BLANK DUAL LAYER DVD
"Blank dual layer DVD" is one of the most popular search terms related to our beloved format, but work is in progress that should dwarf all current notions of mass storage capacity as it relates to the DVD. Time and tide waits for no man, so runs an old proverb, and it is especially applicable to the march of technology; though we are still at the dawn of the Blu-ray Disc, development proceeds apace on optical media of the future that will completely overshadow anything we have today. Exhibit A: the Holographic Versatile Disc, or HVD.
HVDs may one day hold up to six terabytes of information (how's that for "blank dual layer DVD"), though for now experimental prototypes can store a maximum of 500GB. Using a technique called collinear holography, two green-colored laser beams are collimated into one, reading data encoded in a holographic layer near the surface of the disc while a blue laser beam is used as a reference beam to read servoinformation from a regular CD-like aluminum layer near the bottom of the disc. (Servoinformation is data used to control the positioning of the read-head over a disc. On CDs and DVDs this servoinformation is dispersed among all the data.) In between these two layers of an HVD, a mirror layer reflects the green laser while allowing a red laser through, preventing interference from the refraction of the green laser off the data pits of the servoinformation layer on the bottom. This is a great improvement over holographic storage media of the past which underwent too much interference or lacked any servoinformation at all, which rendered them incompatible with contemporary CD and DVD technologies.
Right now an HVD has a maximum transfer rate of one gigabit per second or 125MB each second. Even though HVD specifications and standards were finalized in mid-2007, companies like Sony, Philips, TDK, Maxell, and Panasonic have only somewhat vague plans for releasing discs with one terabyte of capacity within the decade. However, current technology employs a green laser with a power output of one watt, which is quite high for consumer-grade devices. Thus, either the sensitivity of disc polymer must be improved or a laser must be developed that is more suitable for home electronics. The Holography System Development Forum hopes to have improved many other characteristics associated with the format, such as the pulse-to-bit storage ratio, by the time it debuts on the market.
But HVD is not the only technology being researched for our future high-capacity optical storage needs. In fact, quite a surprising number of possible alternatives exist, which future articles in our series of anything remotely DVD will examine – beginning with the very next installment.