More on DVD Recordable

Similar in concept to CD-R, DVD-R (or, DVD-Recordable) 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 DVD-ROM drives and DVD Video players. An early release of DVD-R was important to the development of DVD-ROM titles since software developers needed a simple and relatively cheap way of producing test discs before going into full production.

When it first appeared in the autumn of 1997 DVD-R media had a capacity of 3.95GB. This was later increased to 4.7GB of information on a single-layer, single-sided DVD-R disc. Since the DVD format supports double-sided media, up to 9.4GB can be stored on a single double-sided DVD-R disc. Data can be written to a disc at a DVD "1x" equivalent to 11.08 Mbit/s, 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.

DVD-R, like CD-R, uses a constant linear velocity (CLV) rotation technique to maximise 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 632 RPM on 3.95GB media and 1,475 to 575 RPM 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 centre of one part of the spiral information "track" to an adjacent part of the track, is 0.8 microns (µm), one-half that of CD-R. 4.7GB media uses an even smaller track pitch of 0.74µm.

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 of the lens that focuses it. With CD-R, an infrared laser with a wavelength of 780nm is employed, while DVD-R uses 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 numerical aperture of 0.6. These factors allow DVD-R discs to record marks as small as 0.40µm as compared with the minimum 0.834µm size with CD-R.

The table below 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 moulded, 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 moulded into the pre-groove for synchronising a DVD-R drive's spindle motor during the writing process, and "Land Pre-Pits" (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 carried out 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 as depicted in the diagram. In this case each side must be read directly by flipping the disc over, as dual layer technology is not currently supported.

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 turned 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 650 nm) 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

All DVD discs, recordable 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. The basic recording process is similar to that employed by CD-R technology.

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 Mbit/s during any recording to avoid buffer underrun errors, a condition that can be minimised by the use of a large writing buffer memory. 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 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 likely to be used when authoring video titles due to the large 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.


• Incremental writing is also supported by the DVD-R format. This is very similar in concept to the packet writing technology that is used with CD-R. Incremental writing allows a user to add files directly to a DVD-R disc one recording at a time instead of requiring that all files be accumulated on a hard disk prior to writing as with the disc-at-once method. The minimum recording size must be at least 32KB, (even if the file to be recorded is smaller) as this is the minimum error correction code (ECC) block size for DVD. A disc that is being written to incrementally cannot be considered a complete volume until the final information has been stored or the disc capacity has been reached. The lead-in and lead-out boundary areas therefore cannot be written until either of these two events occur. Such an "unfinalized" disc (one without lead-in, lead-out and complete file system data) can only be read by a DVD-R drive until this process can be completed. After finalisation, a destination playback device can then read a disc, but data can no longer be added to it.

By late-1999, the take-up of DVD-R remained slow and the drives were prohibitively expensive - at around 10 times the cost of a DVD-Rewritable drive - having been further impacted by the appearance of DVD-ROM drives capable of reading DVD-RAM discs in mid-1999. It's large capacity and durability - its media has a typical life expectancy of better than 100 years - make it a good choice for the long-term archival of any information that can be stored digitally. Since DVD discs are dimensionally identical to the CD family of discs, they have the advantage of being compatible with existing CD-based jukebox and changer mechanisms. This allows automated retrieval of recorded DVD-R volumes in networked environments, with a six to seven-fold increase in storage density as compared with CD-R technology.

The DVD Forum's Version 2 specification - finalised in May 2000 - and consequent increase in capacity to 4.7GB, did serve to increase DVD-R's value as a tool for creating master discs before mass production by software houses and in multimedia post-production operations and as a medium for making back-up copies of movie discs. At the same time, it was determined that another type of DVD-R media was necessary for the consumer market, with the result that the format was split into the "DVD-R for Authoring" and "DVD-R for General" formats:

• The DVD-R(A) format continues to be aimed at the professional user and other differences between the formats are consistent with their relative market positioning. Principal amongst these is the DVD-R(A) Cutting Master Format (CMF) feature. This allows 4.7GB "authoring" media to be used as a direct replacement for DLT mastering tapes in the title replication process. This is accomplished by using a portion of the DVD-R's lead-in area to store the DDP (Disc Description Protocol) header information as normally used on DLT master tapes. The elimination of the need to use DLT will result is significant time saving in the final stage of authoring. Since the "authoring" and "general" formats use different recording laser wavelengths - 635nm and 650nm respectively - their media are incompatible for writing. For playback, there is no compatibility consideration and both types of media can be read equally well by DVD Video player or any DVD-ROM drive that supports DVD-R media.
• The key characteristic of the DVD-R(G) format - and quite possibly the key factor in the DVD Forum's decision split the DVD-R format in the first place - is that it contains content protection measures that make it physically impossible to make bit-for-bit copies of CCS encrypted entertainment titles. Other differences are that DVD-R(G) uses decrementing pre-pit addresses, a pre-stamped or pre-recorded control area and allows double-sided discs.

Until mid-2001, DVD-R had been used primarily in professional applications such as video authoring and the storage of imaging data. However, at that time the prospects of wider acceptance of the DVD-R(G) format were substantially boosted by the appearance of Pioneer's DVR-A03 recorder - an all-in-one recording solution capable of writing to all of DVD-R(G), DVD-RW, CD-R and CD-RW media - at an affordable price of around $1000.