MultiLevel Recording (ML) was a technology developed by Calimetrics to increase the storage capacity of prerecorded and writable optical discs. Through a combination of proprietary media, recorder, reader and player modifications, Calimetrics proposed that ML could increase the capacity of a CD-ROM, CD-R or CD-RW to 2 GB, a single-layer DVD, DVD-R, DVD+R, DVD-RW, DVD+RW or DVD-RAM to 7.1 to 10 GB and a single-layer Blu-ray Disc (BD) to as much as 60 GB. An optionally integrated Digital Rights Management (DRM) system entitled MovieGuard was also suggested. An industry group called the ML Alliance was formed in 2000 to help commercialize ML technology. Members eventually included Calimetrics, TDK, Sanyo Semiconductor, Plextor, Matsushita Kotobuki Electronics, Mitsubishi Chemical Corporation, Verbatim, Teac and Yamaha.

Several 2 GB ML CD-based recorders were developed for release in 2002 (TDK's MLCDRW1000 and Plextor's PX-ML3630), but never came to market. This was largely a business decision influenced by the rapid fall of CD-R/RW prices and the simultaneous rise in popularity of writable DVD technology. Calimetrics went on to work on more advanced DVD and Blu-ray versions of their technology, including a proposal to build a next generation version of Enhanced Versatile Disc (EVD). Calimetrics ceased operations in 2004.

Technology

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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. When applied to CD-media, ML-recording can triple storage capacity. When applied to dual-layer DVD, ML-recording can increase capacity by a factor of 1.9. (The relative reduction in gain is due to the tighter tolerances in DVD systems.)

Although simple in principle, implementation of ML was challenging. MultiLevel optical recording is an example of baseband pulse-amplitude modulation. Data storage channels are notorious due to their highly nonlinear writing processes. ML drives were required to perform elaborate laser power-optimization and adaptive media equalization on-the-fly, for both read and write operations.

While Non-binary, or N-ary, modulation is common in the telecom industry, the technique had not been seriously pursued in optical storage prior to Calimetrics.

MultiLevel recording is sometimes confused with multi-layer storage, in which multiple data surfaces are combined into a single disc. The MultiLevel technique can be applied to each layer of such a system.

External links

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• A picture of ML discs
• Results of Blu-ray research with Philips

120 mm discs | 80 mm discs | 60 mm discs | Audio storage | Video storage