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A key aspect in realizing higher recording
speeds is the development of a suitable ‘writing
strategy’ – the timing and power of the laser
pulses in such a way that marks (representing digital zeros
and ones) of the correct length are created in the organic
dye that forms the active layer of recordable DVDs. Doing
this well becomes increasingly difficult at higher recording
speeds, because the available time to heat up and cool
down the dye at the position of a mark becomes ever shorter.
Especially avoiding that post-heating partly erases a written
mark when a neighbouring mark is written is a challenging
task. Philips has developed an efficient write strategy
that not only results in accurate recording results, but
in addition needs only a limited number of parameters to
realize it, allowing disc-drive manufacturers to implement
the algorithm in a straightforward manner.
Besides the write strategy, improvements on the recorder
set-up were needed to realize recording at 16x speed. Especially
the design of an accurate and stable system for tracking
and focusing the laser beam to the right position on the
disc was challenging. Another demanding task was the development
of fast laser driving electronics, which runs at a 420
MHz clock at 16x DVD speed. Furthermore, a prototype high-power
laser was utilized to achieve the recording power needed.
Finally, Ricoh provided the DVD dye discs for the recording
experiments.
The results mark a next step in the speed race for recordable
DVD. It is generally agreed that the now achieved recording
speed is close to the ultimate limit, which is set by the
highest safe rotational velocity of the polycarbonate discs.
At the currently reached 16x recording speed, a disc makes
180 rotations per second, corresponding to a linear velocity
of 56 meters per second (over 200 km/h), while marks are
burned with a precision of less than 0.05 micrometer!
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