Above shows engravings on the polycarbonate plastic of a DVD. The colours are a result of light refracted off the engravings and reflected in the DVD’s foil layer which is layered on top of the plastic.
DVDs are capable of holding a great deal of information. DVDs and CDs have microscopic tracks made up of data pits carved into the polycarbonate plastic of the disc. The data stored in these pits are then read by the DVD or CD laser beam which is reflected off the reflective foil, back through the plastic layer and into the data reader.
DVDs are able to store much more information than CDs because DVDs are able to have two layers of tracks (which can be focussed on individually by the data reader). Additionally, DVD data pits are much smaller (so they can fit more on). Typically, CD data pits are 500nm(nanometers)x900nm, with DVD pits being 320nmx400nm. Just to give some perspective, a millimetre (mm) is the same length as 1,000,000 nanometers – so yep, these pits are tiny. And because the data pits are arranged in a tightly spiralled track, discs can store millions of them. If you were to stretch a double sided DVD track lengthways, it would extend to about 48Km (30miles). That’s 48,000,000,000,000 nanometeres.
Data storage is an increasing issue. With high resolution screens and higher quality productions (particularly video games), a DVD or CD is increasingly insufficient to store enough data and the costs and user experience having multiple discs to store one data set, has forced technology to investigate new ways of getting more data onto single discs.
Blu-ray
Blu-ray discs work the same way as DVDs yet they are able to store 5x more data. The main difference with Blu-ray is the way the data is stored and read. Firstly, the laser used to read the data is different (it’s a blue violet laser, compared to DVD’s red laser) and the size of the data capture aperture lens is larger. The magnification of this lens is twice that of a regular DVD lens. This higher magnification is necessary because the data on a Blu-ray disc is stored far more densely. The tracks of data pits on a blu-ray disc are only 320nm apart (on a DVD, they are 740nm apart and on a CD, 1600nm). Some of the pit lengths are also very small at 138nm (compared to 400nm for a DVD). Therefore, with the combination of higher resolution data capture and increased data amounts, Blu-ray is an increasingly popular format for high-definition playback.
Angular-pits
Known as angular multiplexing, angular data pits is a new technology that could allow storage of up to 1 terabyte of information onto a disc – or the whole series of Friends, rather than having them in a 30 disc box set! The technology works by giving the sides of the data pits (which are usually steep and flat) angles and steps. This increases the surface area onto which data can be stored (angular multiplexing pioneer, Professor P Török’s grant application about angular multiplexing is worth a read).
Reading angular data pits is slightly more complicated because the data has to be read from angled sides but because of light’s property of polarisation, pits can be angled such that they correspond exactly with the angle of polarisation of the laser beam. This means – and this is the best part – the technology allows discs made this way to be backwards compatible with current disc players.
Data storage (showing dual layer, where available):
CD: >900Mb
DVD: 8.5Gb
Blu-ray: 50Gb
Angular pits: 1,000GB
With the combination of more highly compacted data pits, multiple data layers and increased surface area of data storage it is hard to imagine what the maximum data capacity is on a 12cm diameter plastic disc.
nOOpia loves this stuff so much, it’s lost its appetite.
