Re: Floppy disk density

[mjmahon@aol.com (Michael J. Mahon)]

Andy McFadden replied:

>Michael J. Mahon <mjmahon@aol.com> wrote:
>> The inner track has a radius of about 1.375" and the outer track has
>> a radius of about 2.4".  If the motor speed and the recording bit rate
>> are constant, as they are for most floppy drives, then the capacity of
>> the shortest track determines the capacity of all tracks  For a 5.25"
>> diskette, if the motor speed or bit rate were varied to keep bit density
>> approximately constant, the capacity of the diskette would increase
>> by a factor of: 0.5 * ([max radius]/[min radius] + 1), which in this case
>> works out to about a 37% increase in capacity.
>
>The difference between PC and Apple 3.5" disks is much closer (720K vs
>800K, one is 90% of the other).  This may be because the difference in
>circumference for the outside and inside track of a 3.5" disk is less
>significant than for 5.25".

I haven't checked, but they look to have very similar radius ratios.

Perhaps Apple just didn't push it as far as they could have.

>In any event, you'd probably want to break the disk into ZCAV zones
>for practical reasons: any increase in bits per track that results in less
>than a full sector gain doesn't help you.

Exactly.

>I believe the 5.25" floppy drives for the Apple Lisa used some of these
>tricks media to get as much as possible onto the disks (1MB?).  Of course,
>the drives had dedicated CPUs...

Yes, that's my understanding of the Twiggy drives.

-michael

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