Phil Storrs PC Hardware book

DVD - Digital Versatile Disk

Although CDROMs have only been popular for the past few years, the original standards for audio Compact Disks were finalized in 1980 and the current Data Storage format is based on these standards.

The capacity of the current technology, 650 to 680 Mbyte and the Data Transfer Rate of about 150 Kbytes per second, is not sufficient for the storage of movie length high quality video presentations.

The computer and entertainment industries are working on a new CD format, using 1990's technology. DVD, Digital Versatile Disk as it is called, will have capacities as high as 8.5 GByte and data transfer rates as high as 10 Mbits per second. This has been achieved by reducing the spiral track pitch, more than halving the inter-pit spacing (made possible by using a shorter wavelength laser) and relying on more exotic error detection and correction techniques.

Parameter CD data standard DVD data standard
Disk diameter 120 mm 120 mm
Disk thickness 1.2 mm 1.2 mm
Disk structure Single substrate two bonded 0.6 mm substrates
Laser wavelength 780 nm 650 and 635 nm
Numeric aperture 0.45 0.60
Track pitch 1.6 um 0.74 um
Shortest pit/land length 0.83 um 0.4 um
Reference speed 1.2 m/sec CLV 4.0 m/sec CLV
Number of data layers one one or two
Data capacity 680 Mbyte (approx) 4.7 Gbyte one layer
8.5 Gbyte two layer
Reference user data rate 153.6 K/sec (mode 1) 1,108 K/sec (mode 1)
176.4 K/sec (mode 2)
Parameter Video CD DVD-Video
Video data rate 1.44 Mb/s 1 to 10 Mb/s
video, audio, subtitles
Video compression MPEG1 MPEG2
Sound tracks 2 channel MPEG NTSC
2-channel linear PCM
2-channnel/5.1 AC-3
up to eight streams of data
Subtitles one caption only up to 32 languages

What all this means is that DVD Video can hold 133 minutes of MPEG-2 encoded broadcast quality video, together with three sound tracks that can be encoded with Dolby's 5-track surround-sound AC-3 standard.

The only cloud on the horizon of DVD technology is the fear of the entertainment industry, of mass piracy. The data is digitally encoded and so can be copied over and over without degrading the signal, and the standards are independant of national television standards and so one disk can be used all over the world. This means markets can't be manipulated in the way they can today with different video tape standards in use around the world. This has meant the film-makers are trying to get complex hardware anti-copying and nationalisation devices included in the DVD standards. This will have two effects on the implementation of DVD, the cost of drives will be increased by the increased complexity and these measures may effect the performance of Data DVD.

The latest word on DVD as of December 96

The specifications listed above have been enhanced even before DVD has been released to the market. A DVD disc can now have up to four pressed data layers, two on each side, for a total capacity of 17.08 gigabytes on a 12 cm disc, or 5.32 gigabytes on an 8 cm disc. They are made by bonding two .6 mm thick substrates together to form a 1.2 mm to 1.5 mm thick disc with data in the middle, and the same physical size as a CD-Rom. The most likely version for the near future will be the single layer, single sided 12 cm disc with a capacity of 4.70 gigabytes. The spin speed of the single layer disc is a constant linear velocity of 3.49 meters/second, slower than today's multispeed CD-Rom drives. The user data read rate is 11.08 Mb/s (Megabits per second) with an audio/video media payload of 9.8 Mb/s or 1.225 MB/s. That is about equal to the performance of the 8 times CDROM drives.

What are the sizes and capacities of DVD now ?

There are many variations on the DVD theme. There are two physical sizes: 12 cm (4.7 inches) and 8 cm (3.1 inches), both 1.2 mm thick. This is the same form factor as CD. A disc can be single-sided or double-sided. Each side can have one or two layers of data. The amount of video a disc can hold depends on how much audio accompanies it and how heavily the video and audio are compressed.

For reference remember a CDROM holds about 650 Meg byte of data (0.65 Gigabyte).

It takes about two gigabytes to store one hour of average video data.

The above table was taken from a very good FAQ on DVD
Another source of information is from the Sony Corporation

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Copyright © Phil. Storr 4th December 1998