Skew vs. interleave

[Chuck Guzis]

On 7/30/21 2:35 PM, Mike Stein via cctalk wrote:
> There's a small discussion on S100computers about the terms 'skew' and
> 'interleave'.
> 
> In CP/M documentation 'skew' refers to what's usually called interleave
> these days, i.e. offsetting sectors on a track to compensate for the fact
> that by the time the computer has processed a given sector the next one has
> already passed by, so that the computer has to wait an entire revolution
> for it to pass by the head again; in other documentation as in Chuck's
> 22disk for example this is also called 'interleave'.
> 
> However, in later documentation the meaning of 'skew' seems to have changed
> to refer to the offset of sectors between adjacent tracks to compensate for
> the time required to step the head.
> 
> Can anyone (Fred, Chuck?) shed some light on this apparent double meaning
> of 'skew'? And if skew was used to describe sector interleave then what was
> the offsetting of sectors between tracks called?

The CP/M definition of "skew" was the first time I'd ever heard of using
it in that manner.

22Disk's use of "SKEW" was a late cop-out for formatting only.  I
probably should have used the word INTERLEAVE, but that had already been
used in the documentation.   I ran out of synonyms.  In any case, unless
you're formatting floppies, the keyword doesn't matter--and in fact, is
omitted for most of them.  Mea maxima culpa.

The term "interleave" perfectly describes the mechanism.  At a 3:1
interleave, reading sectors consecutively takes three revolutions, as
the sectors for each revolution are *interleaved* with those of the
other two revolution.

"Skew", or offsetting the start of a track perfectly corresponds to the
dictionary definition of (noun) "slant" or "bias", which is what it
is--the track organization is the same as the previous and following
tracks, but it's rotated/slanted with respect to the previous and
succeeding tracks.  Think of it this way, you have a stack of cardboard
discs whose faces are divided into numbered segment.  With the discs
stacked up, draw a vertical line on the side of the stack from top to
bottom.

Now rotate each disc slightly with respect its predecessor, such that
the outside vertical line looks slanted or twisted.  Hence "skew".

I've run into formats where "skew" differs not only from side-to-side,
but also differently from  cylinder-to-cylinder.

In magnetic tape, "skew" refers to the relative misalignment of bit
cells in a given frame.  Lots of attention is paid to "deskewing" hardware.

--Chuck