On Sat, 05 Feb 2000, Laurens Holst wrote:
> With CD-readers the trick is the following: the center tracks of a CD are
> smaller than the outer tracks.
A CD doesn't really have tracks. The data is on a spiral path.
> However, every track contains as much
> information as every other track. So the holes in the inner tracks have less
> space between them than the outer tracks.
It's exactly opposite. The CD surface passes the head at the same speed, so
in the inner region of the CD-ROM there is less information per rotation
than in the outer region.
What you're describing is how floppies and hard disks work.
> If a drive reads at a certain
> speed, the motor has to turn faster for the outer than for the inner tracks.
This is true.
> But then the following trick was discovered: why not let it turn at a
> constant rate? Then it will read the inner tracks faster than the outer
> tracks, but it won't have to use variable speeds.
The outer region would be read faster (more information per rotation). This
does not match with my experience, but it is the logical conclusion,
assuming the motor rotation speed is constant.
Multiple rotation speeds would explain why my CD-ROM drive makes a lot of
noise when reading the table of contents (when a CD is just inserted), but
is not noisy when playing audio (single speed) or accessing files on the
outer edges of the disk (lower rotation speed needed for the same
throughput).
Therefore I think the motor rotation speed is not constant, even for new
drives. However, these probably have a few fixed settings where old drives
could handle a continuous range of speeds. For example, a drive may only
support 1x, 4x, 16x and 32x and not everything inbetween.
> Well, and since then the drive manufacturers give the speed on the inner
> tracks (=the first part of the CD) as the drive speed. But on the outer
> tracks it is much slower. So a REAL 4-speed drive is much faster than a
> 'fake' 4-speed drive.
I use an audio extraction utility which displays the CD speed. My 12x drive
performs audio extraction at about 9x. This value differs a little for
different areas of the disk, but it is always between 8x and 11x. If
rotation speed were completely fixed, the differences would be a lot more
dramatical.
Bye,
Maarten
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