Apple ][ disks

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Re: Apple ][ disks

Post by Malvineous »

Knowing nothing about Apple disk formats this is very interesting, particularly the bit about the quarter tracks. I'm curious though - if you were to step the drive by 1/4 of a track, presumably the write head is still one track wide, so you wouldn't be able to - for example - store four tracks worth of data in the space of one normal track.

All I can see this doing is offsetting a track by 1/4 of a track width, so presumably reading it on a high density drive (where the stepper motor can read double-density half-tracks) would still work?
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Re: Apple ][ disks

Post by IFW »

HD drives that I know of would only step as 96 TPI, ie 80+ track positions.
That would give you the ability to read half-tracks just fine, but you would want to read at 192 TPI for sub-half-track seeking...
Stepping at a finer resolution on them could be possible if the stepper motor would support a finer resolution, and the circuit converting the step pulses to motor signals would be bypassed.
There are drives with a very high resolution accessible for stepping, but we don't have any.
Having said that, professionally duplicated disks (ie most software) would only ever use half-tracks or just normal tracks.
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Re: Apple ][ disks

Post by TeaRex »

Even the Apple drives can usually read data fine if you're just a quarter-track away from the "right" position, thanks to the width of the head; valid data has to be at least one full track away from any other valid data to be readable. Thus, quarter tracks weren't useful for protection purposes on their own. Some (few) titles used them in a "spiral" arrangement together with full and half tracks.
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