The HVD System: Writing Data
A simplified HVD system consists of the following main components:
- Blue or green laser (532-nm wavelength in the test system)
- Beam splitter/merger
- Spatial light modulator (SLM)
- CMOS sensor
- Photopolymer recording medium
The process of writing information onto an HVD begins with encoding the information into binary data to be stored in the SLM.
These data are turned into ones and zeroes represented as opaque or translucent areas on a "page" -- this page is the image that the information beam is going to pass through. Once the page of data is created, the next step is to fire a laser beam into a beam splitter to produce two identical beams. One of the beams is directed away from the SLM -- this beam becomes the reference beam. The other beam is directed toward the SLM and becomes the information beam. When the information beam passes through the SLM, portions of the light are blocked by the opaque areas of the page, and portions pass through the translucent areas. In this way, the information beam carries the image once it passes through the SLM.
When the reference beam and the information beam rejoin on the same axis, they create a pattern of light interference -- the holography data. This joint beam carries the interference pattern to the photopolymer disc and stores it there as a hologram.
The HVD System: Reading Data
To read the data from an HVD, you need to retrieve the light pattern stored in the hologram.
In the HVD read system, the laser projects a light beam onto the hologram -- a light beam that is identical to the reference beam (Read System 1 in the image above). The hologram diffracts this beam according to the specific pattern of light interference it's storing. The resulting light recreates the image of the page data that established the light-interference pattern in the first place. When this beam of light -- the reconstruction beam -- bounces back off the disc (Read System 2), it travels to the CMOS sensor. The CMOS sensor then reproduces the page data.
Page data stored in an HVD (left) and recreated by CMOS sensor (right)
Current optical storage saves one bit per pulse, and the HVD alliance hopes to improve this efficiency with capabilities of around 60,000 bits per pulse in an inverted, truncated cone shape that has a 200 micrometer diameter at the bottom and a 500 micrometer diameter at the top. High densities are possible by moving these closer on the tracks: 100 GB at 18 micrometers separation, 200 GB at 13 micrometers, 500 GB at 8 micrometers and a demonstrated maximum of 3.9 TB for 3 micrometer separation on a 12 cm disc.The system uses green laser, with an output power of 1 watt, a high power for a consumer device laser. So a major challenge of the project for widespread consumer markets is to either improve the sensitivity of the polymer used, or develop and commoditize a laser capable of higher power output and suitable for a consumer unit.
Storage capacity in context
Holographic Versatile Card a variation of the Holographic Versatile Disc
It has been estimated that the books in the U.S. Library of Congress, one of the largest libraries in the world, would contain a total of about 20 terabytes if scanned in text format. Not including images from the books, the content could be stored with capacity to spare on six 3.9 TB discs.
- At 15 meter resolution and 32-bit color (about the resolution found in unpopulated areas on Google Earth), a map of the land masses of Earth would occupy just over 2 TB.
- Using MPEG4 ASP encoding, a 3.9 TB HVD could hold 4,600-11,900 hours of video-just over one year of uninterrupted video at usual encoding rates.
- Using typical satellite radio encoding (CT-aacPlus at 40 kbit/s), a 3.9 TB HVD could hold over 26.5 years of uninterrupted stereo audio.
The HVD Forum
The HVD FORUM (formerly the HVD Alliance) is a coalition of corporations purposed to provide an industry forum for testing and technical discussion of all aspects of HVD design and manufacturing. By cooperating, members of the Forum hope to expedite development and engender a market receptive to HVD technology.
HVD is not the only technology in next-generation, high-capacity optical storage media. InPhase Technologies is developing a rival holographic format called Tapestry Media, which they claim will eventually store 1.6 TB with a data transfer rate of 120 MB/s (960 Mbit), and several companies are developing TB-level discs based on 3D optical data storage technology. Such large optical storage capacities compete favorably with both HD DVD and Blu-ray Disc. However, holographic drives are projected to initially cost around US$15,000, and a single disc around US$120-180, although prices are expected to fall steadily. The market for this format is not initially the common consumer, but enterprises with very large storage needs.
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