On  7 Jul 98 at 11:57, Earl Truss wrote:

> You should understand that I don't have  lot of experience working
> with PC hard drives at this level.  It seems to me that, at least on
> the mainframe disk drives I worked with a few years ago, that the
> number of sectors per track was constant and the difference in the
> length of the tracks was accomodated by increasing the length of the
> space between the sectors.  In the case of a disk where the number
> of sectors per track is constant for all tracks, the inner tracks
> will have a higher transfer rate because the spaces between the
> sectors will be smaller.


  Consider a drive spinning at a fixed rate (RPM), with a fixed head
tranfer rate (bps).  On EVERY track, it reads or writes the same
number of bits in a single rotation.  It isn't the SECTORS that have
more spaces between them on the outer tracks -- its the BITS.
  Note also that although the head is moving the same number of bits,
the outer track of the platter moves past the head at a higher speed
than the inner track.

  Consider, though, that if the outer track diameter is twice that of
the inner track (and this is true even if both happen to be quite
small), the outer track bit-density is only half that of the inner
track.  Typically, a given medium has a maximum bit-density, and the
inner track is about as close to that as can be done reliably.  The
outer track is wasting that property of the medium.

  So one idea is to keep the head transfer rate constant, but instead
of varying the linear speed of the medium past the head, keep the
linear speed constant by varying the rotational speed as the head
moves from track to track.  This requires a more complicated
mechanical mechanism, but gives a uniform bit density over the entire
surface.  I remember reading, 15 years ago or so, about drives that
did this.
  A similar effect, with the complications electronic rather
than mechanical, can be achieved with a constant rotational speed,
but varying the head transfer rate with the track.  Unlike the
mechanical approach, this yields different drive throughput depending
on where on the disk the data resides.  This is what Russ was
describing.

David G