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Establishing the Maximum Speed of a CD-ROM Drive
UnknownSoldier writes "Ever wondered how fast CD-ROM drives can spin their CDs before the CD will self destruct due to centrifugal force? This person was too, and has his results. (So much for those 100x drives)."
...with more lasers.
~shiny
WILL HACK FOR $$$
- 6 replies here and the site already is slashdotted.
Anyway, I think you can make cd drives that spin 4000x if you want, because it might be possible to put the cd in braces to hold it together, and/or to rotate the laser instead. Or how about using multiple lasers?
It's just like silicon transistors: There's always somebody saying there is a final physical limit we'll reach within the five years...
Often, we(they)'ll find a way around the limitation.
--- Hindsight is 20/20, but walking backwards is not the answer.
I realize there are technical hurdles with this idea, but I think they are possible to overcome: use varying luminosity bits.
Right now what they use is On-Reflective Off-Non Reflective. If the laser was able to detect that some of the bits were at 50% reflectivity, then you'd have 2 bits of data for every bit of reflectivity on the surface.
If one were to get fancier, they could use multi-colored bits. Using 2 lasers instead of one, then one laser would read a different value than the other depending on how the surface reacted to the light. They may already be doing that today with DVD's, I'm not really sure. It's been a while since I read up on it.
I guess the real point to what I'm saying is that increasing the density of the data and the spin of the disk aren't the only two options.
"Derp de derp."
The slow seek time doesn't bother me nearly as much as the eternity it takes from the time you insert the CD in the drive until the time it is ready to send data. In fact, I'd probably be happy with an 8X drive if it had a < 1 second delay between hitting the close button and viewing the README file.
You said:
"You probably already know this, but just for the record -- unless you have a defective CD
drive, it shouldn't ever try to spin an audio disc up to full speed unless you're doing digital
audio extraction. If you're merely listening to your CD, it will spin at 1X, just like any
standard CD audio player."
My experience with my CDROM and CDRW drive (Samsung 52X CDROM drive and Sony 16X/10X/40X CDRW drive) is that whenever I put a disk into it, during the SEARCH, the drives will SPIN VERY FAST - I can even hear the whrrrrrlllll sound ! - then it'll slow down, if the drive finds out that the disk is an Audio CD.
What matters is that my OLD audio CDs may NOT even survive the FAST spin during the SEARCH routine.
Muchas Gracias, Señor Edward Snowden !
with seek times limited only by the speed of light!
Figure out how to redirect a beam of light in a couple nanoseconds, and I guarantee you'll win a Nobel prize.
without having to alter existing cd fabrication technologies you could reach much higher speeds if you rotate the cd near its maximum and then rotate the laser in an opposing direction at or near its maximum. Now you can add the two maximums together and you have a MUCH faster cdrom drive.
Much louder too, of course. But getting cdreaders quiet is easy... its just that manufacturers prefer to make cheap drives instead of quiet ones.
Ever need an online dictionary?
The beam will bounce off the pit and either scatter or reflect back up into the mirror striking the focal point
:o)
Thats how you learn how CD`s work in school, but it isn't true. In past it was the classical approach of not telling the whole truth to keep others from copying it.
First the beam is not scattered or reflected, it is _always_ reflected. The CD consits of two layers, the back one is solid and 100% reflective. The distance between the two layers has to be exactly lambda / 4 of the lasers wave length. Now the first layer is semitransparent. Meaning 50% of the light gets through 50% gets reflected. In the first layer you have the pits representing the data. If this layer has a pit 100% of the light gets reflected, but if it hasn't only 50% get through, get reflected at the back layer and then has a destructive interference with the light reflect first. (That's why the distance has to be wavelength/4)
I fear that the interference will not work if the light is not angeled with 90 degree on the disk.
How about using 700 Million lasers, not spinning at all? You could read a CD at once
--
Karma 50, and all I got was this lousy T-Shirt.
Sorry to be a physics geek here, but there's no such thing as "centrifugal" force, unless you're talking about the force caused by a centrifuge dropped from a height.
There IS "centripetal" force, that refers to the force on an object travelling in a circle, which pushes outward from the axis of said circle on an object while it's travelling about the radius.
centripetal force is a force acting toward the centre. in the stone on a string example, it is the force (tension in the string) pulling the stone toward the holder of the string, making it move in a circle. nothing is "travelling about the radius", and nothing is pushing outward from the axis. strings don't push!
centrifugal force is something you get in rotating frames of reference. one doesn't normally use such frames in physics because they are unecessarily complicated. but that is just a matter of calculational convenience; centrifugal forces are real enough in a rotating frame (it is called a fictitious force because it depends on the choice of frame, rather than being intrinsic. see this page). take a fast curve in a car and that fictitious force feels real enough, even if it isn't the simplest way to describe the situation mathematically.
Everything should be made as simple as possible, but not simpler. -- A.E.
There is no "centripetal force." There is, however, a centripetal acceleration, which points *inward*. (Look up the word) You're committing the classic mistake of confusing a force with an acceleration. For example, in your example, the ball's centripetal acceleration is inward. By Newton's 2nd law, a force must be acting on it. The force in this case happens to be the tension of the rope.
There IS centrifugal force. It's a fictional force, which is a sort of misnomer. A fictional force is nothing but a force felt by an object in an accelerating frame of reference, like a ball on a string (since velocity is changing direction), or a car getting on a freeway (since velocity is increasing). The fictional force in your example would be the one felt by the ball, radially outward, with magnitude equal to the tension on the rope.
I think it is you who should have paid attention in physics 101.
If we have to counterbalance it, wouldn't the simple solution be to make the counterbalance a second read head and quadruple the speed for a given RPM, albeit needing rather more powerful motors.
Personally I think this is all rather silly given how little RAM cost now. It would seem more sensible to stick 700MB of consumer DRAM in the drive and cache to it if you need the speed that badly. Cacheing time of 2-3 mins maximum and then many thousand times the original speed with lower power requirements, wear and tear on the disc and drive and noise and vibration levels.
Greg
(Inside a nuclear plant)
Aaaarrrggh! Run! The canary has mutated!
Broadband is nice.
Broadband is nice but expensive. What would you rather pay, $60 for a CD-ROM drive plus an install set, or $200,000 for a house in an area served by broadband?
I rarely use my CD-ROM to install software, since 'apt-get' directly off HTTP is almost as fast
"Rarely" meaning "only for games," right? Most PC games are non-free because artists, musicians, and level designers have a tougher time accepting the free software or open source philosophy than coders do. Because they sell their product at retail, they have 700 MB (capacity of a CD) to work in rather than 20 MB (the maximum attention span of a user behind 56K). (The fact that PC games are available primarily for Windows is beside my point, partly because Wine can run the vast majority of 2D Windows games.)
Will I retire or break 10K?