"Initial" Track on CD Rom Disk (Physical Stop or "Seek")

There is always a limit switch/home position the laser sled returns to to start reading. The start of the spiral track will be directly under the laser at that point. At the end of the disc there is ~2mins of 'stop code' so that when the laser finds itself here, it will know it must trigger 'stop' mode. CD writers are a little different since they can access an earlier porion of the disc for power calibration tests.

AW

Note that the "start" of the spiral track is about 1.6 um wide. The limit switch gets you in the ball park. :)

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My guess is the limit switch (or with the simple magnetic linear motor used, just sending it to one end and waiting until it can't possibly still be in transit) just gets you "inside" the starting track from which point you can start a search in a known direction.

Figure the eccentricity of the disk's rotation is going to be many, many track widths wide, all the real alignment is done by error signals from reading the tracks themselves, it's not mechanically anywhere near that accurate in absolute position.

I could be mistaken, but I think I've actually seen the control loops in simple players respond to an unreadable disk by doing a spindle speed search that includes going all the way down through stop and beyond, so they are spinning in the wrong direction! Could be wron about that, or it could be an artifact of a brushless motor drive circuit rather than the control loop though.