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Thursday, May 19, 2011

DVD Magic

I was thinking about DVD's and it occured to me that tracking the playback groove could be a bit of a trick. The tracks on a CD are microscopic, on a DVD they are even more so. If you had perfect bearings and a perfect fit between the hub and the disk, I suppose you might be able to follow the track, but bearings aren't perfect, and you had your finger in the hole in the disk, you aren't going to have a perfect fit. So how do DVD players follow the track?

I was talking to Jack about this the other day, we came up with a couple of theories:
  1. Use a servo motor to move the playback head back and forth to follow the track as it wobbles around. This suffers from the same kind of mechanical problems that got us here. Unlikely.
  2. Use a MEM (Micro Electrical Machine) mirror to deflect the laser beam to follow the track. Possible.
  3. Use several sensors that would cover the expected range of wobble and use DSP (Digital Signal Processing) to extract the data you want. There is a problem with this approach, and that is that DSP can be very power hungry. I suppose a specialized chip could be made for this purpose that would not use that much power, but without knowing more, I am still concerned.
 So I looked around. And I didn't find anything. I found many copies of the same simplistic explanation of how optical disks work, but none about how they actual manage to follow the track. Email's to Samsung and LG failed to produce even a hint.

Since apparently no one knows how these things work, I am forced to conclude that the optical disk cabal is in league with the forces of darkness.
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1 comment:

  1. CGHillMay 21, 2011 at 9:44:00 AM PDT

    It helps that optical discs of contemporary design are played back at a constant linear velocity: on an audio CD, reading is done at 1.2 meters per second regardless of where the laser is positioned. (This explains why at the start of the program, near the center, the disc is spinning at nearly 500 rpm, but drops to barely over 200 rpm at the end, which is near the edge of the disc.)

    The actuator (the usual magnets and coils) built into the laser carrier is synchronized with the speed control; it therefore knows exactly where the laser should be at any given moment.

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