23.3 CD-RECORDABLE / REWRITABLE

CD-Recordable or CD-R is the compact disc technology that allows the user to create their own CD's. Recordable CD's are "write once" media that can be played (like ordinary CD's) with CD-ROM drives, audio and other CD players installed in the world. Writable CDs, that is, CD-R discs, consist of a microscopic gold reflective layer and a photosensitive dye layer sandwiched between a protective lacquer coating and a polycarbonate substrate. The substrate has a microscopic groove spiraling around the data track of the disc, which helps the laser stay on course. The data burned on a CD-ROM consists of "pits" and "lands." To make a "pit," the CD-R "burns" off pieces of the dye with short bursts of the laser. The untouched part of the disc, the gold portions, become the "lands." The lands are a highly reflective surface, which is then "read" by the laser of a CD-ROM player. The player interprets the data by "reading" the 0’s and 1’s represented by the pits and lands.

CD-ReWritable or CD-RW is the newly standardized Compact Disc on which you can write and rewrite your data. You re-use the disc space of deleted files (like you do with a floppy or hard disk) or erase the complete disc. The CD-ReWritable drive can both write to the current Write Once CD-R discs and new CD-ReWritable discs and of course it can also read CD-ROM discs. CD-ReWritable adds a new level of functionality to the Compact Disc family: The ability to write and rewrite your own Compact Discs. Now we have the option to have next to write once media rewriteable media for personal data storage and back-up applications. The CD-ReWritable drive enables the user to store their large files (up to 650 MB) to rewriteable discs which and can be rewritten over and over.

Using the CD-ReWritable drive you can choose between CD-R or CD-RW media depending on the task at hand. For some applications you want to use Write Once Media while in other cases you would prefer to use CD-RW media. But with both media the data written to disc will probably outlive you. CD-R write once media will be more affordable than the CD-ReWritable media. So if you intend to send somebody a full compatible disc (which probably won't be returned) you will probably prefer to use the more affordable Write Once media.

1. Speed Ratings

The "speed" rating of a CD-R determines how fast it can record data to blank CD-R media. Speed designators, such as "1X", "2X", "4X" and "6X" define multiples of the original playback speed of first generation CD-ROM players:

2. For a CD-ROM player or CD-R/RW, a 1X speed translates to 153,600 Bytes per second. This is usually rounded down to 150 Kb per second. Therefore, a "1X" recorder writes 150 Kb per second to the CD-R media.
3. Likewise, a "2X" recorder records at 300 Kb per second.
4. A "4X" records at 600 Kb per second.
5. A "6X" records at 900 Kb per second. there are some variations in measuring speed, because there are recording modes that provide more than 2,048 data bytes per block (audio is recorded at 2,352 bytes per block).

Some CD-R have a different reading speed than their rated recording speed. for instance, a "2x6" CD-R can write at 2x speed and read back at 6x speed (900 kb per second).

6. Multi-Session CD

A session is defined as an area including lead-in, program data and leadout.

The principal purpose of multiple sessions is to allow additional data to be appended to a previously recorded disc. A CD-R recorder that supports multisession recording can write a disc that will have multiple sessions linked together, each containing their own lead-in, program and lead out. Any multisession-supported CD reader can access the data, whether it was written in the first session or a subsequent session. Contrast this to the recording structure of a pressed CD-ROM or a CD-R written in "Disc at Once" mode that contains just one lead-in area and one lead-out area.

7. Fixation and Finalization

Fixation is the process of writing the lead-in and lead-out information to the disc. This process finishes a writing session and creates a table of contents. Fixation is required for a CD-ROM or CD-Audio player to play the disc. Discs which are "fixated for append" can have additional sessions recorded, with their own session lead-in and lead-out, creating a multisession disc.

When a disc is "Finalized" the absolute lead-in and leadout for the entire disc is written, along with information which tells the reader not to look for subsequent sessions .

8. Disc at once

Disc at Once is a writing mode that requires data to be written continuously, without any interruptions, until the entire data set is transferred to the CD-R.

9. The lead-in, program and lead-out are written in a single event. All of the information to be recorded needs to be staged on the computer's hard disk prior to recording in the Disc at Once mode.
10. This mode is usually preferred for discs that are sent to a CD-ROM replication facility when CD-R is the source media.
11. Recording in the Disc at Once mode eliminates the linking and run-in and run-out blocks associated with multisession and packet recording modes, which often are interpreted as uncorrectable errors during the glass mastering process.
12. Track at once

Track at Once is a writing mode that allows a session to be written in a number of discrete write events, called tracks. The disc may be removed from the writer and read in another writer (given proper software) before the session is fixated. The written sessions contain complete "tracks" of information.

Track at Once writing is a form of incremental write which mandates a minimum track length of 300 blocks and a maximum of 99 tracks per disc. A track written "at once" has 150 blocks of overhead for run-in, run-out, pre-gap and linking. Packet write, on the other hand, is a method whereby several write events are allowed within a track, thus reducing the overhead.

A "session" is a unity on a CD which is bounded by lead-in and lead-out. A session contains one or several (up to 99 per disc) tracks.

There are two types of tracks:

13. The incrementally written type described above.
14. An audio track is usually written in disc at once mode along with several other tracks in one session. These tracks have index numbers in the sub channel which differentiate one from the other and which allow selective playback.

Table 23-1: The Evolution of Compact Disc’s

Disc Type	Recording technology	Data stored in the transitions between:
CD-Digital Audio	Read-only	Physical pits (low reflectance) and lands (high reflectance) in a pressed disc. Data can be read out by CD/DVD laser wavelengths (780 nm/650 nm).
CD-Extra	Read-only	AS CD-Digital Audio
CD-I	Read-only	As CD-Digital Audio
CD-ROM	Read-only	As CD-Digital Audio
Video-CD	Read-only	As CD-Digital Audio
CD-R	Write-once read-many	low-reflectance holes (equivalent to pits) made in a photosensitive organic dye during recording and high-reflectance lands (unaltered region) CD-R discs can be read by standard CD-RD drives and CD players
CD-RW	ReWritable read-many	Phase-change amorphous-phase (low reflectance) regions and polycrystalline-phase high reflectance) regions. When read out, CD-RW's amorphous 'pits' and polycrystalline ' lands' generate electrically similar signals to those from other CD discs. Therefore, data can be read out using a MultiRead CD-ROM or CD player (slightly modified to handle the lower reflectances of a CD-RW disc). Data can be read out by CD/DVD laser wavelengths (780 nm/650 nm. )

23.3.2 CD-ReWritable

By allowing PC users to create and rewrite their own CDs, the new CD-ReWritable (CD-RW) drives and media being introduced by Philips represent a major breakthrough in optical disc storage. Based on recently developed phase-change technology, CD-RW allows discs to be written and rewritten many times over. It's therefore an ideal medium for temporary data storage and for extending PC storage capacity.

Besides being able to read/write CD-R discs and to read CD-ROM discs, CD-ReWritable drives can read/write and rewrite the newest addition to CD media - the ReWritable CD. CD-RW should therefore be seen more as a giant floppy-disk system - ideal for personal data storage (e.g. back-ups). In addition, CD-RW discs generated by the new system will be readable on all CD-ROM drives conforming to the new 'MultiRead' specification. The write-once CD-Recordable (CD-R) medium however will continue to play an important role for low-cost storage in applications such as archiving (e.g. graphics) and data interchange.

 

15. Phase Change Technology

In its original state, the recording layer of a CD-RW disc is polycrystalline. During writing, a focused laser beam selectively heats areas of the phase-change material above the melting temperature (500-700° C), so all the atoms in this area can move rapidly in the liquid state. Then, if cooled sufficiently quickly, the random liquid state is 'frozen-in' and the so-called amorphous state is obtained. If the phase-change layer is heated below the melting temperature but above the crystallization temperature (200° C) for a sufficient time (at least longer than the minimum crystallization time), the atoms revert back to an ordered state (i.e. the crystalline state).The amorphous and crystalline states have different refractive indices, and can therefore be optically distinguished. In the CD-RW system, the amorphous state has a lower reflectance than the crystalline state and, during read-out, this produces a signal similar to that of a regular CD, making it possible to read CD-RW disks on CD-ROM drives.

The phase-change medium consists of a grooved polycarbonate substrate on to which a stack (usually five layers) is sputtered. The chemical composition of the phase-change layer determines the minimum time of crystallization. The disc structure (layer thickness, thermal capacities and thermal conductivities) determines the cooling rate during writing . Precise control of the recording-layer composition is important to obtain the desired recording properties. In general, low recording powers are achieved by using thin layers. The optical properties of the phase-change medium are controlled by the layer thickness' and refractive indices.

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    Fig. 23-16. Section through a 5-layer CD-RW disc.

     

     

     

     

     

     

    By exposure to the heat from a laser, the recording layer can be changed from a polycrystalline (more reflective) state to an amorphous (less reflective) state, and vice versa. The layers are deposited onto a polycarbonate substrate, the latter moulded with a spiral groove for servo guidance, absolute time information, and other data. CD-RW discs are supplied ready-for-use in the polycrystalline state. Like CD-R discs, they can be rotated at different constant linear velocities.

    

    Fig. 23-17. During recording

    During recording the laser output is modulated using three power levels, P_write, P_erase and P_bias, according to a predefined strategy set by the data to be recorded. Unlike CD-R, where the laser is left at its recording power for the time required to form the desired length of 'mark' in the dye, the laser in a CD-RW system has to be pulsed (for thermal considerations) to form well-defined amorphous regions of the desired length. The power level P_erase is used to form the crystalline regions between amorphous marks. The optimum recording powers (P_write, P_erase and P_bias) depend on the disc, the recorder and the recording speed. Their values for individual disc/recorder combinations and different recording speeds are set by the Optimum Power Control (OPC) procedure.

    

    Fig. 23-18. Recording strategy example.

    Shown here are the laser modulation strategy to record the shortest 'pit’ defined in the CD system, the '3T pit', and the strategy to record a 'land.’ Recording a pit means creating a low-reflectivity (amorphous) region in the active layer; recording a land means creating a high-reflectivity (polycrystalline) region as illustrated in the graphs.

    This strategy has two parts:

17. a pulsed part (pulsing is necessary to write amorphous pits').
18. a non-pulsed part, when the strategy writes crystalline lands' between 'pits.

Note that both strategies involve writing data to the disc. In CD-RW, the traditional concept of erasure does not exist. New data is simply written over existing data in a single-pass of the laser beam. CD-RW is therefore known as a Direct Overwrite (DOW) system.

Thus, the write strategy writes new data to the disc while simultaneously overwriting old data. This process can be repeated several thousand times (at least 1000 rewrites). Further, for the specified write strategy, a CD-RW disc can be written using a standard CD-R laser of 30-40 mW (Source Power).

The recorded tracks on a CD-RW disc are read in the same way as regular CD tracks. By detecting transitions between low and high reflectance, and measuring the length of the periods between the transitions. The only difference is that the reflectance is lower than for regular CDs. This does however mean that CD-RW discs cannot be read by many current CD-ROM drives or CD players. A solution to this inconvenience has already been found.

To outline the solution, it is helpful to consider the original CD reflectance specifications: 70% minimum for lands, 28% maximum for pits, that were introduced to allow the relatively insensitive photodiodes of the early 1980s to read the signal pattern reliably. But with today's photodiodes able to detect much smaller reflectance differences, these stringent specifications are no longer necessary.

The CD-RW disc has a reflectance of 15-25% for lands. The CD-ReWritable system, therefore, works at reflectances about one-third of those of the original CD specification. However, as explained, with modern photodiodes this presents no problem . All that is needed to reliably read the recorded pattern is extra amplification. Looking to the future, the CD-RW phase-change technology is significantly independent of the recording/read-out laser wavelength. CD-RW discs can be read out by the 650 nm lasers used in DVD systems as well as by the present 780 nm lasers used in other CD drives. Clearly, this creates additional options or drive manufacturers.

So, except for the reflectance levels (which are no longer an issue), CD-RW conforms fully to the original CD specification. The discs are the same size and store the same quantity of data. The drives include a small modification but otherwise are identical to CD-R drives. In high-volume production, CD-RW drives should be only slightly more expensive than usual CD-R drives.

23.3.4 The ReWritable Discs

CD-ReWritable discs use phase-change technology to store data. The phase-change medium consists of a grooved polycarbonate substrate on to which a stack (usually five layers) is deposited, see Fig.23-16. The phase-change (recording) layer is sandwiched between dielectric layers. A commonly used phase-change material is Ag-In-Sb-Te alloy. The chemical composition of the phase-change layer determines the minimum time of crystallization. The disc structure (layer thicknesses , thermal capacities and thermal conductivities) determines the cooling rate during writing. Precise control of the phase-change layer composition and of the disc structure is important to obtain the desired recording properties. In general, low recording powers are achieved by using thin layers. The optical properties of the phase-change medium is controlled by the layer thicknesses and refractive indices. Current CD-RW discs are suitable for 2x nominal CD recording speed. In 1998, CD-RW discs will be available for 1x, 2x and 4x nominal CD recording speeds.

CD-RW discs are manufactured using equipment similar to that already found in conventional CD factories. Some dedicated equipment is however needed. For instance, the active layer formed by sputtering is amorphous, and has to be converted to the polycrystalline phase by heating it with a high DC power. This process, termed initialization, ensures that every new disc reaching users is in the high reflectance phase, which makes CD-RW discs optically correlated with CD-R discs.

As supplied to the market, all of the recording layer of a CD-RW disc is polycrystalline. During recording, a laser selectively heats areas of the recording track above the layer's melting point. The melted crystals along the track flow into the amorphous phase which is then 'frozen-in' by quickly cooling the layer. The reflectance of the amorphous areas is much lower than that of the crystalline areas which, during read-out, gives rise to a signal similar to that produced from the pits and lands of a regular CD.

During rewrite, some amorphous areas along the track are returned to the crystalline phase by annealing below the melting point; others are rewritten as amorphous areas (the data to be written determines what happens to each area). Similarly, some crystalline areas are converted to the amorphous phase by heating above the melting point, then quenching as described above, others are rewritten as crystalline regions. This process can be repeated several thousand times per disc.