We are all aware that high-definition DVDs have been a long time coming, and as ever with A/V recording technology, two incompatible formats are vying for the hearts of consumers. In the case of high-definition DVD, the two formats are Blu-ray and HD-DVD. The arguments about the superiority of one format over the other are marketing and "electro-political" issues. Let's take a look at some of the technical aspects of the two formats.
Both of these formats are intended to store hig definition video and multichannel audio data on digital optical disks that, at 12 centimeters in diameter, are the same size as compact disks and standard DVDs. The digital data is recorded on the disks in the same fundamental manner as that employed on CDs and standard DVDs: a laser is used to burn a series of pits into the substrate material. Coding schemes aside, two binary states--positive and negative--must be represented on the surface of the base material, and these states are denoted respectively by the presence of a pit that does not reflect light back to the lens, or the absence of a pit, in which case, light is reflected back to the lens. (User-recordable CDs and DVDs have their "pits" "burned" into a dye that coats the substrate surface, rather than being literal pits in the substrate.)
The standard compact disk contains digital audio recorded in uncompressed form. The amount of data required to represent video, on the other hand, is too great to be practicably recorded in its uncompressed form on a 12 centimeter optical disc. The standard DVD uses MPEG-2 video compression to reduce the data content required to represent a video program of reasonable length. As a frame of reference, a CD can contain about 700 MB of data, while a standard DVD can contain about 4 GB of data.
We are well aware that the quantity of data generated by high-definition video requires another leap in data storage capacity to make a high-definition DVD a practical reality.
VIOLETS ARE BLUE
Either of the high-definition DVD formats can accommodate compression coding using AVC or VC-1, which permit equivalent video quality at higher compression ratios than those afforded by MPEG-2, (as well as MPEG-2 itself). This is helpful, but not sufficient.
Fortunately, we now have blue-violet lasers. A semiconductor laser has two properties that are fundamental to its use as a DVD-burning device--it emits light at a single, narrow spectral wavelength, and the light that it emits is phase-coherent. Phase coherency concentrates the energy of the emitted photons so they may be used to burn pits in a substrate, while the emission of a single wavelength facilitates control of the physical size of the resultant pits. The standard DVD uses a red-light laser with a wavelength of 650 nanometers. For those with an RF orientation, 650 nanometers corresponds to a frequency of about 461,220 GHz. A blue-violet laser emits phase-coherent light at a wavelength of 405 nanometers, or a frequency of about 740,230 GHz. This is about 0.6 the wavelength of the 650 nanometer light, and it facilitates burning a smaller pit.
The brand name of one of the high-definition DVD formats notwithstanding, both formats use the 405 nanometer blue-violet laser. It is at this point that the formats diverge. The two formats use different lens systems to focus the laser light, and these systems have different numerical apertures.
Numerical aperture is a measure of the lens system's ability to gather light and thus resolve fine detail at a fixed object distance.
When light hits an object, or a pit, it diffracts, splitting into several diffraction orders that are bent at increasing angles from the original beam. So when light is reflected off the substrate surface, it scatters at various angles rather than reflecting directly into the lens.
The light beam reflected off a pit in the substrate reaches the lens in the shape of an inverted cone; the point of the cone is at the pit, and the broad base of the cone, at the lens surface. The angle of the side of this light cone is called the "angular aperture." The numerical aperture is equal to i(sin q), where q is one-half the angular aperture, and i is the refractive index of the medium between the object and the lens. In this case, the medium between the object and the lens is air and the value of i is 1.0, so the numerical aperture is simply sin q.
The numerical aperture of the HD-DVD format is 0.65, while that of the Blu-ray format is 0.85. The HD-DVD disk has a track pitch of 40 micrometers and a minimum pit length of 204 nanometers, while the Blu-ray disk has a track pitch of 32 micrometers, and a minimum pit length of 160 nanometers.
The geometry of the optics used in the Blu-ray format requires the pit tracks to be located closer to the surface of the disk than those of the other format. The practical result of this is that the disk's outer surface is more in focus than in other formats where the tracks are located farther away from the surface, and the optical system of the player is thus less able to see through surface dirt and scratches while focusing on the pits. This produces the requirement for a cleaner surface, and a stronger polymer coating that is more resistant to scratching and chipping.
Those are the fundamental technological differences between the two high-definition DVD formats. What materializes in the marketplace is still an open question.
