VanLUG Email Archive
Dave Martindale (davem@groucho.cs.ubc.ca)
Sun, 26 Sep 1999 10:37:01 -0700
Kevin Chu <kevin@portal.ca> writes:
>Seems to me that with constant angular velocity your bit rate would
>vary with where you're looking on the disc, unless you vary the bit
>density with the location on the disc, which would then create
>compatibility problems. Am I thinking along the right track here?
If the angular velocity is constant, then the data clock frequency
changes continuously as the disk is read. That doesn't necessarily
create any problems - it's just a matter of designing the drive so
that the clock information read from the head is used to adjust the
decoder clock rate, instead of adjusting the rotational speed of the
disc. This probably makes it harder to design the decoder, that's all.
The big advantage of CAV is that the head can seek from anywhere on
the disc to anywhere else and start reading immediately, without
waiting to adjust the rotational speed of the disc. CLV drives always
seem to be changing speed unless you're reading a large contiguous
file (like video) and thus seeks take forever.
>Does it matter that your bit rate varies? Hard drives are another
>story.
Hard drives are all CAV - it would take far too long to keep changing
speeds. In the old days, this meant that the outer tracks just
wasted a lot of potential information storage space. Now, drives are
divided into zones and the data clock rate is different for each
zone, allowing an approximation to constant information density over
the whole surface, but giving higher transfer rates at the outer
edge of the disk.
Dave
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