Archive for January 19th, 2010

The CD That Wouldn’t Play

Misshapen CD Hole

Misshapen CD Hole

Mary popped a CD into the boom box, poked the Go button, and the display read “No Disc”… which was odd, as the larger player in the living room had gotten halfway through it with no trouble.

A bit of diagnostic winnowing revealed a ding on one side of the CD’s hole, as though it had been mashed by a heavy object. These CDs (it’s 13 of 16 in an audio book) aren’t new, but they’ve been reasonably well treated by all parties. It looks like it might have been crunched in a player, which you’d think would be impossible.

The disc seemed to seat firmly on the player’s hub, so I suspect the ding put the CD far enough off-center to defeat the player’s track acquisition and following algorithm.

A long time ago I wrote exactly that firmware for a prototype video disk player: find a one-micron track with a one-micron beam while the track wobbulates a few hundred microns as the disk spins at 3600 rpm. After that, mapping the track eccentricity and following it around the disk was a simple matter of software…

In this case, a bit of razor-knife surgery removed the plastic intruding into the hole and set everything to rights.

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