Re: Write Signal on Disk II Question

[glen herrmannsfeldt <gah@ugcs.caltech.edu>]

Bryan Parkoff wrote:

(snip)

>     QA is ideal to show digital bit stream.  Write Signal is analog bit 
> stream.  It controls the current in the coil of the read/write head.  Also, 
> it has second head like as erase head..  The high/low state of the Write 
> Signal determines the direction of the magnetic field set up in the 
> read/write head.  Please explain more about the field reversal and the lack 
> of field reversal why Write Signal is required to toggle while QA is always 
> set for all 8 digital bits.

When writing the current in the write coil is toggled.  On reading,
a current pulse is generated when the head goes over a transition.
The important characteristics are the minimum distance between 
transitions and the ratio of the maximum to the minimum distance.
Because of bit shifting and disk speed variations the ratio can't
get so large and accurately detect which bit cell a transition
belongs in.

-- glen