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CD-RW
compatibility |
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The added value of
CD-RW compatible mechanisms
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The CD
reflectance specifications : 70% minimum for lands,
28% maximum for pits, was introduced to allow the
relatively insensitive photodiodes of the early
eighties to read the signal pattern reliably. But
with today’s photodiodes able to detect much
smaller reflectance differences, these stringent
specifications are no longer necessary. Moreover,
for CD-RW, the specifications leave little energy in
reserve (only 30% after 70% is reflected) to produce
phase changes in the recording material.
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The CD-RW
recording layer (a mixture of silver, indium,
antimony and tellurium) has a maximum reflectance
(in the crystalline phase) of about 20% and a
minimum reflectance (in the amorphous phase) of only
5%. The CD-ReWritable system, therefore, works at
reflectance of slightly less than one-third of the
original CD figure. With modern photodiodes, this
presents no problem. Hence the advantage of CD-ReWritable
compatible mechanisms which do include these new
generation photodiodes with high sensitivity. As a
result, these mechanisms can read all CD related
media, i.e. CD DA, CD-R, CD-RW…Besides, these new
sensitive photodiodes are important to reduce laser
light on CD-RW discs and so to improve laser
lifetime.
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