Desktop version

Home arrow Computer Science

  • Increase font
  • Decrease font


<<   CONTENTS   >>

Digital Versatile Disk (DVD)

The enormous success of the CD technology, and its wide acceptability in the user domain encouraged the designer to further go ahead in quest of even more storage capability that has eventually led to the emergence of high-capacity digital versatile disk (DVD) optical disk within a few years. It truly replaces the existing analog VHS video tape being played with a video cassette recorder (VCRs), and actually begins the digital era of the videos. While the physical size of a DVD (1.2-mm thick and 120 mm in diameter) is the same as that of a CD, but the storage capacity is currently as much as seven times of a CD-ROM. This huge storage capacity further facilitates to enrich the multimedia culture, thereby allowing more videos to incorporate into any software for regular use as well as software into videos for demonstration purposes.

Before the advent of DVD and Blu-ray, video-CD (VCD) introduced sometime in 1993 became the first format for distributing digitally encoded films on standard 120 mm optical disks (its predecessor, CD-video used analog video encoding). In the same year, two new optical disk storage formats were being developed. One was the Multimedia Compact Disk (MMCD), backed by Philips and Sony, and the other was the Super Density (SD) disk, supported by Toshiba, Time Warner, Matsushita, Hitachi, Mitsubishi Electric, Pioneer, Thomson, and JVC. However, the DVD disk storage format invented and developed by Philips, Sony, Panasonic, and Toshiba sometime in 1995 has achieved much larger storage by incorporating several distinct changes in the concept of existing CD design. Some notable ones are the following:

  • 1. A laser diode red light with a wavelength of 635 pm (micron) is used instead of the infrared light laser used in CDs having a wavelength of 780 pm. This shorter wavelength makes it possible to focus the laser light on a relatively smaller spot. Consequently,
  • • Bits are made packed more closely on a DVD.
  • • Moreover, the spacing between the loops of a spiral track on a DVD is 0.74 pm, whereas in CD, it is 1.6 pm.
  • • The minimum distance between pits along the spiral in a DVD is 0.4 pm, whereas in a CD, it is 0.834 pm.
  • • The minimum width of the pits in a DVD is 0.4 pm, whereas in a CD, it is 0.5 pm.

The ultimate result of these improvements is about a 7-fold increase in capacity, to about 4.7 GB (= 7 x 680 MB). This single-layered single-sided disk is defined in the standard as DVD-5. Further increases in capacity have been achieved by way of approaching towards making a two-layered and two-sided disks.

2. A double-layered disk makes use of two layers on which tracks are implemented on top of each other. Here, the pits of the second layer lands on the top of the first. The first layer is the clear base, as in CD disks. But, instead of using reflecting aluminium, the lands and pits of this layer are covered by a translucent material that acts as a semi-reflective layer. The surface of this material is then also programmed with indented pits to store data. A reflective material is placed on top of the second layer of pits and lands. The disk is read by adjusting the focus of the laser beam on the desired layers. The lasers in DVD drives can read each layer separately. When the beam is focused on the first layer, sufficient light is reflected by the translucent (semi-reflective) material to detect the stored binary patterns. When the beam is focused on the second layer, the light reflected by the reflective material corresponds to the information stored on this second layer. In both cases, the layer on which the beam is not focused reflects a much smaller intensity of light which is subsequently eliminated by the detector circuit considering it as noise. This technique almost doubles the capacity of the disk to about 8.5-8.7 GB, instead of the expected exact double 9.4 (2 x 4.7 = 9.4). This happens due to the lower reflectivity of the second layer which affects its storage capacity to a little extent that limits to achieve a straightaway full doubling. This disk is, however, called DVD-9 in the standard.

Two single-sided disks can be put together to form a sandwich-like structure where the top disk is turned upside down. This can be done with two single-layered disk giving rise to a composite disk, known as double-sided single-layered disk providing a capacity of

9.4 GB (2 x 4.7 = 9.4). This disk is called DVD-10 in the standard.

The above approach can be carried out with two double-layered disks giving rise to a double-sided double-layered disk that yields a total capacity up to 17 GB. This disk is specified as DVD-18 in the standard. A layered view of a double-sided dual-layered DVD is depicted in Figure 4.12 (make a comparison with the corresponding CD layered-view as depicted in Figure 4.10). The notable characteristics and specifications of different types of

FIGURE 4.12

DVD-ROM double-side dual-layer capacity (17 GB).

TABLE 4.4

Specifications of DVD Disks

Read Mechanism

650nm laser,10.5 Mbits/s (lx)

Write Mechanism

10.5 Mbits/s (lx)

Capacity

  • 4.7 GB (single-sided, single-layer common) 8.5-8.7 GB (single-sided, double layer) 9.4 GB (double-sided, single layer)
  • 17.08 GB (double-sided, double-layer rare)

Standard

DVD Forum's, DVD Books, and DVD+RW Alliance specifications

DVD disks are summarized in Table 4.4. However, the fundamental characteristics, such as, pit size, inter-spiral track gap, etc. are all same for all types of DVD disks, and, hence, are not included in this Table 4.4.

Writing speeds for the DVD were initially lx, that is, 1,385 KB/s, in the first drives and media models. More recent models, at 18x or 20x, have 18 or 20 times of that speed. Note that for CD drives, lx means 153.6 KB/s, about one-ninth as swift. The DVD drives provide an access time that is similar to that of CD drives. However, when the DVD disk rotates at the same speed, the data-transfer rate is much higher because of the higher density of pits available in DVD disks.

Pre-recorded DVDs are mass-produced using moulding machines that physically stamp data onto the DVD. Such disks, similar to CD-ROMs are known as DVD-ROM, because data can only be read and not written nor erased. Blank recordable DVD disks (DVD-R and DVD+R) can be recorded once, similar to CD-R, using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM), similar to CD-RW can be recorded and erased multiple times. The specifications of these disks are summarized in Table 4.4.

The DVD video format was first introduced by Toshiba in November 1996 in Japan, in the United States in March 1997 (test marketed), in Europe in October 1998, and in Australia in February 1999. In May 1997, the existing DVD Consortium was replaced by the DVD Forum, which is open to all other companies.

High-Definition Optical Disk: HD-DVD and Blu-Ray

High-definition optical disks (HD DVDs) are designed to provide even more storage capacity compared with traditional DVDs by further reducing the size of the pits used for storing data, thereby achieving higher bit density, and that became possible with the use of an even shorter wavelength laser beam of 405 nm in the blue-violet range.

In 2006, two competing new disk formats called HD DVD and Blu-ray disk were released as the successor to DVD. The HD DVD, however, competed unsuccessfully with Blu-ray disk in the format war of 2006-2008. The Blu-ray scheme ultimately achieved market dominance, and became the successor to the DVD format. The single-layer/single-sided HD DVD scheme can store 15 GB and a dual layer HD DVD can store up to 30GB, while a single-layer/single-sided Blu-ray disk can store 25 GB and a dual layer Blu-ray disk can hold up to 50GB. This high capacity of Blu-ray is mainly due to its positioning of the data layer on the disk much closer to the laser when compared with DVDs, that facilitates a tighter focus that, in turn, permits to use further smaller pits and closer tracks with lesser distortion. However, three categories of Blu-ray disk are available, namely, read-only (BD-ROM), recordable-once (BD-R), and re-recordable (BD-RE).

In spite of availability of higher-capacity disk with changed format, the standard DVD still remained dominant as of 2012, mainly due to the fact that the Blu-ray technology was not sufficiently matured to replace the existing culture. Moreover, write and read speeds of Blu-ray were relatively poor, and also the fact that the necessary hardware was still expensive, and not very readily available. That is why, a large majority of consumers were fully contended with the existing DVD practice, although the Blu-ray players and the now-defunct format HD DVD players were designed to be downward compatible, thereby allowing older DVDs to play at will with these devices, since the media are physically identical. Nevertheless, it was commonplace for major releases issued in "combo pack" format that included both a DVD and a Blu-ray disk (as well as, in many cases, a third disk with an authorized digital copy). Also, some multi-disk sets have used Blu-ray for the main feature, but DVDs for supplementary features (examples of this include the Harry Potter "Ultimate Edition" collections, the 2009 re-release of the 1967 The Pioneer TV series, and a 2007 collection related to Blade Runner). Another reason cited (as of 2012) for the slower transition to Blu-ray from the existing standard DVD was the necessity of, and confusion over “firmware updates" that needs an access permission while performing updates over the internet.

 
<<   CONTENTS   >>

Related topics