Technology
To store information onto a physical surface, the data must be transformed into a series of marks, using a modulation code. The codes used in most optical disc systems are binary, meaning that resulting surface has only two states: marks and non-marks. The following figure illustrates the EFM code used in CDs and DVDs:
Because the edges are positioned on a grid that is finer than the minimum mark size, EFM achieves about 1.5 bits per minimum-mark, even though it is a binary code.
MultiLevel recording refers to the use of multiple reflectivity values to encode data onto an optical disc. By using more than two levels, more information can be put into the minimum feature size. The following figure illustrates a MultiLevel code (note: colors are used only to represent differences in intensity):
The 8-level code used on the prototype systems is a convolutional code, storing about 2.5 bits per data cell. By using this code in combination with a smaller mark size and a more efficient error-correction code, the capacity of CD media was tripled. When applied to dual-layer DVD, ML-recording can increase capacity by a factor of 1.9.
MultiLevel optical recording is an example of baseband pulse-amplitude modulation. While Non-binary, or M-ary, modulation is common in the telecom industry, the technique was originally developed and patented for optical disc recording at Optex Corporation in the early 1990s (in conjunction with the University of Rochester) for use with their Electron Trapping Optical Media (ETOM). Although simple in principle, implementation of ML was challenging, in large part because data storage channels are highly nonlinear. The overall ML-system response is much more sensitive to variations in its individual components (operating temperature, media uniformity, read-head fluctuation, etc.) than a conventional CD/DVD system. To compensate, the ML logical-format devotes a substantial portion of bits (as forward-error correction coding) to enhance robustness against media defects and signal noise. ML-drives used sophisticated power-optimization during writing and adaptive equalization during reading.
MultiLevel recording is sometimes confused with multi-layer storage, in which multiple data surfaces are combined into a single disc. Multi-layer and multiLevel technique can be combined (as in dual-layer ML-DVD ROM), where ML-modulation is applied to each individual layer of the disc.
Read more about this topic: MultiLevel Recording
Famous quotes containing the word technology:
“Radio put technology into storytelling and made it sick. TV killed it. Then you were locked into somebody else’s sighting of that story. You no longer had the benefit of making that picture for yourself, using your imagination. Storytelling brings back that humanness that we have lost with TV. You talk to children and they don’t hear you. They are television addicts. Mamas bring them home from the hospital and drag them up in front of the set and the great stare-out begins.”
—Jackie Torrence (b. 1944)
“Our technology forces us to live mythically, but we continue to think fragmentarily, and on single, separate planes.”
—Marshall McLuhan (1911–1980)
“Technology is not an image of the world but a way of operating on reality. The nihilism of technology lies not only in the fact that it is the most perfect expression of the will to power ... but also in the fact that it lacks meaning.”
—Octavio Paz (b. 1914)