Blu-ray vs. HD-DVD

Authors: spath & H3rB3i
Date: March 2005

2005 is here and the companies promoting the next generation storage formats are ready for the last and most important rounds in the ‘war", which will probably answer the question: which format will become the next standard for movie and data storage?

HD-DVD and Blu-ray: What are the technologies?

These two formats are heralded as the successor to the current DVD technology. Blu-ray and HD-DVD have both been developed to enable recording, playback and rewriting of high definition video and data. The key to these technologies is the blue-violet laser that is used to write the data to the disc. This blue laser has a much shorter wavelength than the current red laser DVD system, which makes it possible to read and write smaller pits, as a result, these discs can hold up to 15 GB (HD-DVD) and 25 GB (Blu-ray) of data on a single sided single layer 12cm disc.

This additional storage capacity will be essential when HDTV becomes mainstream, in order to allow storage of HD TV shows or movies on an optical disc in the same high quality. You can record about 13 hours of standard TV but only a bit more than 2 hours of uncompressed high definition TV on a 25GB disc.

When we take a closer look at the two competing formats, we can quickly see that the dimensions of the discs and the use of a blue laser are about the only similarities the two formats share.

But before we start to compare the two formats to find out the differences, let us explain some terms we will need to know, to understand how it is possible to store that high amount of data on discs that are looking like conventional CDs or DVDs.

Numerical Aperture (NA)

The numerical aperture of an optical lens is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance.

Together with the wavelength, this is how the NA or the resolution of an optical system is determined. The NA and the wavelength also define the size of the laser beam; the result of a higher NA and a shorter wavelength is a smaller laser beam.

This allows focusing the beam with much higher precision and a reduction of the Track pitch.

Track Pitch

The data track of an optical disc is a spiral starting from the centre of the disc; the distance (in micro meters) between two rows of the track is the track pitch. It is measured from the centre of the row to the centre of the next row.

When the Track Pitch can be reduced, this means that the distance between the rows is lower and that the resulting track is much longer, thus you can store more data.