Archived from groups: rec.audio.high-end (
More info?)
In article <crp2ka01qo1@news1.newsguy.com>,
"campdog" <petealmquist@wmconnect.com> wrote:
> if the recording track were actually off center i would have to guess
> that in order to follow the track the laser would have to wobble to
> keep it lined up. could this possibly cause wow or flutter or some
> other kind of distortion? either that or it simply couldnt play the
> track at all. this didn't seem to be the case. i used to work at
> nimbus recordings manufacturing plant in virginia and any discs that
> showed that type of problem were routinely trashed. most likely the
> paper lable was the only thing off center.
Paper label? CDs don't have them, unless you stick one on yourself.
Prepressed CDs have a screen-printed graphic on one side.
> the only other problem i
> could forsee would be the disc being out of balance and vibrating in
> the drive sort of like an out-of-balance tire. that would also
> probably effect the playback quality.
CD drives are designed to deal with this since even near-perfect discs
have some vibration and/or warpage. The laser and its focusing optics
are mounted in a linear voice coil motor, allowing the drive to adjust
the distance of the laser from the disc. This serves as the focus
adjustment -- the lens itself has a fixed focal point and the entire
lens/laser assembly is moved up and down to place the focal point on the
surface of the disc. This assembly is controlled by a servo system
which keeps the laser focused at all times.
Note that this doesn't maintain playback quality so much as it maintains
the ability to play back discs at all. The amount of allowable slop in
the focal length is very, very small. If the system is out of focus,
the laser spot on the surface of the disc becomes too big to resolve
individual tracks, which of course means the drive can't read data at
all. So, the effect of actually losing focus would likely be big
dropouts, nothing subtle like changes in frequency or frequency
response, etc. (Most drives will insert silence whenever they can't
read data from the disc.)
--
Tim