Likewise, Blu-ray Discs (BDs) and HD DVDs have the potential to provide the highest resolution that the current high-def video standard allows. They could even provide unprecedented audio performance by using various advanced capabilities, but these still weren't finalized as of this writing (height channels, anyone?). Both systems also promise seamless integration with the Internet and extensive new capabilities for videogame producers.
All these new powers stem from two breakthroughs. First is an increase in the maximum data rate for streaming information off the discs — 36 megabits per second (Mbps) for both BD and HD DVD compared with 11 Mbps for standard DVD — which allows these discs to provide even better picture quality than current high-definition TV broadcasts. They'll also be able to handle several applications simultaneously, letting you do things like call up background audio and video commentaries without interrupting the movie or choose from multiple camera angles on the fly for the scene you're watching.
The second breakthrough is far greater storage capacity than DVD. While a single-sided, single-layer DVD can hold 4.7 gigabytes (GB), a single-sided, single-layer HD DVD can hold 15 GB, and a comparable Blu-ray Disc can hold 25 GB (see the table on page 81 for detailed technical comparisons between the four formats). While each high-def format starts with a new blue laser to achieve these breakthroughs, they follow radically different paths from there.
Honey, I Shrunk the Pits
The key technology behind both HD DVD and Blu-ray is the blue laser, an evolutionary development of the infrared and red lasers used in CD and DVD players. The wavelength of the light coming from the blue lasers in both high-def disc formats is 405 nanometers (billionths of a meter), which is shorter than the DVD wavelength of 650 nanometers (a pure red) and nearly half the CD wavelength of 780 nanometers (in the near-visible infrared). Using a shorter laser wavelength allows a much smaller spot to be focused onto the reflective data layer (see “Laser spot sizes” in the diagram below).
Top, the red and blue laser spots used to read the four optical-disc formats are shown to scale, though greatly enlarged. Middle, the data surfaces are shown to scale in simulations of atomic-force-microscope scans of the pit layers. Bottom, vertically to-scale renditions of the disc layers used by the four formats.
Using a smaller spot lets you shrink everything on the disc pro- portionally to increase the amount of data you can pack in. This happened once before with the move from the CD's infrared laser to DVD's shorter-wavelength red laser. The data-carrying pits and the spacing between revolutions of the pit trail (the “track pitch”) are both substantially smaller with Blu-ray and HD DVD than they were with DVD. (Click to see the format specifications PDF and the 'Data surfaces' in the diagram above. It's a delicious historical irony that some of the atomic-force microscope photos on which the diagrams are based were made by dragging a microscopic stylus over the disc — shades of the phonograph!)
Optically, the HD DVD system was arrived at by taking a standard DVD and shrinking the dimensions of the data layer as much as the new laser wavelength allows. DVDs and HD DVDs have the same diameter, and both use the same sandwich construction made up of two 0.6-mm substrates, only one of which usually carries data, bonded with an adhesive to create a disc 1.2-mm thick (same as a CD). DVDs and HD DVDs have the same thickness as CDs to retain compatibility with current disc-loading mechanisms. Otherwise, DVDs and HD DVDs could be half as thick.
Small Size, Big Problems
But having a disc that thin would have compounded a problem that arises when you shrink both the laser spot size and the width of the data tracks: the thinner the disc, the more sensitive it is to “tilt” and other optical imperfections that can distort or redirect the laser beam. Rather than use a diagram to illustrate this problem, I'm going to ask you to try an experiment.
Take the bottom side of a DVD or CD, hold it up at arm's length, and adjust its position so that you're looking straight on at the reflection of your eye. This represents the ideal disc-scanning geometry, where the laser beam hits the disc at right angles and is reflected directly back into the scanning lens (your eye).
Now, with the disc still at arm's length and with you still looking straight ahead, slowly turn or tilt the disc until you can no longer see your eye's reflection. Remember the angle at which you lose the reflection. You've just simulated what happens to a player's scanning mechanism when a disc is tilted with respect to the laser.
Disc tilt can come from many sources, such as a mechanical misalignment — one reason you don't want to have to redesign a properly working disc mechanism. A more likely source of deviations from perfect flatness is poor disc molding during manufacture or warping from poor storage afterward. In the worst cases, disc tilt can cause a laser to lose track of the trail of data pits completely, just as you lost view of your eye when you tilted the mirror.
In milder cases, slight disc tilt and other misalignments cause the laser spot to change from a circle to an ellipse. The problem with ellipse-shaped laser spots is that they can pick up more than one track of disc data at a time. Auxiliary elliptical spots are sometimes used, in fact, to help center the main reading beam on the data track.
In any case, a deformed reading spot probably won't be able to register the shape of the passing pits precisely, causing the scanner's electronics to produce a misshaped waveform in which it's difficult to tell a recorded “1” from a “0.” This makes disc error rates go up, possibly beyond the point where the player's error-correction systems can compensate. And this is only with DVDs — things get even more dicey when you're using the reduced laser spot, pits, and track pitch of the new high-def formats.
There are two ways to combat disc tilt. You can tighten manufacturing tolerances for disc flatness (for instance, HD DVD's limits for disc tilt are a few percent tighter than for DVDs), or you can move the reflecting surface closer to the laser. Pick up your disc again. This time, hold it only 6 inches or less from your face. While looking straight at the surface, tilt it again. You'll notice this time that the angle at which you can hold the disc before you lose the reflection of your eye is much greater than before.
You've just experienced what a DVD player's scanning mechanism encounters when you switch from playing a CD, in which the laser has to shine through a 1.2-mm substrate to get to the reflective data surface, to playing a DVD or HD DVD, where the data surface is only 0.6 mm beneath the disc's surface — a huge change of optical distance. While the pits and track pitch are smaller on a DVD than a CD, having the reflecting surface closer to the lens reduces the system's sensitivity to tilt. In fact, both DVDs and HD DVDs use a sandwich construction to address the optical necessity of moving the reflecting surface closer to the lens and the mechanical necessity of making a disc as thick as a CD.
So Near and Yet So Far
To achieve an even higher data capacity than HD DVD while using the same laser wavelength, Blu-ray lasers produce an even smaller spot, allowing for an even smaller pit size and track pitch. But to do this the reflecting layer has to be even closer to the lens. And to focus so closely, you have to use a considerably different lens design than the one in DVD players. The data layer in a BD is only 0.1 mm underneath the disc surface — one-sixth the distance of the same layer in a DVD or HD DVD (see “Disc cross sections” in the diagram below).
Top, the red and blue laser spots used to read the four optical-disc formats are shown to scale, though greatly enlarged. Middle, the data surfaces are shown to scale in simulations of atomic-force-microscope scans of the pit layers. Bottom, vertically to-scale renditions of the disc layers used by the four formats.
Aside from considerably increasing data capacity, Blu-ray's more radical approach to a high-def format has several notable consequences. The first is that most dreaded of consumer-electronics words: incompatibility. The problems begin at the factory. The equipment for manufacturing DVDs and HD DVDs is designed to handle separate, 0.6-mm-thick disc layers, which are then glued together for the final disc. But BDs use a single, 1.1-mm-thick substrate and a 0.1-mm protective layer through which the laser shines. This requires an expensive changeover to all-new disc-pressing and manufacturing processes and machines.
Also, DVD optical-scanning mechanisms are optimized for focusing on the deeper data layers in DVDs and HD DVDs. The laser-pickup mechanisms in Blu-ray players and recorders also need to be able to read CDs and DVDs as well, and possibly even HD DVDs. So laser pickups that can handle a number of disc formats are likely to be more expensive than ones optimized just for BD.
Bringing the data layer closer to the disc surface also increases Blu-ray's sensitivit y to surface defects like scratches, dust, and fingerprints. With CDs, DVDs, and HD DVDs, the defects are out of focus by the time the laser spot hits the data layer inside the disc, minimizing interference. The first Blu-ray prototype discs had disc cartridges to guard against surface contamination, but recent prototypes have had a “hard-coat” layer (which TDK uses on some of its recordable DVDs) that reduces data-corrupting disc damage. Nonetheless, for reliable operation, it'll probably be important to keep BDs as clean and scratch-free as possible (not a bad idea for any optical format).
While the impending format war between Blu-ray and HD DVD stems from their incompatible optical systems, the only thing audio- and videophiles really care about is what's recorded on the discs. In the months leading up to the new formats' launches, we'll run articles and columns examining the substantially improved video and audio expected from the use of advanced encoding systems that are being standardized even as I write. We'll also look at the new worlds opened up by the extensive game, menu, and Internet interactivity that's being designed into these systems from the start. For years, the differences between products and even formats have been insignificant. But now the excitement is back. Whatever Blu-ray and HD DVD prove to offer us, it's not going to be “same old, same old.”