Chapter 6 DVD-R and DVD-RW
| 6.1 History of Specification Publications |
The specification for the write-once DVD-R disc, DVD-R (3.95 GB) Book version 1.0 was released in 1997, and Pioneer began selling industrial DVD-R discs and drives that same year. A preliminary specification for a 4.7 GB disc, Book version 1.9, was released in November 1998. However, as a result of discussions about issues related to copy protection, the 4.7 GB DVD-R specification was divided into two specifications, DVD-R for Authoring version 2.0, and DVD-R for General version 2.0, which were published in February and May of 2000, respectively. Following that, supplemental information documents were published as addendums to the General specification in September and December of 2000, bringing the DVD-R specification to the form in which it exists today.
The specification for the rewritable DVD-RW disc, on the other hand, targeted a 4.7 GB capacity from the start. Version 1.0 of that specification was published in November 1999. In September of 2000 revision information was released to bring the specification to version 1.1, with a supplemental information addendum also published in September and December, bringing that specification to the form in which it exists today.
The content of those specifications will be described later in this chapter. A broad range of DVD-R discs, DVD-RW discs, DVD recorders, and DVD-R/RW drives based on these specifications are now being introduced to the market.
Figure 1 History of DVD-R/RW disc specification publications
The details of the copy management mechanism are described in Figure 7.
To begin with, each DVD specification provides a mechanism for recognizing the recording media, and this is common to the entire DVD family. That is, each disc contains a flag called Book Type which indicates to which specification the disc is compliant. Recordable DVD media have a particular wobbling track, which is used as a means to recognize the recordable media. Detection of this wobbling track allows the player to reject media with an illegal Book Type; that is, media that has been recorded improperly.
Besides this, discs also are specified to have a read-only area, as has was described above, as another mechanism for preventing the recording of copy-protected information.
In DVD-R for General and DVD-RW version 1.1, by recording specified data (a Media Block Key, or MKB) to this read-only area, and by further adding a bar code style signal (called NBCA) to the disc's inner tracks, allows the disc to support a copy generation management scheme (with support for copy-once media) called Content Protection for Recordable Media, or CPRM. This additional generation management information is placed on the disc by the disc manufacturer, and is an optional feature of the specifications.
| 6.2 Basic Concept |
The basic concept behind DVD-R and DVD-RW are now well-known. The most fundamental concept is that these discs are compatible with read-only DVD discs. This is exactly equivalent to the relationship between CD-ROM discs, and CD-R and CD-RW discs. The basic discs are high-volume pressed versions, with compatible recordable media also playing a major role. Therefore, the 4.7 GB DVD-R disc is specified such that after recording, its signal characteristics and format of the recorded data are the same as those defined by the DVD-ROM specification. The same is true of DVD-RW; with the exception that the reflectivity of DVD-RW discs are the same as that of dual-layer DVD-ROM discs, DVD-RW discs are like DVD-R discs with the addition of rewrite capability.
Since all these specifications are defined based on this fundamental concept, DVD-R and DVD-RW discs can be easily played in players designed for read-only DVDs and in DVD-ROM drives.
Figure 2 Basic Concept
| 6.3 Basic Specifications |
This section will describe the basic specifications that are common to 4.7 GB DVD-R and DVD-RW discs.
As explained previously, the basic playback specifications of DVD-R and DVD-RW discs after recording are the same as those for DVD-ROM discs. As shown in the table below, the reflectivity of DVD-RW discs differs from that of single-layer DVD-ROM discs (but is the same as that of dual-layer DVD-ROM discs). With that exception, other parameters such as recording capacity, density (track pitch, minimum pit length), and recorded signal playback quality follow suit with the parameters of single-layer DVD-ROM discs.
| DVD standard | single-layer DVD-ROM |
DVD-R | DVD-RW | dual-layer DVD-ROM |
|---|---|---|---|---|
| Laser Wavelength | 635 / 650 nm | |||
| Objective lens NA | 0.60 | |||
| Reflectivity | 45 to 85 % | 18 to 30 % | ||
| Modulated amplitude | 0.60 min. | |||
| Data track form | Single spiral track | |||
| Track pitch | 0.74  |
|||
| Tracking method | DPD (Differential Phase Detection) | |||
| Minimum pit length | 0.40 |
0.44 |
||
| Data modulation | 8 / 16, RLL(2,10) | |||
| Error correction | RS-PC (Reed-Solomon Product Code) | |||
| Channel bit rate | 26.16 Mbps | |||
| Scanning velocity | 3.49 m/s (CLV) | 3.84 m/s (CLV) |
||
| User data capacity | 4.70 Gbytes / side | 4.25 Gbytes/layer |
||
As shown in Figure 3, the recording tracks (grooves) "wobble" at a fixed frequency, and address pits called Land Pre-Pits are positioned between the recording tracks. (The details of this structure will be explained later.) These two types of addressing are used during recording to control disc rotation and generate the recording clock, as well as providing information such as recording address, which is necessary in the recording process.
After recording, the disc Information Area, the playback region, has exactly the same structure as that of a DVD-ROM disc, and the data format is also exactly the same. Closer to the center of the disc than the Information Area is another region, called the R-Information Area, which is peculiar to DVD-R and DVD-RW discs. This area contains an area called the PCA (Power Calibration Area), which is used for laser power calibration, and an area called the RMA (Recording Management Area), which contains recording management information necessary for the recording device. This information is provided to prevent problems in playing these writable discs in ordinary players and drives.
6.3.1 Preformat and addressing
DVD-R and DVD-RW discs use groove wobble and land pre-pits as a means for providing pre-format. As shown in Figure 3, the disc substrate contains grooves which are used to guide the laser beam, and the data is recorded in these grooves. The areas between adjacent grooves are called lands. Groove wobbling means that the grooves wander back and forth with a fixed amplitude and spacial frequency. Land pre-pits are isolated pits formed on the lands in a fixed pattern. The major uses of these two features are as follows:
| Groove wobbling | ||
| - | generation of a spindle motor control signal | |
| - | generation of a gate signal used in detection of the land pre-pits Land pre-pits |
|
| - | high-precision positioning when writing data | |
| - | acquisition of recording address and other information necessary for recording |
This pre-format signal is obtained through differential detection of the laser light reflected from the disc during tracking, but is designed so as not to be detected in the overall signal ordinarily used to obtain user data from an optical disc. That is, the pre-format signal used during recording is designed so that it has no effect on the playback of data the user has recorded to the grooves.
Figure 4 shows a basic diagram of the detected signal. As seen in the figure, the groove wobble signal is detected as a sine wave, while the land pre-pit signal is coincident with the first three peaks of the wobble signal in each synch frame (an odd or even frame). These three pulses make up one set. These three pits are assigned in a fixed pattern. The first bit always exists, and the sync pattern in the recorded data is synchronized with this pre-pit. This method of recording user data while providing phase compensation to the recording clock based on the pre-pit position makes it possible to append data or write data with much greater precision than is possible with CD-R and similar format.
Today, there are three types of linking loss area (the loss of user data which accompanies appending or rewriting of data), with magnitudes of 32 kB, 2 kB, and 0 bytes. Use of the above-described method makes it possible to do "lossless linking" (in DVD-R for General and DVD-RW).
6.3.2 Write strategy
As one means of increasing recording performance when actually writing a signal to the disc, the DVD-R and DVD-RW specifications define a type of laser output multi-pulse modulation called write strategy, which rely on the fact that DVD-R and DVD-RW use heat for recording. Figure 5 shows the basic write strategy for these two types of disc. For both media types, the laser pulse for a single recording mark is divided into a top pulse and a series of multiple pulses, which provides control of distribution of heat generated in forming the mark. DVD-R uses two levels of laser power, while DVD-RW uses three levels to enable overwriting. For the latter in particular, the series of pulses is ended with a cooling pulse.
This method alleviates heat interference between adjacent and the heat accumulation at the trailing edge of the formed mark, and makes it possible to obtain a playback signal with good characteristics.
The DVD-R and DVD-RW specifications also provide the option of encoding the optimal pulse width for a particular media type and embedding that information in the land pre-pit, as a mechanism for further increasing system margin.
Figure 5 Write Strategy
| 6.4 Features of the Specifications |
As mentioned above, the 4.7 GB DVD-R specification was divided and published as two specifications (DVD-R for Authoring version 2.0 and DVD-R for General version 2.0), out of concern for copy protection issues. The former specification is restricted to professional authoring applications, while the latter specification is available for general consumer applications. The DVD-RW specification was also designed for consumer applications, and was revised from version 1.0 to version 1.1 to perfect its copy control mechanisms. At the same time, playback compatibility with DVD-ROM was also improved.
This section will describe the commonalities and differences between these three specifications.
6.4.1 Blank disc structure
Figure 6 shows the specified characteristics of the different types of blank discs. From the figure we can see that the DVD-R for General, for consumer applications, and the DVD-RW discs have the same structure. That is, the recording laser wavelength and distribution of recording addresses included in the land pre-pit data are the same, and both require playback-only areas (areas which cannot be written) for use in preventing the recording of copy-protected data. Further, these discs contain an area near the center of the disc for (optional) NBCA copy generation management data (the details of which will be explained below).
The main difference between these and DVD-R for Authoring is in whether or not copy management mechanisms physically exist on the disc. The professional and consumer discs also use different laser wavelengths and addressing schemes so that the each types of disc is not compatible with the other style of recorder.
The means of creating a read-only region on the surface of the recording disc is currently different for DVD-R and DVD-RW. Since DVD-R for General is a write-once medium, the disc manufacturer creates the read-only area by writing to that area as part of the manufacturing process. DVD-RW, on the other hand, is a rewritable medium. In DVD-RW, the read-only area is pre-recorded with embossed pits. The quality of the signal read from the embossed pits was not specified in the DVD-RW version 1.0 specification. In revising the specification to version 1.1 it was determined that to complete the copy management mechanism that signal quality should follow suit with that of the DVD-ROM specification. As a result, the modification to the specification also helped maintain compatibility with DVD-ROM discs.
6.4.2 Copy management technology
| 6.5 Conclusion |
DVD-R and DVD-RW are recordable and re-recordable DVD standards recognized by the DVD Forum, defined in accordance with the basic concept of providing compatibility with read-only discs.
As applications for these recordable media, the DVD Forum has established the Video Recording format, which provides excellent support for features such as video editing, and the DVD Video format (for non-copy-protected data only), which is primarily concerned with providing compatibility with the existing DVD-Video specification. These specification are available today for use in certain applications.
The DVD-R and DVD-RW specifications were promulgated somewhat later than the read-only specification, and as a result there are some compatibility issues with some of the earlier DVD players and drives. (For example, some players cannot recognize DVD-R/RW discs because the aforementioned Book Type flag is different from those of ROM discs, or because DVD-RW discs are single-layer discs but have the same reflectivity as dual-layer discs.) Recently, however, as a result of increased recognition of these specifications, standardization efforts such as RWPPI (the RW Products Promotion Initiative), and due to continuing efforts by manufacturers to improve product quality, compatibility problems and user inconveniences in the market should be considerably reduced.
Hand in hand with the increasing popularity of DVD video, audio, and ROM, we anticipate that DVD-R and DVD-RW will also quickly establish a large recordable DVD market.
Chapter 2 Physical Format of Read-Only Discs