Vanessa, I think I got to that in my follow-up.

We are mainly talking about the Zip Disk that been around for awhile and may 
have used an older scheme then the hard drives that came out in 1995 (IIRC 
about when Zip Disks first came out).

Like I said in a follow up - it can be PRML or even EPRML. Modern HDs uses 
this scheme and possibly a custom encoding schem on top of that. I agree 
with Thomas as well, I think it can be a proprietary/custom encoding scheme 
(or one of many encoding systems out there) running on top of the PRML/EPRML 
recording system. As for the use of GCR.... It be suprising to see being 
used today as it is primarily a "Retro" thing, nowadays.

"Vanessa E."  wrote in message 
news:gmfbho$rd8$1{at}aioe.org...
> Rick Balkins wrote:
>
>> It is probably an RLL scheme. MFM is for FAT/FAT12 disks.
5.25" & 3.5"
>> floppy disks and very early hard drives. New hard drives uses an RLL 
>> scheme.
>> If anything, for legacy sake, this would be an MFM/RLL style scheme 
>> versus a
>> GCR/RLL scheme.
>
> Only older hard disks with the ST-506 interface used RLL and MFM.
>
> Modern hard disks use something different.  Anything made between the 
> ST-506
> era and roughly 2006 uses parallel recording of course, but not the 
> RLL/MFM
> schemes.  What exactly is used probably depends on the manufacturer.  From
> 2006 onwards, drives are starting to appear that use perpendicular 
> recording,
> where the magnetic particles are oriented vertically with respect to the
> media.
>
> On top of the recording scheme are something called PRML (which helps make
> sure reads are consistent despite noisy media), and Reed-Solomon error
> correction (same scheme as used on CD and DVD media).
>
> On top of that is the interface layer, the bit we all know as IDE, SATA, 
> or
> SCSI.  The controller in the computer has no idea at all about the 
> recording
> scheme.
>
>> FAT was built around and on top of MFM coding ...
>
> It might have been designed with floppy disks in mind, but FAT is a 
> filesystem
> layer, independent of the underlying recording scheme.  These days, it is
> most commonly found on memory cards, thumbdrives, mp3 players, and so on.
>
> FAT is used so commonly because practically everything made in the last 20
> years or so can read it.  Other filesystems can theoretically be used as
> well, as long as the disk is low-level formatted properly and the
> filesystem's metadata will actually fit into the 1.4 MB space available.
>
>> If you take a look at Iomega SuperDisk project,
>
> SuperDisk was a 3M/Imation technology, not Iomega.  That aside...
>
>> it alludes to MFM and MFM/RLL encoding scehemes versus GCR because it
>> doesn't read older GCR disks but MFM disks. So, this is probably the 
>> case.
>
> You're talking about the floppy disk read capability.  The Imation 
> SuperDisk
> project is a variation on the older Floptical technology, which used RLL 
> for
> the recording scheme, with a laser to guide the head from track to track.
>
> As Iomega disks are entirely magnetic, RLL probably also forms the basis 
> of
> the recording scheme, but it likely differs enough from the sort used on
> floppy disks that it can no longer be called "RLL".
>
>> GCR was Commodore, Apple II, and few IBM based systems:
>> http://en.wikipedia.org/wiki/Group_Code_Recording
>
> According to the above link, GCR can be used on top of RLL, which does 
> make
> sense.  Remember that RLL and MFM are the low-level recording schemes, 
> while
> GCR is really more of an encoding scheme, which is why we have raw
> GCR-encoded 1541 disk images out there (.G64), in addition to the usual
> unecoded version (.D64).
>
> As to the ZIP drive, I would have to agree with Thomas - the low-level
> recording scheme could be just about anything, since it is abstracted away 
> by
> the interface.
>
> -- 
> "There are some things in life worth obsessing over.  Most
> things aren't, and when you learn that, life improves."
> http://starbase.globalpc.net/~vanessa/
> Vanessa Ezekowitz 
>
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