Re: Comparitive speed of DVD vs CD drives

From: Paul Floyd (root@127.0.0.1)
Date: 04/02/03 

From: Paul Floyd <root@127.0.0.1>
Date: 2 Apr 2003 19:55:31 GMT

On Tue, 01 Apr 2003 23:46:21 GMT, Rich Teer
   <rich.teer@rite-group.com> wrote:
> On 1 Apr 2003, Paul Floyd wrote:
>
>> Single speed CD-ROM is 130KiB/s. Single speed DVD-ROM is about 1.4MiB/s.
>> However, the sped-up versions are almost always a bit deceptive. CD-ROM
>> uses constant linear velocity, so that the disc spins at a lower angular
>> velocity as the laser moves from the centre to the edge. The drives are
>> usually not capable of spinning at the angular velocty that would give
>> the 50x or whatever speed-up at the inner disc. So the real speed up
>> is usually at the ID is around half the advertised speed up. However,
>> since most of the data is at the OD, the real average speed up isn't
>> that far from the claims.
>
> Interesting. Are you saying that data CDs are recorded from the outside
> in? Audio CDs are recorded from the inside out, for some reason.

As I said, the disc spins at a lower angular velocity as the laser moves
from the centre to the edge, i.e., starts at the centre at a high
angular velocity, and moves outwards to the edge of the disc. Usually
though there spin speed is on a plateau to begin with - as fast as the
drive can go. So the initial radial movement translates to an increased
data rate (as the radius increases). At some point, it reaches the
fastest data rate that it can cope with, so as it continues to move
outward, the angular velocity decreases to keep the data rate constant.

A bientot
Paul 

-- 
Paul Floyd                 http://paulf.free.fr (for what it's worth)
Netgear: the worst technical support I've ever encountered.

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