Re: Disk II Phase Question

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

"Bryan Parkoff" <nospam@nospam.com> writes:
>     I am not sure that I can understand your comment.  Is it true that head 
> writes data on track 5 which it contains valid data while extra unused data 
> can be written on track 4.5 and track 5.5.  Extra unused data is like 
> invalid bit stream.  How can you draw a picture to explain what head gap is 
> for.

Tracks have a particular physical width.  The track spacing is 48 tracks
per inch.  Thus the width of a track must be less than 0.529 mm (1/48 inch).
Half-tracks are 0.265 mm.

I don't know what the actual head gap size is, but it's more than 0.265 mm,
and less than 0.529 mm.

Therefore, if you have a disk that has data on tracks 0 and 1, and you
write some data on track 0.5, it will corrupt the data on tracks 0 and
1 because the physical position of the head is such that the gap overlaps
parts of tracks 0 and 1.

So you can write and reliably read back a disk like this:

     0  1.5  2.5  3.5  4.5

Or like this:

     0  1.5  3  4.5  6

Or any other combination you like that does NOT have two recorded tracks
with less than one full track of separation.

Thus you cannot reliably use half tracks to get more data on the disk.
The only reason to use them is for copy protection.