Everything You Wanted To Know About DVD-R.

This article is the first in a two-part series.

DVD Recordable (DVD-R) technology allows anyone to create DVD discs at the desktop. Similar in concept to Compact Disc Recordable (CD-R), DVD-R is a write-once medium that can contain any type of information normally stored on mass produced DVD discs--video, audio, images, data files, multimedia programs, and so on. Depending on the type of information recorded, DVD-R discs are usable on virtually any compatible DVD playback device, including most DVD-ROM drives and DVD Video players.

A DVD-R disc is able to contain a maximum of either 4.7 or 3.95 billion bytes of information on each side, depending on the type of blank media used. Since the DVD format supports double-sided media, up to 9.4 gigabytes can be stored on a single double-sided DVD-R disc. Note that the term "gigabyte" refers to one billion bytes in DVD Forum specifications.

Data can be written to a disc at a DVD "1X" equivalent of 11.08 megabits per second (Mbps), which is roughly equivalent to nine times the transfer rate of CD-ROM's "1X" speed. After recording, DVD-R discs can be read at the same rate as mass-produced replicated discs, depending on the "X" factor of the DVD-ROM drive used. These transfer rates, coupled with DVD-R's capacity and conformance to worldwide DVD standards, makes it an extremely viable and cost effective storage medium.

DVD-R Technology

DVD-R is a write-once format, meaning that data can be written to a disc and stored without fear of accidental erasure. The fundamental technology employed is similar to that used by CD-R, except that data is written at a higher rate and density.

lDVD-R, like CD-R, uses a constant linear velocity rotation technique to maximize the storage density on the disc surface. This results in a variable number of Revolutions Per Minute (RPM) as disc writing/reading progresses from one end to the other. Recording begins at the inner radius and ends at the outer. At "1X" speeds, rotation of the disc varies from 1,623 to 632RPM on 3.95GB media and 1,475 to 575RPM on 4.7GB media, depending on the record/playback head's position over the surface. On 3.95GB media, the track pitch or the distance from the center of one part of the spiral information "track" to an adjacent part of the track is 0.8 microns, one-half that of CD-R 4.7GB media uses an even smaller track pitch of 0.74 microns.

DVD-Recordable Disc Dimensions

To help achieve a six to seven-fold increase in storage density over CD-R, two key components of the writing hardware needed to be altered: the wavelength of the recording laser and the numerical aperture (n.a.) of the lens that focuses it.

With CD-R, Ian infrared laser with a wavelength of 780 nanometers (nm) is employed, while DVD-R versions 1.0 and 1.9 use a red laser with a wavelength of 635nm. At the same time, the numerical aperture of a typical CD-R drive's objective lens is 0.5, while a DVD-R drive uses lenses with a n.a. of 0.6. These factors allow DVD-R discs to record marks as small as 0.40[micro]m as compared with the minimum 0.834[micro]m size with CD-R.

The Table highlights the differences between some basic parameters of both media formats. Recording on DVD-R discs is accomplished through the use of a dye recording layer that is permanently transformed by a highly focused red laser beam. This dye substance is spin-coated onto a clear polycarbonate substrate that forms one side of the "body" of a complete disc. The substrate is injection molded and has a microscopic, "pre-grooved" spiral track formed onto its surface. This groove is used by a DVD-R drive to guide the recording laser beam during the writing process and also contains recorded information after writing is completed. An undulating "wobble" signal is molded into the pre-groove for synchronizing a DVD-R drive's spindle motor during the writing process and "Land PrePits" (LPP) are also contained in the land areas between grooves for addressing purposes.

A thin layer of metal is, then, sputtered onto the recording layer so that a reading laser can be reflected off the disc during playback. A protective layer is, then, applied to the metal surface, which prepares the side for the bonding process.

These steps are done for each side of a disc that will be used for recording. If only a single recording side is required, then the opposite side can contain a label or some other visible information such as pit art. If both sides are needed for recording, then two recordable sides can be bonded together. In this case, each side must be read directly by flipping the disc over, as the DVD-R format does not currently support dual layer technology.

The recording action takes place by momentarily exposing the recording layer to a high power (approximately 8-10 milliwatt) laser beam that is tightly focused onto its surface. As the dye layer is heated, it is permanently altered such that microscopic marks are formed in the pre-groove. These recorded marks differ in length depending on how long the write laser is tamed on and off, which is how information is stored on the disc. The light sensitivity of the recording layer has been tuned to an appropriate wavelength of light so that exposure to ambient light or playback lasers will not damage a recording.

Playback occurs by focusing a lower power laser of the same approximate wavelength (635 or 650nm) onto the surface of the disc. The "land" areas between marks are reflective, meaning that most of the light is returned to the player's optical head. Conversely, recorded marks are not very reflective, meaning that very little of the light is returned. This "on-off' pattern is, thereby, interpreted as the modulated signal, which is, then, decoded into the original user data by the playback device.

Expected Life Of DVD-R Media

Life expectancy is a key issue when considering the use of DVD-R for applications such as document imaging and other archival applications. Although each disc media manufacturer has its own life expectancy rating, Pioneer DVD-R media is currently rated at better than 100 years.

Compatibility

Properly recorded DVD-R discs should be playable on nearly any destination device that can properly make use of whatever data is written. The Fig illustrates this.

DVD-R Compatibility

Recorded DVD Video discs can be played on a DVD video player, as well as a computer that is equipped with a DVD-ROM drive, a DVD-compliant MPEG decoder card (or decoder software), and application software that emulates a video player's control functions. A recorded DVD-ROM disc can be read by a computer equipped with a DVD-ROM drive, as well as a computer equipped for DVD video playback, as described above. DVD Video components are not necessary, however, if DVD Video material is not accessed or is not present on a disc.

Recorded DVD-R discs support a new file system called "UDF Bridge". This is a hybrid approach that provides both the UDF (Universal Disc Format) system, as well as the older ISO9660 system used by the CDROM format. This allows DVD discs to be used with computer operating systems that do not have any provision for UDF support.

Recording A Disc

The basic recording process for DVD-R discs should be familiar to any user of CD-R technology. Like CDR, blank DVD-R discs are recorded in a DVD-R drive that is controlled by a host computer. The recording process is orchestrated by application software that allows a user to specify which files will be transferred to the disc, as well as conducting the actual recording itself.

All DVD discs, record-able or not, must have three basic areas recorded on them: lead-in, user data, and lead-out. The lead-in and lead-out areas are boundaries that indicate to a playback device where the inner and outer limits of a recording are, respectively. They contain no user accessible information, but are critical to the proper functioning of a disc.

There are two methods of writing a DVD-R disc: disc-at-once and incremental writing. Disc-at-once, as its name implies, is the process of writing an entire disc's worth of data, up to 4.7GB, at one time. A host computer must consistently provide data at a full 11.08 megabits per second during any recording to avoid buffer underrun errors. Buffer underruns can be minimized by the use of a large writing buffer memory in a DVD-R drive; Pioneer's second generation drive, in fact, provides a 6.75MB buffer that can absorb bit stream interruptions of more than four seconds in duration. As faster writing speeds are developed in future products, host computer performance and drive buffer requirements may also need to increase accordingly.

DVD-R disc-at-once writing is performed such that the lead-in, data area, and lead-out areas are all written sequentially. This differs from how track-at-once CD-R discs are typically written, where the data area is written first, followed by the lead-in/table of contents and lead Out areas.

Disc-at-once recording is typically used when authoring and recording video titles due to the structure and size of these programs. It can also be used for multimedia or other software titles intended for publishing, as these works are normally assembled on hard drives as a finished image file prior to testing them on DVD optical discs.

The second part of this article will appear in the March issue of CTR.

Andy Parsons is the senior vice president of product development and technical support at Pioneer New Media Technologies (Long Beach, CA).

Parameter                      DVD-R (1.0, 1.9)
Media Type                     Write-once
Wavelength(Recording)          635-645 nm
Wavelength(Reading)            635-650 nm
Recording Power                6-12 mw
Numerical Aperture (Recording) 0.60
Numerical Aperture (Reading)   0.60
Reflectivity                   [R.sub.14H] [greater than] 0.6
Parameter                      CD-R
Media Type                     Write-once
Wavelength(Recording)          775-795 nm
Wavelength(Reading)            770-830 nm
Recording Power                4.8 mw
Numerical Aperture (Recording) 0.50
Numerical Aperture (Reading)   0.45
Reflectivity                   [R.sub.TOP] [greater than] 0.65