Hardware question.
 

Please help settle a question, to wit;
Does turning the power off on your CD, while it is playing,
scratch the surface being read?
I say no.
She says yes, and specifically says "It's like stopping an
old record player and having the needle scratch the
surface."
Now, I do not pretend to be an expert, however I always
thought CD's read the data through the use of light
(http://electronics.howstuffworks.com/cd.htm).
I thought all data corruption would occur through mechanical
means, like placing a CD on a rough surface, dust, and the like.
Please settle this, as I lack sufficient credibility.
If I'm wrong or right, I just want to know the truth.
Thanks!:thumbsup:

You've got it right. The only thingthat physically contacts the cd during playback is the spindle, which goes nowhere near the playing surface. There's nothing to scratch the surface.

However, depending on the design of the individual cd player in question, it is theoretically possible that rotational instability caused by the sudden deceleration could cause the disc to come into contact with any exposed hardware. It's not very likely, though.

Nothing touches the CD while it's playing. The middle is on a spindle but the actual data disc is untouched. You're right, she's wrong.

When it comes to the rotational instability, I saw something on
Mythbusters about it. They got the RPMs up to such a degree
that the CD's flew apart.
Thanks!
I'll keep reading for dissenting opinion, though.

You are right. The RPM you are talking about is around 100x read speed - CDs fly apart around that rotational speed. That's why you don't see much past 52x CD-ROMs. They start going unstable past that approximate threshold (warping, etc.)

Turning it off during the middle of an access or whatever will do nothing harmful.

It doesn't suddenly stop, either. The mechanics of every *-ROM I have owned (to confess, only four) have gentily slowed down the disc. It wouldn't make sense if it did stop it suddenly, and it certainly doesn't make sense that the motor locks without power. There is a back-EMF in the motor if a closed circuit exists, counter-acting the rotation. In my honest opinion, it would not be strong enough to cause such a deceleration to make a disc warp thus contacting any stray parts in the ROM.

The motor doesn't lock, however the dc electric motors typically used in this application apply resistance when not active; it is, as you noted, inherent in the design. I wouldn't really be concerned about the disc warping so much as any shuddering or vibration. I'm not sufficiently invested to actually crunch the numbers, but I doubt the amount of energy we're talking here is sufficient to cause any appreciable warp in the disc, even in the fastest cd-rom drives on the market (let alone your standard cd player, which rotates at a much lower speed). However, the disc does undergo a deceleration as the sudden cut in power to the motor results in an abrupt change from a constant rotational velocity to a relatively rapid move towards rest. I should think most of the kinetic energy of the rotational mass would be dissipated through heat in the motor, but it is possible that some of it could be redirected into a vertical acceleration of the disc (ie a vibration). Given that most players give the disc a fair amount of vertical play it is possible that a poorly designed player with an excessively high deck and/or any components extending toward the disc could result in the disc contacting said deck (and/or components) which, in turn, could result in a scratch. The probability of this, however, is quite low, as noted above.