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What causes some transports to oscillate?

Have you ever had a CD or CD/DVD player that will oscillate audibly, especially near the end of a disc? I have and I have contacted those in the know and the only thing I have been told is that the little magnetic disc clamp may be getting weak. I am not so sure.

My theory is that some discs have a center hole that is very slightly off center. Discs rotate more slowly as they near the end of play. If the disc is slightly off balance due to an off-center hole, they may wobble like a top slowing down which causes the disc clamp to wobble and contact the top plate of the transport producing a clicking or ticking noise - sometimes quite audibly.

But if the motor bearings were precise enough and rigid enough, should they not hold the entire process on center and prevent the wobble? My guess is that top-loading players either never wobble or do not do so audibly.

Also, I think that there are many more discs out there with off-center holes that we realize.

Would it not be possible for transport designers to take this into account and design transports that can deal with off-center holes more gracefully?

If my thinking is correct, why is this not more widely discussed and taken into consideration?

Thank you!
poetcatullus
xti16
1,525 posts
 
Thanks for correcting me Al. Guess I had it backwards
George
geoffkait's avatar
geoffkait
23,365 posts
 
If the CD and transport are not absolutely level during play there will a tendency for the disc to wobble and this can be audible, but perhaps not as "oscillation," even when the level is off by only a couple degrees. The only fail-proof way to obtain correct level is to place a small bubble level directly on top of the CD spindle or transport surface around the spindle. The level of the top of the chassis is often unreliable.
poetcatullus OP
53 posts
 
Thanks, Geoff!
I thought about this but then dismissed the idea, erroneously thinking that nobody would design a disc player assuming that it would be placed on a perfectly level surface - houses and furniture being what they are. Oh well . . .

I remember turntables with bubble levels built into them.
kijanki's avatar
kijanki
4,428 posts
 
First track is closest to center (AFAIK) and since CD has constant linear speed, angular speed at first track has to be the highest. I experienced CD vibration few times but it disappeared after I opened and closed tray again. It might have something to do with magnetic clamp. This clamp is lifted and lowered by tray mechanism. Bottom of this clamp has ring of felt to protect CD. My player had this felt worn out causing poor clamping.
poetcatullus OP
53 posts
 
Kijanki is on the right track. Careful observation has revealed that the transport itself is NOT oscillating. The spindle and disc clamp are perfectly concentric and plumb. What is happening is that the little upper part of the clamp that goes along for the ride as the CD is spinning does not always land concentrically after the CD loads. Thus, sometimes it will brush the sides of its mounting hole as it spins and produce extraneous noises. Unfortunately, said noises can get annoying at times. This could be considered a design flaw where the engineering was brilliant on paper, but not 100% in the field. Oh well . . .
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