David Scheidt <[EMAIL PROTECTED]> wrote:
>Almost all fast (>12X or so) CD-ROM drives are variable speed.
To put this a different way, the data on CD's is stored at a constant
lineal density (closely related to the wavelength of the laser).
Audio CDs are read using constant-linear-velocity, the angular
(rotational) speed reduces from the inside edge to the outside edge,
to maintain a constant data rate. CD-ROM drives initially just copied
this approach.
Almost all fast CD-ROM drives now use constant-angular-velocity (which
simplifies the rotational mechanics and should improve seek times, at
the expense of slightly more complex read logic). This means the the
lineal velocity (and hence data rate) increases from the inside
(start) to the outside (end) of the disk.
The quoted CD-ROM speeds are always based on the peak data rate
(and the better manufacturers actually quote the speed range).
Peter
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