that now includes Ricoh, Philips, Sony, Yamaha, Hewlett-Packard, and Mitsubishi Chemical Corporation, announced CD-ReWritable to the world. CD-ReWritable is the next stop on theCompact Disc road map that started in 1980 with the introduction of CompactDisc Digital Audio by Philips and Sony.
The CD-ReWritable disc is based on Phase-Change technology, and can store 650 Megabytes of computer data or 74 minutes of music on a standard CD-sizedisc. There are no new formats associated with CD-RW; the existing CDformats such as CD-ROM, CD-Audio, PhotoCD, CD-I, etc. can all be used with CD-RW media. The CD-RW disc can be erasedand re-written about 1,000 times (called cycleability), making it ideal for applications such as back-up and prototyping. The CD-ReWritable recorders are multifunction drives capable of recording CD-RW discs as well as CD-R discs, and reading/playing both types of recordable media and stamped discs.
CD-ReWritable is a "bridge" technology. The new CD-RW media is designed to be played back on new generation CD-ROM drives, including DVD drives. It is still not clear if these DVD drives will be compatible with CD-R media. With its multifunction capability, a CD-ReWritable recorder can create CD-RW discs that are readable on the new DVD readers, as well as CD-R discs that are fully compatible with the current generation of CD-ROM drives and CD players. The recorder will also "bridge" the current and future generation readers.
Standards have been established for CD-ReWritable to ensure that all compatible recorders and readers can interchange the media: A new chapter has been added to the Orange Book--Part III--that specifically addresses CD-ReWritable. A logical interchange standard has also emerged: Universal Disc Format (UDF), which brings packet writing into a cohesive standard. Finally, as part of the CD-ReWritable Consortium, a new certification program called Multi-Read (MR) has been established to help CD-ROM drive manufacturers make future-generation CD-ROM drives CD-RW Ec UDF-compatible.
The cost of the media helps define the applications for CD-RW and CD-R: the CD-RW media has an initial MSRP of $20 - 25, and CD-R media is now well below the $5 mark. Because of its low cost and easy interchange with standard CD-ROM drives, the CD-R disc continues to be the best way to distribute data. In addition, because of its write-once nature, the CD-R disc is well-suited for archiving data because the files cannot be modified. The new CD-RW media is ideal for back up and prototyping, and other applications where the disc is likely to stay in-house.
The CD-ReWritable drive is best described as a multifunction CD recorder that can:
* Record and read/play CD-ReWritable (CD-RW) discs
* Record and read/play CD-Recordable (CD-R) discs
* Read/play commercially replicated discs
The system can be thought of as "CD-R Plus", since it really is just a CD recorder, but with the added function of using the new CD-RW discs. A CD-ReWritable system would consist of a CD-ReWritable recorder, host computer, interface card (SCSI or ATAPI), mastering software, and media. In addition to being able to rewrite CD-RW discs, the recorders can utilize the Universal Disc Format (UDF) logical file system instead of ISO9660 to store data on the CD-R or CD-RW discs using packet writing. Packet writing has long been heralded as the best way to record a CD since the recorder can start and stop the recording at will, eliminating buffer underrun.
CD-R drives write data to the disc in a permanent way, the laser burns the pits (actually bumps) into the organic dye layer of the CD-R disc by heating the spot to be recorded to about 300 - 400 degrees Celsius (temperature will depend on what speed the disc is recorded at). These recorded bumps have lower reflectivity than the surrounding lands, and are optically very close the pits found in a stamped CD, having the same light scattering effects. Generally, the reflectivity of a CD-R disc is around 65%, meaning that 65% of the laser beam's light is reflected back to the read head.
In contrast, a Phase-Change recorder uses a higher-power laser to change the phase of the recording layer from its highly reflective "crystalline" state to a lower reflective "amorphous" state. This is accomplished by heating the spot to be recorded with a higher-power laser to about 600 degrees Celsius. As the spot cools, it becomes a amorphous "mark" that is very close optically to the pit on a stamped CD. To change the spot back to the crystalline state, the laser uses a lower power setting and heats the amorphous mark to its glass transition temperature (about 200 degrees) and the spot will transform back to the crystalline state. The CD-RW system is Direct Overwrite-compatible (DOW), which means that new data can be recorded directly over the old, eliminating the need to pre-erase.
Unfortunately, the Phase-Change disc has very low reflectivity, around 25%. This is what prevents older CD-ROM drives from reading the Phase-Change CD-RW disc, the read heads in these drives cannot compensate for the lower reflectivity. The Red Book (the grandfather CD standard) specifies that discs will have 70% reflectivity. CD-R discs with 65% reflectivity are close enough optically that CD-ROM drives and CD players can read them with few exceptions. Newer "Multi-Read" (MR) drives have a simple circuit called Automatic Gain Control (AGC) that boosts the gain of the CD-ROM drive's read head to compensate for the lower reflectivity of the CD-RW Phase-Change disc. DVD-ROM drives will also have this AGC circuit.
The Phase-Change system employed in CD-ReWritable is actually the reverse of other Phase-Change systems in use. Most other systems record a crystalline mark on amorphous background (called burn bright) as opposed to CD-RW which records a amorphous mark on a crystalline background (burn dark). This is because the burn dark system is optically closer to the higher reflective lands and light scattering effects of pits on a stamped CD.
How the Recorder Works
The CD-RW recorder is only slightly different from a regular CD-R recorder. The changes are mostly in electronics. The CD-RW recorder must use a different "Write Strategy" to record on the CD-RW disc than is used on the CD-R disc. When recording on the CD-R disc, the OPC procedure is performed to select the correct laser power for that recording, and then the laser is turned off and on (modulated) to form the correct size (there are 9 possible bump lengths, just like pits on the stamped disc) bumps in the dye as it records the data. With CD-RW media, the laser is not only modulated off and on to form the correct size "marks," it is also modulated between write, erase and bias laser powers. Bias power is the same power used to read the disc and during recording it is modulated to keep the recording layer from absorbing too much heat, which would make distorted marks. The laser is modulated with the erase power because it is capable of directly overwriting old data. This process is called the Write Pulse Train (WPT). Based on the media inserted in the recorder, the firmware will set the correct Write Strategy. The laser power for recording CD-RW discs is higher than it is for CD-ROMs. For example, a CD-R disc recorded at 2X needs a laser in the 8 - 10 milliwatt (mw) power range. For recording a CD-RW disc at double speed, a laser in the 8 - 14mw range is required. In the case of CD-R or CD-RW discs, the laser wavelength remains 780mn. 1n addition, the CD-ReWritable recorders implement the AGC circuit to read back CD-RW compatible discs.
The OPC (Optimum Power Calibration) procedure for CD-RW discs is much like it's CD-R cousins. The recommended recording laser power is encoded in the Absolute Time In Pre- groove (ATIP) of the disc and is read by the recorder's firmware. The recorder then performs a test write of 6 sectors in the PCA (Power Calibration Area) of the CD-R disc with several different laser power values based on the recommended power, then reads the test sectors back and selects the laser power that produced the sector with the best reflectivity. With CD-RW media, it is much the same, except in addition to the recommended write power being encoded in ATIP, the recommended erase power and bias powers are also encoded and read by the recorder. Running OPC, a technique developed for CD-R that can alter the laser power on the fly to compensate for media irregularities such as dust or fingerprints can also be applied to CD-RW media.
CD-RW Media Functionality The only difference in the construction of a CD-RW disc and a CD-R disc is the recording layer. CD-R discs are constructed by injection molding the polycarbonate disc with it's wobbled "pre-groove." Next, the organic dye is applied by spin-coating the dye onto the disc to ensure a even distribution of the dye. A gold reflective layer is then applied, followed by a protective coat of lacquer to protect the gold and dye. The printing on top of the lacquer layer is the last to be applied. When recording, the laser enters the polycarbonate disc and uses the wobbled pre-groove for tracking and rotation control. The recording takes place in the pre-groove on the dye layer.
With CD-RW discs, the disc starts out with the same polycarbonate base with the pre-groove, next comes the recording layer, which actually consists of four layers: the lower dielectric layer, the recording laser, the upper dielectric layer, and the reflective layer. The upper and lower dielectric layers draw away heat from the recording layer to keep heat from building up. Instead of organic dye, the recording layer is made up of a quaternary Phase-Change alloy consisting of silver, indium, antimony, and tellurium (Ag-In-Sb-Te) These elements make the Phase-Change recording possible. The reflective part of the recording layer is an aluminum alloy, just like in stamped discs. Gold was required by CD-R discs to get the reflectivity as high as possible, and the organic dye tended to oxidize aluminum. After the recording layer, a layer of lacquer is applied to protect the recording layer "sandwich", and the printing is applied on top of the lacquer.
As with CD-R, the laser beam from the recorder enters the polycarbonate disc and uses the wobbled pre-groove for tracking and rotation control. The recording takes place in the pregroove, but the real action takes place in the alloy recording layer of the sandwich where the marks are recorded.
Until now, CD-R discs had to be recorded all in one session (disc-at-once), or track at a time (track-at-once). Disc-at-once is primarily used for mastering discs to be replicated by the thousands. Track-at-once is used to store data on the disc in a more casual manner called multi- session, which allows data to be added to the disc in separate recordings. The down side is that for a disc to be readable on a CD-ROM drive or CD player, the session must be closed, which writes the disc lead-in with table of contents, and lead-out. This takes about 15 Megabytes of space on the disc, as well as about 3 minutes to complete, no matter how little data was recorded.
Packet writing allows the recorder to write files at a time, instead of tracks at a time. Packet -- writing can be of fixed packet size (all 512Kb sectors for example), or variable length packet size (mixed sized packets to fit file size). Fixed packet is the closest to the way a hard disk drive stores data, sectors on hard disks are always the same size. A track is created on the disc and left open. Packets are added to the track (files usually have to be split up like a hard drive does to fit the file into several packets) at will. Because the drive is just recording individual sectors, it can start and stop recording at will. The biggest benefit is that there is no more buffer underrun. This makes CD recording very "usable" for the end user. 1t also paves the way for "save as" file saving directly to the disc, or dragging and dropping files from the File Manager/Explorer or Finder to the icon of the disc. Packet writing does not require the traditional pre-mastering software used to record track-at-once or disc-at-once discs. A driver or hidden application is used to split up the files into logical packets, and records them; all the complexity is hidden from the user.
A new file system, called Universal Disc Format (UDF), was developed specifically with CD-R/CD-RW packet writing in mind. It is much more flexible that ISO9660, which required all the files in a session to be recorded at the same time.
Packet writing is one of the keys to success of CD-ReWritable, since it appears to the user like a big floppy; they can store data how they normally do, and erase/overwrite at will.
CD-R Drive Compatibility
CD-ReWritable media is not compatible for recording on regular CD-R drives.
CD-RW media is also not generally readable in older CD-R drives, but because of the sensitive nature of CD-R drive read heads, some CD-R drives are able to read the CD-RW media once it has been recorded.
As previously explained, CD-RW media has much lower reflectivity (about 25%) than regular stamped discs, or even CD-R discs. Because CD-ROM drives lack the AGC circuit that allows the CD-ROM pick-up to compensate for the lower reflectivity, the CD-RW discs are not compatible with the CD-ROM drives. However, just like with some CD-R drives, there may be some of the faster CD-ROM readers that have sensitive enough pick-ups to read a recorded CD-RW disc.
Because of the economics of the CD-R/CD-RW media, the compatibility with CD-ROM drives is not e as big an issue as it once was. If a user is creating a disc for distribution to someone else, he or she will likely use a CD-R disc in the recorder because it costs about 90% less than a CD-RW disc.
DVD Compatibility - There is considerable attention being given to the compatibility of CD-R discs and DVD players. It's true that the organic dye used in CD-R discs is tuned for the 780nm laser wavelength. It's also true that when the 650nm laser that is used in DVD players tries to read a CD-R disc, the reflected light is out of phase. This basically means that CD-R discs are not compatible with DVD players using only a 650nm laser. Some DVD player manufacturers are using a two-optics approach in their drives to maintain backward compatibility with CD-R discs. Lower-density stamped discs pose no problems for DVD players, the light is in phase. There is a new effort by the Optical Storage Technology Association (OSTA) to make reading CD-R discs part of the requirements for a DVD manufacturer to put the Multi-Read logo on their drives.
CD-RW discs are fully compatible with the 650nm lasers used in DVD players. The Phase-Change alloy recording layer is not wavelength specific like the dye on CD-R discs. So long as the DVD player supports the lower density (650Mb) discs, then it should have no problem to read CD-RW discs.
The Multi-Read program was developed to bring CD-RW-compatible drives into the mass market. It is a self-certification that CD-ROM drive manufactures can advertise once they are compliant. In short, a MR-compatible CD-ROM drive will be able to:
* Read the lower reflective CD-RW discs
* Read a packet recorded disc
Because of the highly competitive nature of the CD-ROM drive market, most manufacturers support Multi-Read as a way to differentiate their products. Several of the top CD-ROM drive manufacturers have committed to support MR in their new models, including:
* Toshiba
* Plextor
* Sony
* Hitachi
* Panasonic
* Mitsumi
It's important to recognize that each of these manufacturers is on a different schedule to implement MR Drives that comply with the MR program will be advertised as such on the packaging/literature, or on the drive itself.
The MR program can also be applied to DVD drives, and there is a new effort by OSTA to make reading of CD-R discs an additional requirement for certifying DVD drives as MR-compliant. This will provide potential DVD drive buyers with the assurance that the drive can read all available CD media.
The MR program does not really apply to CD audio players, although it seems logical that some high end CD player manufactures may wish to make their products compatible with the lower-reflective.
CD-RW media. Music professionals will have some use for CD-RW media to back up sound files, but most will continue to make reference discs on CD-R discs playable at their client's location.
Back-up
The CD-RW media is ideal for back-up applications. Back ups typically happen incrementally (once a day, week, month, etc.), and the disc can be used to record a back-up session in packet, or track at once mode. Once the end of the disc has been reached, the back-up can start again at the beginning of the disc and overwrite the old data. With CD-R, once the end of the disc is reached, it must be thrown away or stored, it is not suitable for back up.
Low-cost Personal Storage
The CD-RW disc can be used like an optical drive with packet writing for low-cost personal data storage. The cost per megabyte of CD-RW media compared to MO, PD, Jazz, or similar media is much lower.
Software Prototyping
Software developers who distribute their software on CD have typically burn up plenty of CD-R discs during development of their product. They create a disc, then test the software. When they find a problem, they must throw away the CD-R disc, modify the software, then create another disc and start testing again. If this process were used with CD-RW media, then the disc could be rewritten each time a change was made. When the software was stable, they could load CD-R discs to create beta copies to send out to their outside testers.
Multifunction Jukeboxes
The CD-RW recorder is an excellent jukebox mechanism because it is able to handle three different media types: stamped discs, recordable, and rewritable discs. On a dedicated workstation, or over a network, users have the ability to read data, publish discs for distribution, and back-up; all on one drive.
Physical Standards: The Color Books CD-ReWritable is fully compatible with the Red Book (CD Audio standard), Yellow Book (CD- ROM standard), Green Book (CD-I standard), White Book (Video CD Standard), Orange Book Part 1I (CD recordable standard) in every way except reflectivity. The CD-RW disc is the same physical size as the other discs, data is stored in the same formats, and is played in the same fashion. However, as previously explained, the reflectivity is lower for the CD-RW disc, about 25%. Orange Book Part III specifically addresses CD-ReWritable technology. Orange Book Part III defines the lower reflective CD-RW disc, and how to record on it.
ISO 9660 is the original file system developed for CD-ROM drives. It provides for cross- platform interchange of discs on PC, Macintosh, and UNIX systems that were ISO 9660-compliant. One of the severe limitations of ISO 9660 is that the file system needs to know about all the files to be recorded in advance so the "Path Tables" and "Primary Volume Descriptor" (PVD) could be generated. These are the logical table of contents of what data is stored on the disc, and where it is located. Other limitations of ISO9660 included:
* File names must be only in capital letters, A - Z
* File names are limited to the DOS 8 + 3 format
* No more than 8 directory levels _ Only legal character besides letters is
the under score ( )
ISO 9660 was fine for creating discs with track-at-once or disc-at-once recording modes where pre-mastering software was used to select all the files in advance of recording, however, with the advent of packet writing, a more flexible file system was needed.
In 1995, the Optical Storage Technology Association (OSTA) began searching for a new logical file system that would be more flexible that ISO9660. As OSTA is an organization comprised of representatives from the major optical manufacturers, discussions led to the adoption of the Universal Disc Format (UDF). Essentially, UDF version 1.5 is very similar to ISO9660 in that it is capable of being cross platform. It does not, however, have the name and directory restrictions that ISO9660 imposed. With UDF, packet writing is handled in one of two ways:
* Track mode
* Random mode
In track mode UDF, a track on the CD-R/CD-RW disc is opened and the recording must take place sequentially in the track. There are areas in the track where a temporary PVD and path table are kept for updates to files. The track can be closed at any time and a session written around the packet track, using the temporary PVD and path table to build the PVD and path table for the session, or the track can stay in it's native state. A device driver would be required to read the disc back on a standard CD-ROM drive today unless the session was closed in 1SO 9660 format. UDF track mode can be used with CD-R or CD-RW media.
In Random mode, the disc is "formatted" into logical sectors, and these sectors are similar to those on a hard disk drive or optical drive. They can be randomly be written (and in the case of CD-RW media. randomly rewritten). Random mode UDF also provides for the defect management. and can remap bad sectors so data isn't stored in them. This is ideal for CD-RW as the cyclability is currently 1,000 overwrites. If a sector gets worn out, it can be mapped out and not used anymore, and a spare sector can be used. Random mode is clearly the most desirable way to use the disc, but has the most interchange problems, a device driver is definitely required to read this disc.
The good news is that ultimately UDF will be supported by the various operating systems such as MS Windows 95, NT, Macintosh, etc., then no device drivers will be required. In the meantime, UDF will allow packet writing to become a reality and interchangeable on CD-RW systems.
In the past, CD-R discs were written with Pre-mastering software which basically assembled all the files into an ISO9660 CD-ROM session, or formatting and arranging audio files for creating a music disc. This software was not always easy to figure out, and it was a standalone application, separate from the software used to create the content to be recorded. With the advent of UDF, packet writing, and rewritable media, the software used to record a CD has evolved.
The user of a multifunction CD-ReWritable drive will most likely use a combination of software to record discs, depending on the user's application: * Traditional Pre-Mastering Software * UDF Packet-Writing Software Back-Up Application Software
Traditional Pre-Mastering Software Traditional CD pre-mastering software, such as Easy CD-Pro from Adaptec, or GEAR from Elektroson, have been adapted for CD-ReWritable in a very easy way. When a CD-R disc is, inserted into the CD-RW recorder, the software and drive act just like a traditional CD-R drive. To support rewritable discs, a simple "erase" button is added into the software. If a CD-RW disc is inserted into the drive, it too is recorded just like a CD-R disc. All disc formats can be recorded, and disc-at-once, track-at-once, and multi-session recording works just like with a CD- R disc. For example, if the user is creating a prototype disc for testing and a new version is to be recorded, the erase button is used to erase the PMA (temporary table of contents). Depending on implementation, this can take a few seconds or a few minutes. Once completed, the CD-RW disc is ready to be rewritten with the new information. With the PMA area erased, the recorder thinks the disc is blank, the tracks and sessions present on the disc will be overwritten with the new data. If desired, the entire CD-RW disc can be erased, but this can take up to 40 minutes, and is not necessary. The traditional pre-mastering software will be used to create true pre- mastered CD-R or CD-RW discs in the CD-ROM (modes 1 & 2, forms 1 & 2), and CD Audio discs.
UDF Packet-Writing Software UDF is a much better file system for CD-R and CD-RW discs that will be recorded with packet writing. Packet writing allows the CD-R or CD-RW disc to appear as a drive letter or as an icon on the desktop for dragging and dropping files to them, just like a user would do with a hard disk or floppy disc. Because this type of software interface requires no learning curve for the user, it is highly desirable. Software such as Adaptec's DirectCD uses this type of approach. The current implementation for CD-R and CD-RW recorders uses the track incremental recording approach. This means that the data is recorded sequentially within an open track. The track can be closed at any time, or left open. Readback on a CD-ROM drive will require closing the track and session (for TOC to be written). If further recording is desired, a new track can be opened and recording can be continued. As with traditional pre-mastering software, an "erase" button is inserted in the DirectCD software so a user can erase the disc.
With track incremental, it is not possible to erase a file in the track. If a user drags a modified version of a existing file on the disc, the new version is recorded, and references to the old file are deleted. To reclaim the disc space, the disc must be erased (again, the time to erase can be a few seconds or a few minutes, depending on implementation). So, with track incremental, the CD-RW media acts just like the CD-R media, except the CD-RW media can be erased when the disc is full, or if the user wants to start over. Random mode UDF has also been implemented by the hardware and software. Random mode allows the disc to be "formatted" into logical sectors of the same size, then the recorder can randomly record and erase any individual sector, just like a floppy or hard disk drive. This will allow modification or deletion of specific files, and the ability to reclaim any disc space from "erased" sectors. Packet writing can only be used to create CD-ROM discs, and not ones to be interchanged. Discs created using packet writing are mainly used for personal file storage and kept with the drive. Some packet writing software allows the disc to be "finalized" in ISO9660 format, which would be suitable for interchange (CD-R discs mainly).
Back-Up is one of the key applications for CD-RW media. Most of the software companies producing back-up applications that support CD recorders are using track incremental packet writing. This is fine, because the disc acts just like a tape, and tapes are recorded sequentially. The software interface for back-up software is just the same as traditional back-up software. For supporting CD-RW discs, there is the "erase" button that will erase the CD-RW disc. The biggest benefit of using CD-RW discs is that if a file needs to be restored to the hard disk, access to the file on the CD-RW disc is direct random access, unlike the traditional way of streaming the tape to the place where the file resides. So a user can back-up on the CD-RW disc, taking advantage of compression that is part of many software packages, getting up to 1.3Gb on a 650Mb CD-RW disc. The software also usually supports CD-R discs as well, allowing a permanent archive of files, with compression and random access restoration. Because the recording is sequential, when it reaches the end of the disc, the user can simply start back at the beginning of the disc and overwrite the older back-up sessions.
CD-RW vs. MO, PD, Jazz, tape, etc. The best thing about CD-ReWritable is that its a Compact Disc. There are many other storage solutions that might be able to store 650Mb of data on a removable disk, but none of them have a standard like the Compact Disc does. Even though the CD-RW media may not be fully interchangeable with the CD-ROM drives of today, the recorder can produce a CD-R disc that is, and soon to be released CD-ROM and DVD drives will be able to read the CD-RW disc. Here is how CD-RW compares with the popular high-capacity removable storage solutions in the market:
* CD-RW vs. PD - PD is the closest real competition to CD-RW, as both use Phase-Change technology. However, the PD rewritable disc is of a proprietary format and can only be interchanged with other PD drives. Also, the PD drive is a slow CD-ROM reader, only 4X reading. All the CD-RW compatible recorders are at least 6X readers. Also consider cost per Megabyte. PD discs cost around $30, making the cost per Megabyte about $.05. CD-RW discs cost less than $20, making the cost per Megabyte $.03. Both store 650Mb on a disc.
* CD-RW vs. MO - MO is a close cousin to Phase-Change, but the high cost of the drives and media have limited them the certain niche markets. MO drives have unique cartridges that can only be read back in other MO drives (and with the same device driver). Also, the cost per Megabyte is higher. The current standard for 3.5" MO is 640Mb. And the cartridge costs around $15, so the cost per Mb is $.06, and CD-RW is only $.03 per Mb. Even the newest 5.25" MO drives with 5.2 Gb capacity cost per MB is $.04 ($100 cartridge), and the drive cost is three times higher than a CD-RW recorder. * CD-RW vs. Jaz - Like the other systems, Jaz requires another Jaz drive to read the data on the jaz cartridge, and the cartridges are expensive, $100 for a 1.0Gb cartridge, making their cost per MB $.10. Also, Jaz is a magnetic disk, which means the disk is only good for a few years before the magnetic properties of the disk deteriorate. CD- RW is good for 30 years or more.
* CD-RW vs. Zip - Magnetic cartridge drives like the Zip drives are relatively low capacity (120Mb). Because they use proprietary cartridges, they can only be interchanged with like drives. Additionally, the cost per megabyte of these magnetic cartridges is high, more than $.12 per Mb! Because Zip are magnetic technologies, the data is only good for a few years, CD-RW is good for 30 years or more.
* CD-RW vs. tape - Tape provides sequential access, you must stream to the point on the tape with the data, no direct random access. The only viable use is archiving; and even there, it is poor because the magnetic properties of the tape are only good for a few years, as opposed to being able to store data reliably on a CD-RW disc for 30 years or more.
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