Q I am a student at Casey TAFE and I have nearly completed a 3-year, part-time certificate in IT. I am currently in the middle of a peripheral device module. Part of the assessment is to choose any peripheral, and do an assignment and a small presentation on it. I chose DVD-ROM. PC World has given me a wealth of useful and relevant information on the subject of DVD and so has a book I have, called The PC Bible.

A The Digital Versatile Disc (DVD) evolved from CD-ROM, and is the new generation of optical disc storage that boasts high performance and increased capacity. DVD-ROM is a variety of DVD that is designed specifically to hold computer data and to enhance the audio and video capabilities of a PC.

A DVD disc looks very much like a CD. ItÆs a silver disc that is 1.2mm thick, 12cm in diameter, and has a hole in the centre. Data is recorded onto a DVD disc in a spiral groove of tiny pits, which can be read by a laser beam and converted to an analogue signal that you can see or hear. The increased capacity of DVD over a CD is achieved by a combination of smaller pit lengths, tighter tracks, shorter wavelength lasers, and more efficient data compression, channel encoding, and error correction technologies.

In addition, a DVD disc can contain up to four layers of data, with two layers on each side of the disc.

All DVD discs are moulded from polycarbonate, and consist of two thin 0.6mm platters which are bonded together to create a single disc. The bonding process makes the disc rigid, and also creates two separate sides on it. Advances in the manufacturing process and semiconductor laser technology allow each side to hold two layers of data, which are usually referred to as the inner and outer layers. Consequently, a DVD disc can be manufactured in one of four ways: single-sided with one layer of data and a dummy base; doubled-sided with one layer of data on each side; single-sided with dual layers of data; and doubled-sided with dual layers of data. Usually the capacity of a dual-layer disc is slightly less than double that of a single-layer disc because the minimum pit length of both layers is increased slightly to reduce inter-layer crosstalk. As you might imagine, the laser in DVD players is quite sophisticated, and is able to focus on each of the two different levels of data separately. Because the outer layer of a disc can be partially reflective and partially transmissive, light from the laser is able to pass through it to the inner layer, which is totally reflective. The levels mentioned in your textbook are most likely referring to these different layers of data.

What your textbook may not have mentioned though, is that when it comes to measuring DVD capacities, the units of measurement in the world of DVD are slightly different to those in the rest of the computer world, and the two are easily confused. DVD capacities are often quoted in "G bytes", which are based on multiples of 1000 (one G byte is one billion bytes, or 10⁹). In the computer world, however, measurements are based on multiples of 1024, so that one gigabyte (GB) is 1,073,741 824 bytes or 2³⁰.

The four types of DVD discs and their capacities can be summarised as follows: