White Paper: DVD-RAM: Is it a reliable technology?

DVD-RAM is a high capacity storage technology that could become the true successor to the older CD-R and CD-RW formats. In...

DVD-RAM is a high capacity storage technology that could become the true successor to the older CD-R and CD-RW formats. In addition to additional capacity, the format has certain built-in features that make it much more reliable.

DVD-RAM is a rewriteable data storage format that was designed by the DVD Forum to provide a reliable and affordable storage solution to the emerging multimedia world. The ever-increasing storage requirements in the enterprise market, along with the advent of digital video and movies at home, are driving demand for large capacity rewriteable storage. DVD-RAM offers a highly reliable format designed with future expandability in mind.

In July 1997, the DVD Forum published the specification for DVD-RAM (DVD-RAM Book 1.0). DVD-RAM is an optical disk technology that provides 2.6Gb of fully rewriteable data storage capacity on each disk surface. Drives and media conforming to the DVD-RAM specification are now available from multiple manufacturers. As a result, the capabilities of DVD, the new optical data storage standard, now include both data distribution and data storage.

The DVD-RAM format specification was defined by Working Group Five of the DVD Forum. Members of Working Group Five, who defined the DVD-RAM standard are: Eastman Kodak Company, Hitachi Ltd, Matsushita Electric Industrial Co, Ltd, Mitsubishi Electric Corp, Philips Electronics NV, Pioneer Electronics Corp, Sony Corp, Thomson Multimedia, Toshiba Corp and Victor Company of Japan. Since its founding, in December 1995 (as the DVD Consortium) the DVD Forum has defined and published specifications for DVD-ROM, DVD-Video, DVD-Recordable and DVD-RAM. It has also grown to include more than 120 member companies.

As part of its multi-year, multi-product specification effort, the DVD Forum consults regularly with other industry groups, and has submitted its published specifications to international standards organisations. One of the most important of these informal groups is the computer industry Technical Working Group, made up of the leading manufacturers of personal computers and servers. In 1995, this group defined a set of requirements for optical disk storage technologies that would replace the 650Mb CD technologies used throughout the industry. The DVD Forum has responded to the requirements of the Technical Working Group in the development of all DVD specifications.

The goals of the DVD Forum program in defining the DVD-RAM Format were:

1) To assure interchangeability of media between drives manufactured to the defined specifications. The DVD-RAM Book 1.0 specification was ratified after a three-month evaluation period, during which nine media suppliers and 13 drive manufacturers confirmed the technical details and feasibility of the specification.

2) To provide the highest levels of compatibility between read-only and recordable DVD technologies. Thus, the rewriteable DVD-RAM format shares common format features with read-only DVD-ROM. This allows future DVD-ROM and DVD-Video drives to read DVD-RAM disks.

3) To provide reliable and fast access to data written anywhere on the disk. Zoned Constant Linear Velocity (ZCLV) recording and multiple techniques for sector header identification are used to optimise random access data performance.

4) To achieve drive and media cost levels that allow widespread adoption by manufacturers of personal computer and consumer electronics products. Phase change media technology and land/groove recording methodology allow for cost effective media and drive implementations.

The program goals were achieved and multiple manufacturers brought DVD-RAM products to market in a relatively short time.

The starting point of assured compatibility with other DVD technology is in the physical structure of DVD-RAM media. Specifications were defined to match, in all possible cases, the DVD-ROM media specification.

Rewriteable DVD-RAM uses a land/groove structure and phase change material to record data. The land/groove combination forms a continuous spiral track, with data recorded alternately on land and groove. This results in a logical track pitch of 0.74 µm. With this approach, the physical track pitch, or distance from a land track to the next land track, is 1.48 µm. Working Group Five also evaluated land only and groove only recording techniques. With those alternatives, the physical track pitch would be only 0.8 µm. Such a narrower pitch is more difficult to manufacture, and would likely be more costly than the land/groove solution.

To maintain nearly constant data density across the surface of the DVD-RAM disk, the recording surface is divided into zones. This scheme, combined with the land/groove technique, maximises disk capacity, with a total recording capacity per disk side of 2.6Gb. The disk is operated in ZCLV mode. Using this method, disk rotational speed is kept constant within recording zones but varied from zone to zone. This results in constant data rates through-out the disk surface. ZCLV provides for constant data recording rates, nearly constant recording density, and optimised random access performance.

DVD-RAM media is protected by a cartridge and may be used as a bare disk. It is strongly advised to use a cartridge whenever possible, to provide maximum protection to data at all times. Cartridges with single-sided disks allow removal of the disk, to enable operation as a bare disk.

Phase change material is used as the recording medium in DVD-RAM. The phase change material can take on two distinct states, amorphous or crystalline, with different levels of reflectivity. These different levels of reflectivity are detected by the optical readers of both DVD-RAM and DVD-ROM drives, enabling data to be read from the disk.

Prior to recording, the disk surface material is in a crystalline state exhibiting high reflectivity. The light intensity of a laser beam is modulated to write data marks on the disk's surface. The energy of the laser pulse quickly raises the surface temperature of the disk above the material's melting point. Then the laser power is quickly lowered to its cooling point, rapidly cooling the disk surface.

The material in the area affected by the laser pulse remains in amorphous state. This area of low reflectivity is a recorded data mark. To erase this data mark, a lower power laser beam is used to return the disk surface material to their initial crystalline state. The laser causes the surface temperature to rise to its crystallisation point enabling the material to revert to the initial crystalline state thus erasing the data mark.

DVD-RAM drives use a multi-pulse waveform write strategy to form recording marks reliably. The duration of laser pulses is varied to write different marks. All of the waveforms use an identical cooling pulse. Data is recorded using the same 8 to 16 modulation code used for DVD-ROM. This enables future DVD-ROM drives to playback DVD-RAM disks.

The challenge of achieving reliable, high-density recording of data on DVD-RAM phase change media, while providing for read-only compatibility with DVD-ROM drives, was met through a combination of techniques contributed by different members of the DVD Forum. The resulting wobbled land and groove recording format is detailed below. Three key aspects of the format are wobble-linked ID detection, sector format and header format.

The DVD-RAM disk surface is divided into 24 annular zones. This zoned surface layout keeps the length of a sector and the recording density almost constant throughout the disk. Each zone has 1888 tracks (944 land tracks and 944 groove tracks) and each track is divided into sectors. The innermost zone has 17 sectors per track, and the number of sectors per track increases by one in each succeeding zone. Thus, the outermost zone features 40 sectors per track. Each sector begins with a read-only identification (ID) field, embossed onto the disk surface. This ID field, the header, is used to identify the physical location of the sector and is kept separate from the user-recordable data field, to assure that it is permanently readable.

All types of DVD disks include a control area at the inner diameter of the disk. This area contains embossed, read-only data that contains information identifying the type of media, e.g. DVD-RAM, DVD-ROM or DVD-R. The information allows drives to identify which type of media is inserted into a drive, and is designed to support compatibility among the DVD product family. Data marks are written on both land or groove tracks.

Land and groove tracks alternate on every revolution of the disk, forming a continuous single-spiral track. This single spiral feature allows for the continuous recording of large data streams, such as video data.

The embossed sector header includes four ID fields, each containing identical information to provide redundancy in identifying sector location. These embossed ID fields are offset from the track center by half a track width. Two are offset toward the inner diameter, two towards the outer diameter. The recordable area of a sector is slightly wobbled, with a period of 1/232 of the physical sector length. This feature is used to produce a constant clock signal that allows the drive to find the user data in the sector in the event that a sector ID cannot be read.

The sector format of DVD-RAM media has data blocks that are identical to DVD-ROM sectors. Each physical sector consists of a 130-byte-length header part and 2567-byte-length recording field, which includes gap field, guard fields, synchronisation signals and buffer fields. Guard fields protect both ends of the recording data field from degradation due to repetitive overwriting.

DVD-RAM media requires 2697 bytes for each user recordable data sector. This provides for a gap between each 2418 byte block of recordable area. However, the sector data configuration itself is the same for DVD-RAM and DVD-ROM. The recording code (8 to 16) and error correction code (Reed-Solomon product code) are the same for both types of media. Thus, the information recorded on the DVD-RAM disk can easily be played back using the readout circuit implemented on DVD-ROM drives.

The DVD-RAM media header format is designed to assure reliable recording and playback. As noted earlier, the header ID information is permanently embossed on the disk. Each header is divided into two parts. One part of each header is shifted by half a track towards the outer diameter of the disk, while the second part is shifted by half a track towards the inner diameter. ID field offsets are dependent on whether the following recordable area is on a land or groove track. "Land sectors" start with ID1/2 disk diameter (ID) and ID3/4 are offset towards the outer disk diameter (OD).

"Groove sectors" start with ID1/2 offset towards the OD and ID3/4 are offset towards the ID. This change of ID offset sequence between land and groove tracks allows the drive to reliably switch its tracking servo from land to groove tracking and vice versa.

DVD-RAM has both rewriteable data areas and embossed data areas, while DVD-ROM only has an embossed data area. The embossed data for DVD-RAM is located at the innermost part of the disk, which is a part of lead-in area. The lead-in areas of all DVD disks contain information that identifies the type of media, such as DVD-RAM, DVD-ROM or DVD-R. The data structure of this area is common across the DVD-family, in order to ensure future compatibility.

The rewriteable data area is divided into 24 zones, each of which has an associated spare area. The spare area is used for the replacement of any defective sector in the user area. Defective sector addressing is handled using four defect management areas, two of which are located in the lead-in area and the others are located in the lead-out area. Those four areas contain the same information, providing an additional level of redundancy to improve reliability. Each defect management area has lists of defective sectors and their replacement sectors.

Defect management is performed by drives in order to provide an error free storage environment for file systems. DVD-RAM media is designed to support rewriting of data more than 100,000 times in the lifetime of a disk. This is achieved by implementing two methods for randomising the location and the polarity of data in recordable sectors.

The DVD-RAM format specification calls for a different start position of written user data each time a data sector is overwritten. The recording start position is randomly shifted by 0 to 15 channel bits, and the lengths of the guard fields are changed by 0 to 7 bytes.

The second overwrite technique is based on the fact that a recorded mark to code a data bit, or "1" is represented by the edge of the mark. (This principle is called mark edge recording.) Since the leading and trailing edge of a mark are treated equally, the polarity (mark or space) of a recorded mark is unimportant and does not effect the ability to retrieve data. By applying random inversion, the location of marks and spaces becomes almost uniform when averaged throughout the overwrite cycles. This uniformity greatly improves the overwrite cyclability.

( Hitachi 1999

Compiled by Agith Ram

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