> Subject: Re: 800k discs > > > >An 800K Mac floppy drive doesn't use a strong enough magnetic field to > >properly record data onto 1.44 MB disks. > > > >Is the glass half empty or half full? > > That's why you can't format 800K disks as 1.4MB; the reason you can't use disks > the other way 'round is due to differences in the media. > -- > > the pickle
To clear up the persistent confusion and superstition about 800K vs. 1.44MB media, here's the correct story: There is about a ~10% difference in the magnetization thresholds (called "coercivity") for the two media, with the 800K stuff having the lower value. If you want to get technical, 800K media have a nominal coercivity of 650 oersteds, versus 720 - 730 oersteds for 1.4MB media. So, 800K drives may find it difficult to write on 1.44MB media. However, 10% is not a large difference, and in fact, is about the same as normal variations within a batch from a given manufacturing run. Plus, coercivity varies with temperature, too. So, the two media are not as wholly incompatible as lore has it. However, if a 1.4MB disk has *ever* been written on by a 1.4MB drive (and this includes formatting), an 800K drive's weaker write fields may not be strong enough to reliably over-write the existing data, and you'll have flaky behavior (particularly if you're unlucky enough to have a drive with write currents at the low end of the spec, trying to write on a floppy with coercivity at the high end of spec). But if the floppy is virgin, you'll rarely see any problems at all. [You can "re-virginize" floppies if you have a good demagnetizer handy.] Now, if you go the other way, by melting or punching (don't drill!) an extra hole to trick drives into thinking an 800K floppy is really 1.4MB, there's no problem with the drive's ability to flip magnetizations properly. However, the higher density is achieved by packing adjacent bits more tightly together on a given track (but the number of tracks per side is the same -- 80 -- for 400K/720K/800K/1.4MB media), and the lower density media may not have fine enough particles to do the job well (and the lower magnetic field strength of those particles further degrades margin). That's why many advise against doing this operation. Since both media types are readily available (the 800K stuff is the same as 720K media from the PC world, but you'll have to reformat as Mac if they come preformatted, as they usually do nowadays), there's no real reason to do any of these things. But, every once in a while, you'll find these hacks useful in an emergency. 400K floppies use exactly the same media as do 800K floppies. The former are simply one-sided. Be aware that many 400K drives actually damage the surface that isn't used (because the media rides on a post which frequently isn't all that smooth or clean), so you may not be able to reformat them as good 800K floppies ever again. Finally, the tricks that squeeze a few percent more data from a floppy frequently do so by simply going beyond the 80 normally allocated tracks (this is also done in some copy protection schemes -- most disk copy software automatically stops copying at the 80th track). If you combine that trick with somewhat more efficient formatting and data management schemes, you can easily boost the useful density by 10%-20% (such as is done with Microsoft's 1.6MB DMF disks, which don't go beyond 80 tracks), without stressing the capabilities of the media at all. While on the subject of superstition, people are overly (almost irrationally) frightened of a compact mac's crt energy, but insufficiently worried about the main power supply filter caps. You can read more about this subject at http://www.applefritter.com/thefritter/13/drwebster.html --Cheers, Tom -- Prof. Thomas H. Lee Center for Integrated Systems, CIS-205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 http://www-smirc.stanford.edu 650-725-3709 ph, -3383 fax -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> The FAQ: <http://macfaq.org/> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
