Has anyone experienced CD warp, excluding mistreatment?
Yes. In the early days of CD where the 'warp' was caused by a combination of various factors.
I shall explain.
Two distinct processes for disc manufacturing resulted in very slightly different centre hole parameters. The Sanyo and Sony method moulded the disc and centre hole in one go, resulting in the stepped moulding, but perfect centre hole, sealed both on the inside and outside edges. Track spiral eccentricity was pretty good but not perfect. The spring clamps of the Japanese machines were very good at self centering and leveling the disc- warp was not a problem and the lateral tracking systems were able to easily compensate. Coarse tracking would increment the sled, and fine tracking could deal with eccentric data spiral issues.
The Philips method involved optically centering the track spiral after manufacture, machining the centre hole and trimming the edges of the disc. This left the edge of the aluminium layer open to the atmosphere and caused the dreaded 'laser-rot' as the aluminium oxidised from the inner and outer edges. It also sometimes resulted in rough centre holes which didn't mate perfectly with some clamps, resulting in the disc spinning, but slightly moving up and down like a warp as it rotated. Focus servos were unable to compensate, particularly at the outside of the disc, where the effect was exacerbated.
Many machines were incorrectly diagnosed as having focus/tracking issues when it was the discs themselves that were at fault. Many early Polygram discs were returned to retailers as faulty, due to swarf or rough holes not mating with clamps well. Yields and tolerances improved once Philips/Polygram changed their disc manufacturing processes to bring them in line with the Japanese.
If you want to see how much warp there is on CDs, put an oscilloscope probe onto the focus coil drive and monitor the waveform- it is amazing how warped they actually are. All CDs have warps, some more so than others.
