[ale] OT: CD drive and revolution speed correlation?

[aaron]

On 2009, Mar, 04, , at 12:48 PM, Jeff Hubbs wrote:

> Well, CTV would be more pedantically correct than CLV.  But yes, if
> the drive is running at constant speed, the data rate should increase
> as the radius increases.
>


Traditional terms for this (from way back in the analog video laser
disk days, before all this diabolical digital delirium) are CLV =>
Constant Linear Velocity and CAV => Constant Angular Velocity.

With CLV, the disk spins faster as the read/write point is closer
to the hub and slower as the read/write point tracks closer to
the outside edge. The result in that the data density is constant
throughout the track.  This is the way that the authentic DVD
standard works (DVD-R).  It is also the CD standard.

With CAV, which is used by some of the DVD-RAM / DVD-RW / DVD+R
adaptations and/or aberrations of the authentic DVD standard,
the disk spins at a constant (angular) speed, so data density
in the "track" is higher near the hub and lower at the edges.

MY PET THEORY FOR THE DATA RATE MYSTERY:
-- At very high spin speeds, CLV would be more difficult to
maintain against forces of inertia, so with higher speed reads
"lazy" drives might just keep the disk spinning at a constant
speed and let the data rate vary from front of disk to back.
(They may also spin at a single constant rate when given
a speed instruction)


The link below shows some curve graphs of (CLV) CD data rate
when the drive is running in CAV mode It's draws a nice curve:
    http://www.cdspeed2000.com/faq.html

I would imagine that today's CD/DVD drives have a wide range
of speeds at which they can accurately read or write data and
that they will use the simplest mode (CAV) whenever possible.


peace
aaron



> On Wed, Mar 4, 2009 at 11:53 AM, Devnull <devnull at iamdevnull.info>  
> wrote:
>> On Wed, Mar 4, 2009 at 10:57, Jeff Hubbs <jeffrey.hubbs at gmail.com>  
>> wrote:
>>> *Audio CDs* practice Constant Tangential Velocity in playback.  For
>>> data CDs, it's more arbitrary; my understanding was that a "52x"
>>> CD-ROM drive is only 52x at or near the outer edge, where the  
>>> drive's
>>> maximum rotational speed corresponds to the highest data rate.  It's
>>> reasonable to say that CDs of all sorts have a constant tangential
>>> data density independent of radius.
>>
>> I am not all that familiar with Constant Tangential Velocity, from
>> what you have described here, I am guessing that it is similar to
>> Constant Linear Velocity. Since the CD under testing was a data CD,
>> does that mean that Jim was correct, insofar as the CD drive spins at
>> the same constant physical speed during the entire read process. If
>> that is correct, then it would be dependent on some pi-based  
>> function,
>> right?
>>
>>> I recall that we've at least one person on the list who has had a CD
>>> shatter under load inside a drive.  I have had a CD *eject* at speed
>>> once.  I ran.
>> (!)
>>
>> --
>> -
>> /dev/null
>> We are the Pentium of Borg. Division is futile. You will be  
>> approximated.
>>
>> _______________________________________________
>> Ale mailing list
>> Ale at ale.org
>> http://mail.ale.org/mailman/listinfo/ale
>>
>
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://mail.ale.org/mailman/listinfo/ale