#: 12199 S11/OS9/6809 (Non-CoCo)
    12-Sep-91  00:13:03
Sb: #12187-#80/40 tracks
Fm: Pete Lyall 76703,4230
To: Steve Wegert 76703,4255 (X)

Steve,

Eh? You'll need an 80 tracker to read 80's and 40's. A 40 tracker is
only good for 40's (and 35's, of course).

Reason why? Well, in order to get 80 tracks in the same space as 40,
they had to cut down both the intertrack gap size and the track width,
so an 80 track head cuts a narrower swath (magnetically). Imagine
that this is a 40 track 'data' track:

    4040404040404040404040404040404040404040
    4040404040404040404040404040404040404040
    4040404040404040404040404040404040404040

And this is an 80 tracker's data:

    8080808080808080808080808080808080808080

If you wrote on a diskette that had been perviously treated (written;
formatted) by a 40 tracker, you'd get this:


    404040404040404040404040404040404040404040
    808080808080808080808080808080808080808080
    404040404040404040404040404040404040404040

In other words, the 80 track narrow data track would be imposed upon
the existing wider 40 track path. You're okay if you only read this
with the narrower 80 track head, which will reject the side garbage
that's leftover from the 40 track data. HOWEVER, if you reinsert this
in a 40 track drive and try to read it, the wider 40 track head MAY
see both the new 80 track written data and the older, corrupt, 40 track
data... it's really a hairy issue of luck and (mis)alignment if it works.
In the process, it may render the disk unreadable by anything but the
80 track drive.

While some of this was simplified for the sake of explanation, this
is basically the problem.

Pete

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