Date   : Thu, 14 Sep 2006 22:32:06 +0100
From   : Philip Pemberton <philpem@...>
Subject: Re: Domesday Disaster

Jules Richardson wrote:
> Incidentally, I presume that CD and DVD players have a similar mirror 
> arrangement - don't know for sure, though.

CD/DVD drive laser pickup heads typically have the laser diode, a 
photo-transistor or photo-diode detector, a beam splitter / mirror to send the 
laser beam up through the optics but let the return beam go straight down to 
the detector, and the focussing optics.

The lens itself has a small magnet mounted on it - typically neodymium - and a 
coil around the outside. To focus the laser beam on the disc, the laser is 
powered up, then a servo system in one of the player's LSIs tries to pick up 
the data track. It keeps varying the focus until the error rate is fairly 
decent, then steps into normal operating mode. Usually while it's running 
normally, the servo system keeps running - if the error rate goes up too far, 
it'll try and refocus the laser slightly (I guess this accounts for slightly 
warped discs to some degree). If it can't read the block at all, it'll seek 
back to block 0, refocus, then seek back to the block of data it was trying to 
read. Usually the drive gives up after three or four recalibration cycles.

Some drives (most of the LiteOn writers and some of the Plextors and BenQs) 
will actually return error rate, jitter and pit size information when probed. 
This tends to be useful for checking the quality of a burned CD/DVD, and 
occasionally for data recovery ("give me that damaged data block without 
running EDC/ECC over it, and I'll fix it myself").

That's how I understand it anyway. I reserve the right to be wrong - YMMV, 
E&OE and all that.

-- 
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