ix255@sdccs6.UUCP (ix255) (05/20/84)
Rumor is that Apple (most Shugard drives in fact) are capable of
double density recording butr controllers are behind the times. As an
Apple, I would be very interested in drives with 630k/side. (80 tracks,
32 sector/track). Of course I don't expect Apple to tell me about this, so
does anyone out there know of a way (if any) to:
1 Allow the apple/Quentin/etc. drives to use all 80 tracks
(its claimed the existing drives can do this now)
2 Allow the apple to use something called MFM recording
All info is most helpfull. ..!noscvax!sdcsvax!sdccs6!ix255clt@pur-phy.UUCP (Carrick Talmadge) (05/22/84)
The way I understand it the Apple ][ analog board does use the
MFM recording scheme. Also as Dave Seaman reported, the problem is in
the Tandem drive itself. It can half-track so as to give an appearance
of 80 track drives, but, as I understand it, there are two reasons why
this can't be utilitized to double the amount of info stored on a disk.
First, the width of the track written to disk by the magnetic head is
too wide to allow adjacent half tracks to be written without overlap, and
secondly, even if the magnetic head could produce a sufficiently "thin"
recording track, the head positioning mechanism is not precise enough to
take advantage of it. Some computer manufacturers such as Commodore get
around this by writing more than 16 sectors to disk on the outer tracks,
but this leads to an anoumously high diskette failure rate...
Carrick Talmadge
UUCP: {decvax,ucbvax,harpo,allegra,inuxc,seismo,teklabs}!pur-ee!Physics:clt
INTERNET: clt @ pur-phy.UUCPcmf@cwruecmp.UUCP (Carl Fongheiser) (06/04/84)
There seems to be some confusion (this may be an understatement) about exactly what an Apple disk drive is and what it will do. This may be a bit technical, but here goes... The mechanics (i.e. head, motors, chassis, etc.) of the Apple drives are equivalent to those of a Shugart SA400. Shugarts were used for a while, but reliability problems started cropping up, and they switched to ALPS drives. In any case, these are your standard, run-of-the-mill, 48 TPI floppy drives. Only 35 out of the 40 available tracks are used, probably because 35 track drives are cheaper, being older. The recording method used is FM. However, they pack 4K on a track, which is the same density as most MFM disks. In fact, if the Apple drive used 40 tracks and both sides (like some of the Rana and other drives), it would have 320K per disk, just like an IBM PC. Now, you may ask, how does the Apple get MFM data density with FM data rates and encoding? The answer is a very clever recording method called MGCR (stands for Modified Group Code Recording). Thanks to some wizardry (no lesser adjective is appropriate) on the part of Steve Wozniak, this is done with a handful of cheap chips (look closely at the Apple controller sometime). The basic idea involved in FM is to have a stream of clock bits with a stream of data bits interleaved. MFM takes this and fudges with the clock bits (sorry for the lack of technical accuracy, folks), sometimes dropping them altogether in order to fit in the data. On the average, there are more flux changes per unit length, and thus a higher bit density. MGCR takes the data bit stream and breaks it up into 6-bit groups. It maps these groups to 8-bit groups (hence the name "group code recording") which represent flux changes on the disk. No clock bits are used. Instead, each 8-bit group conforms to a simple set of restrictions regarding number of consecutive 0 bits and others (including the high bit being high (no pun intended)). These restrictions allow the nifto Woz state machine on the controller to keep track of everything and stay synced up. This technique is used for high-density tape drives and on the MAC. In short, it gives you double-density with single-density physical bit density (flux changes per unit length), and hence, greater reliabilty. It's too bad everyone else uses flakey old MFM. Oh, well. On the issue of 96 TPI drives, etc.: The stepper for the head on Apple-compatible drives is controlled directly by the 6502. The existing code in DOS will step at 96 TPI even with a 48 TPI mechanism. Several copy-protection schemes rely on this so-called "half-tracking" ability. In fact, with appropriate incantations, you can step the head at 192 TPI. There's only one small catch: a 48 TPI head is twice as wide as the one on a 96 TPI mechanism. Therefore, a standard Apple drive will not quite handle 96 TPI. A 96 TPI mechanism will. This is the trick by which the Rana and other 96 TPI drives will boot and run 48 TPI (standard) disks without complaining. They look like normal drives, but can handle the higher track density. One wonders why Apple doesn't bring out 80 track, double sided drives. Again, Oh well. --Clayton Elwell ...!cbosgd!osu-dbs!elwell despite the address on the header of this message. P.S. Flames are welcome, but do not send to cwruecmp!cmf...