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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
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almarg
9,670 posts
 
George (Xti16), the OP's statement about speed variation was correct. From this Philips paper on CD technology:
The pits in a CD are 0.6 micron wide (1 micron is 1/1000th of a mm), 0.12 micron deep and 0.9 to 3.3 micron long. A disc full of these pits, with a track pitch of 1.6 micron, has a capacity of 650 to 700 MByte. Since the information is recorded on a spiral track, and is read out at a constant speed of 1.25 m/s from the inside to the outside of the disc, the rotational speed decreases as the disc is played from 500 to 200 r.p.m.
I too have never encountered or heard of the phenomenon the OP has experienced.

Regards,
-- Al
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mapman
21,838 posts
 
I've had it happen on occasion with drives on some older laptops with some CDs, never on a CD player.

I'd attribute it to a combo of drives and disks that are not manufactured within certain common modern quality assurance tolerances, in other words imperfections in drive build, CD product, or both together having particularly bad karma.

Device sitting square on a rigid table or platform always seems to help keep resulting vibration to a minimum.
poetcatullus OP
53 posts
 
Thanks again!
I consider the Matsushita/Panasonic DVD-A110U/CA transport and its successors to be quality equipment. They are certainly used in some high-end players. But then again, as Wyle E. Coyote said: "Even a genius can have an off day." (Yes, there was a episode where he actually spoke.)
xti16
1,525 posts
 
Thanks for correcting me Al. Guess I had it backwards
George
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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.
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