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Re: blank DVDs no recognized, other media OK

On Tue, Aug 29, 2006 at 10:05:06AM -0400, Matthias Julius wrote:
> Are you sure?  I thought they both use red lasers.

DVD is red, CD is infrared.

This is why DVD has more space on it.  The tracks are narrowe, the pits
shorter, and the focal depth much shorter.  A CD reflects off the other
side of the media, a DVD reflects off the center layer of the DVD (or
1/4 of the way down for the second layer of a dual layer DVD).

Older DVD players only had one laser and used lens tricks to read CDs,
but could really only read stamped CDs since the lens trick required a
lot of reflected light to read, so most CD-R disks and CD-RW disks were
unreadable by those players.  Most newer DVD players have dual lasers
just like DVD drives in computers always had.

The wavelengths used are:
CD: 785nm
DVD: 650nm
HD-DVD and BD: 405nm

Len Sorensen

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