There has been a lot of discussion lately regarding the two most prominent recording formats being promoted for HDTV use. Looking back at the recent past we could say that the same groups that had fought over DVD-Video formats before it was finally standardized, are still fighting for two new formats. On one side of the fence we have two Japanese giants of the consumer electronics industry, NEC and Toshiba. They have jointly developed the "AOD" (Advanced Optical Disc) format for HDTV recording applications. The main argument for AOD is that it is play-compatible with DVD-Video, and so it is often called "HD-DVD" for that reason. Sony and Philips along with Hitachi, Sharp and Samsung stand on the other side. They are promoting the "BD" format, which is better known as the "Blu-Ray Disc". Both formats initiate evolutionary higher capacities than current DVD-Video.

Structurally, HD-DVD is identical to DVD-Video, employing two 0.60mm thick discs that are bonded together to form the HD disc. It can store 15GB on a single layer DVD. HD-DVD recordings can therefore be replicated on the same equipment and manufacturing infrastructure used to replicate standard DVD-Video recordings. As a result, this minimizes disc production costs.

BD Main Specifications

	Read-only	Rewritable
Data capacity	Single-layer 15GB/sideDual-layer 30GB/side	Single-layer 20GB/sideDual-layer 40GB/side
File format	UDF
User bit rate	36Mbps
Disc size	120mm (diagonal), 1.2mm (thickness: 0.6mm x 2)
Laser wavelength	405nm
Lens numerical aperture (NA)	0.65
Track structure	--	Land & groove
Signal processing	PRML
Modulation	2/3 conversion

Toshiba and NEC insist that the provided 15GB of capacity is more than adequate for HDTV software applications. The compression algorithms used in HD-DVD are different from those currently in use, so there are claims that the 2/3 conversion could reduce picture quality. On the other hand, the format features the same 0.6 numerical aperture for the object lens and disk structure as the current DVD disk system. The companies stressed the importance of backward compatibility. Taking advantage of this compatibility, NEC has developed a HD DVD drive that employs a single optical head. The head has blue laser and red laser diodes as light sources. However, the lasers share the same object lens. Newly developed ICs handle the physical difference between DVD and HD DVD.

NEC claims the single optical head structure enables production of smaller and thinner HD DVDs at lower cost. Using the single head, NEC developed prototypes of full-and half-height drives that achieved 15 Gbytes for a single-layer ROM disk, 30 Gbytes for a double-layer ROM disk and 20 Gbytes for rewritable disks.