Bose Life CD5 Series I Service Manual page 9

CD TERMS
Eccentricity: The extent to which the hole in the middle of the disc is not located in the geo-
metric center. In other words, the amount that the disc moves in and out as it rotates. Eccen-
tricity is measured as the distance between the center of the hole and the center of the disc (as
determined by the spiral tracks).
Mistracking: When a CD player fails to play the disc in a continuous manner. This may be
caused by a large enough optical defect (scratch, etc.) or by vibration.
Optical Defect: A defect on the surface of the disc which adversely affects the reading of
information by the laser pickup. There are four basic types of optical defects: voids, black dots,
fingerprints, and scratches.
Skipping: When a CD mistracks backwards and gets caught in an "endless loop". This is
usually caused by a large scratch or other optical defect.
Warp: The extent to which the surface of the disc is not parallel to the seating plane of the disc
(at the center). In other words, the amount that the disc wobbles up and down. Warp is mea-
sured as the vertical deviation between the seating plane and the particular point on the disc.
Alignment Terms
CD Alignment: The process of adjusting a CD player for optimum performance, particularly
with respect to its playability.
Free Run Frequency: The VCO frequency in the absence of any signal from the disc. Accu-
rate frequency adjustment is required for the player to be able to read data off the disc. VCO
misalignment results in poor track access and longer access times.
Tracking Offset: The DC offset voltage present at the output of the tracking servo in the
absence of any input signal. For best results, the offset should be adjusted near 0 to keep the
laser positioned in the center of the track. Negative offset causes the laser to be positioned
towards the inside of the track. Misalignment of this parameter can cause the player to skip or
mistrack when playing a dirty or "black dot" disc, especially if the disc is also eccentric.
Scratches and voids may also cause the problem.
Focus Offset: The DC offset voltage present at the output of the focus servo in the absence of
any input signal. For best results, the offset should be adjusted near 0 to keep the laser exactly
in focus. Misalignment of this parameter usually causes dropouts when playing a dirty or "black
dot" disc, especially if the disc is also warped. Scratches may also cause problems. Note that
on the CD5, the focus offset is preset and the adjustment pot is not loaded, however, there is a
spot on the PCB for it.
E-F Balance: The DC offset that results from driving both the E and the F elements of the
photodetector with equal signals. For best results, the offset should be adjusted near 0 to keep
the laser positioned in the center of the track. As with track offset, negative offset causes the
laser to be positioned towards the inside of the track. Misalignment of this parameter can
cause any number of problems including: 1. Poor or slow track access even when playing a
good disc, and 2. Skipping or mistracking when playing a scratched or void disc, especially if
the disc is also eccentric.
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