Sam Goldwasser wrote:
"Cliff Top" writes:

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.

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

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.