When DVD hit the home video market a decade ago, it changed the whole conception of home entertainment with its superior video and sound quality. The old VHS tape was no match to DVDs which came with high resolution capabilities, superior colour quality and other features like digital sound effects, extra commentary tracks, zooming and movie selection features. The full length movie could be recorded in a single DVD and this was not a small feat that time. DVD’s popularity soon led to the demise of VHS tape and adoption of DVD format by the film industry in the following years.
However, a standard DVD had a major disadvantage. A high density optical disk though it is, a DVD could store only standard definition videos. At best, DVDs can store and display movies with a resolution of 720×480 pixels either in interlaced or progressive scanned formats. This is much lower resolution by the high definition standards. The main reason behind it is its storage capacity which is much less than what is required to record a full high-definition movie.
This setback was soon overcome by the emergence of two high definition formats – HD DVD and Blu ray. These competing formats are also incompatible with each other that led to format war with companies and entertainment industry is quite divided over their support and adoption of these formats.
Just as we compare HD DVD with other high density optical discs and to find out its advantages, it is worthwhile to understand the basics of HD DVD format and HD-DVD discs
What is HD-DVD format?
High-Definition DVD (HD-DVD) is an optical disc just like a DVD, but it comes with better storage capacity so that it can record and playback high definition movies. HD-DVD disc has the same diameter (12 cm) and same thickness (1.2 mm). However, a HD-DVD can hold much more information than a DVD. To make it more clear, a single layered HD-DVD can hold 15GB of data as against 4.7 GB capacity of single layer DVD. In other words, a DVD can store a movie (standard video) of two hours duration while a HD-DVD can store more than twenty such standard videos.
Although both DVD and HD-DVD store data in the form of pits on their surface, there are a lot of differences as to how closely the pits and tracks are arranged and how data is recorded or read etc.
In a HD-DVD the pits are made much smaller and the spiral tracks are more tightly packed. HD-DVDs have less than half of the track pitch of standard DVDs (0.40 micrometers Vs 0.74 micrometers). To store and read data from such minuscule pits, HD-DVD uses blue laser (or violet) of smaller wave length (405 nanometer).
Most HD-DVDs use MPEG-4 video compression standard to store information. This enables HD-DVDs to compress large data into small files using less space on the disc. HD-DVD is also compatible with MPEG-2 compression format which is a standard encoding method for DVDs.
Types of HD-DVDs
There are four types of HD-DVD discs based on their capacities and read-write features:
- HD-DVD ROM
- HD-DVD RW
- HD-DVD R
- HD-DVD RAM
Except HD-DVD RAM, all other types have 15 GB capacity per layer. Thus, a double layered double side HD-DVD will have a capacity of 30 GB.
HD-DVD RAM is a high capacity disc that can hold 20 GB on a single layer and 40 GB on a double layer.
Video and audio formats
For high definition recording, HD-DVDs use various HDTV resolutions and video compression formats.
The range of HDTV resolutions that HD-DVD can support include 720p, 1080i, 1080p while the standard DVD video format (480p) is also supported by most HD-DVDs. Most HD-DVDs are capable of compressing in both MPEG-2 and MPEG-4 formats along with other more efficient formats like VC-1 and AVC.
HD-DVDs can support up to eight sound channels at 24 bit or192KHZ and audio can be decoded using a variety of digital audio formats that include Dolby Digital AC-3, DTS, Dolby TruHD and DTS-HD Master Audio.
HD-DVD Movies
There were about 330 movie titles available in HD-DVD format by the end of 2007. Along with HD-DVD only discs, dual-format discs and hybrid discs have also been used by some of the companies for movie release. In a dual-format disc, there is movie in DVD format (4.7 GB layer) on one side of the disc and HD-DVD version (15 GB layer) on the other side.
Hybrid discs come in two versions: HD-DVD/DVD hybrid disc and HD-DVD/Blu-ray hybrid disc. These discs support two formats and are playable by HD-DVD player and blu-ray disc player.
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