According to Sony, 120 million CD players and three billion CDs have been sold in the US alone. The success of the CD - as if nobody knew it - is total. When was the last time you heard the word 'record'? Now a new revolution is upon us: the digital video disc or DVD. There is the DVD for video and the DVD-Rom for computer data. The new medium looks almost exactly like the old; it is 120 millimetres in diameter and it is 1.2 mm thick. Looks, however, can be deceiving. The new DVD format holds seven times the data of the standard CD. That translates into 4.7 gigabytes per layer - and there can be two layers with DVD. A disc with two layers can hold 8.5 gigabytes on one side. The increase in data capacity is accomplished in a number of ways. First, the pit size is much smaller on DVD. The pits are the holes or troughs that make up the bits of the data. When enlarged they look a bit like graphic representations of Morse code, a not altogether false analogy. The minimum length of a pit on a standard CD is 0.83 micrometres. The minimum length for a DVD pit is 0.4 micrometers. The second thing is that the tracks are much closer together. The tracks on a standard CD are 1.6 micrometres apart whereas those on the new DVD are 0.74 micrometers away. To access these smaller pits a different kind of laser is needed. The conventional one used in CDs is an infrared light with a wavelength of 780 nanometres. The DVD players and DVD-Rom drives use a red light laser with a wavelength of 650 and 635 nm. This shorter wavelength can pick up the smaller pits with ease. Together with this change in laser comes a new laser assembly. The DVD uses a higher numerical aperture (NA) lens which also helps in focusing the laser beam more accurately for the narrower tightly packed pits. The new DVD uses digital modulation and error-correction schemes that have been specifically designed for the increased capacity of the new medium. DVD uses the 8 to 16 (EFM PLUS) modulation scheme, assuring it backward compatibility with today's discs. The error correction system is the Reed Solomon Product Code (RS-PC) which experts agree is 10 times more robust than at present. As if this increase were not enough, the new DVD also supports dual-layer discs. These are discs with two layers on one side, not to be confused with double-sided discs. The one closest to the laser is a 'semi-transmissive layer', permitting the laser to focus 'through' the layer and on to a second layer that is deeper. This makes it possible to double the amount of data on a single side of the disc. It also means that it is not necessary to flip the disc as one does, for example, with laser discs for films. This double-layered disc can hold up to 8.5 gigabytes of data, giving it 12 times the capacity of today's CDs. The way the laser seamlessly changes focus between the two layers is that it uses a data buffer that can stay ahead so that no time is lost when the laser must refocus to the deeper layer. The makers of DVD claim higher standards in the quality of video as well. Sony says that the video quality 'approaches D-1', the CCIR-601 studio production standard. The CCIR-601 digital video standard has a video rate of 167 megabits per second. To achieve this, DVD must use some form of data compression. (If it did not, a 4.7 gigabyte disc would hold only a paltry four minutes of video.) DVD uses the Motion Picture Experts Group 2 (MPEG2) standard. This uses something called 'redundancy' to achieve extremely high compression rates. Redundancy is just how two particular pictures 'differ'. It is much easier to save only those parts of a picture that make it different from another. In video, nearly every picture uses most of the data of the previous one. All this data being the same, it would be redundant to save it more than once. By using MPEG2, a single-sided, single-layer DVD disc can hold up to two hours and 13 minutes of video. Even with this there is enough digital space left over to handle discrete 5.1 channel digital sound. It would seem that this new format is here to stay. But things do change rather quickly these days.