[NTLUG:Discuss] speed question on CD-ROM, CD-R, and CR-RW

[Steve Baker]

Fred James wrote:
> 
> This is my first venture into writing CD's, so this may seem a silly
> question, but I just have to ask...
> 
> Are the read and record speeds independent of each other? 

Yes.

> I mean for instance, what determines the read speed?

The performance of the CD drive depends on the ability of the drive to spin
the disk fast enough (that's not hard) - but also on the amount of power that
the solid state laser can deliver.  There has to be enough light reflected from
one teeny tiny 'bit' on the disk drive to be detectable - and obviously the faster
the disk spins, the less time there is for the detector to pick up enough light to
decide whether this is a zero or a one...hence, the brighter the laser, the faster
you can spin the disk and the faster the data rate is.

When you write to the disk, you are causing a state change to the dye layer that's
embedded in the middle of the disk. That's done by shining laser light at a higher
intensity and for a longer period of time than you do when you read the disk.
(Obviously there has to be more light delivered to the surface of the disk
when you write to it than when you read from it - because otherwise the act of
reading the disk would also erase it).

All of this means that drives can generally read disks faster than they can
write them.

However, no matter what speed the disk was written at, it should be readable
an any speed that your drive supports.

The speed you can write is also determined by the rate at which you can fill
the input buffer on the CD drive - because if that buffer ever empties
completely, the disk will be ruined.  However, modern CPU's can generally
keep the drive's buffer full with no problems - even when they are running
other programs at the same time.  It's probably still good advice not to
run 'heavy' programs at the same time the CD is writing though - you'll
usually get away with it - but I prefer not to take a chance on that -
especially at higher write speeds.

> If I record at one speed will it read at another?

Yes.

----------------------------- Steve Baker -------------------------------
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