Keith M wrote:
Can you put some rough ballpark numbers on how much
total (worst case)
shifting you are talking about, altogether? I'm assuming the shift
would show up as distance/time between pulses, right?
Yes. If the system that
wrote the disk is doing write precomp with
reasonable values, the shifting should be relatively small, under 0.5
us. However, I've seen much more than that. (At least, I think I
have. It was consistent enough and pattern-sensitive enough on some
disks that I can't explain it by instantaneous speed variation.)
So you are saying that the shift could be enough, for
example, to make
two pulses that were written 4us apart appear as if they are 6us apart?
Usually
having 4 get close to 6 would indicate that the writing system
was doing too much precomp.
Note that the peak shifting varies with the head position; it's worse on
the inner tracks because they have the highest linear bit density.
(Same reason that 8" drives need reduced write current on tracks beyond
43.) The precomp is generally set based on an expectation of the inner
track behavior, so it can be too much on the outer tracks.
Any good book references for data separation
algorithms? Would this
topic be covered in detail, for instance, in some other generic
engineering type reference, or ... ?
I've never found any useful published
information beyond the data sheets
and app notes for the FDC and data separator chips.
Best regards,
Eric