cctech Digest, Vol 12, Issue 33, Re: Drive Alignment

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Tue Aug 24 17:47:30 2004

>From: "Barry Watzman" <Watzman_at_neo.rr.com>
---snip---
>
>The "modern" (about 1981) way was with a digital alignment diskette. This
>was a normal data disk, almost, but various tracks on the diskette were
>either correctly aligned, or intentionally mis-aligned by varying known
>amounts. By seeing which tracks could be read, and which could not, you
>could determine the alignment status of the drive. This was useful for
>checking alignment, but not nearly as good for setting it. SOME of these
>disks ALSO had the analog patter on track 38. You needed the digital
>alignment disk and software that knew what to do with it.
>
---snip---

Hi
 This track is created by having two signals recorded a half
track apart. One signal has one extra cycle in it per revolution
of the disk. This makes the cats eye pattern when the two signals
add together in the head.
 Another method used was to microstep 5 times across the track
with a tone. The tone centers on the track would be the highest.
If you take my ascii art and lay it on its side, that would be
what u see on a scope:
     |
     #
     #
    ###
    ###
   #####
   #####
    ###
    ###
     #
     #
     |
     #
     #
    ###
    ###
   #####
   #####
    ###
    ###
     #
     #
     |

 A misaligned track might look like:
 
     |
     #
     #
    ###
    ###
   #####
   #####
   #####
   #####
     #
     #
     |
     #
     #
    ###
    ###
   #####
   #####
   #####
   #####
     #
     #
     |
 This tended to look more blocky than smooth like the cats eye
pattern but didn't require a tone to be exactly one cycle different
per revolution of the disk. One could make there own disk
like this with a micro stepping controller for the stepper
and a micrometer to set the track position relative to the center
of the disk.
Dwight
Received on Tue Aug 24 2004 - 17:47:30 BST