There are significant differences between MFM and GCR that might affect the useable lifespan of disks. The first is that technically, Apple's GCR only records at single density (not double density) media bit rates. MFM records a '1' bit (aka, a transition) as a clocking bit between every data bit. GCR doesn't add as many 'clocking' bits, but rather encodes the data in a manner that there are a limited number of consecutive '0' bits (no transition).
What that means in practice is that on the disk media, the following bit (transition) streams represent 3 bytes. These are examples, not necessarily the exact data that would be written. I'm using a ! to represent a dedicated clock bit (always a '1') and 0 or 1 for data bits. Note that the data bits are not identical because of the GCR encoding, the data would have to be decoded in order to see the original data. MFM = !0!1!0!1!1!1!0!0!1!1!1!0!0!0!0!1!0!0!1!0!0!1!1!1!0 GCR = !0111001!0011101!1011001!1101010 Notice that MFM uses 48 bits on disk (including 24 clock bits), while Apple's GCR uses only 32 bits on disk (with only 4 dedicated clock bits). In Apple's implementation, the first bit of each byte on disk must be a '1', effectively making it a 'clock' bit, however, some of the Apple // copy protection schemes don't enforce that restriction which is one of the things that makes some disks very hard to copy. The way that PC's using MFM record more data on a 5.25" floppy is that it is actually writing bits to the media twice as fast (500Kb/s, aka Double Density) as Apple's disk drive (250Kb/s, aka Single Density). In practice, this means disks written on an Apple // have up to twice as much space between transitions on disk as do MFM disks, and that may contribute to longer term readability. GCR is a close relative (and predecessor) of the RLL encoding system used on all modern hard disks and almost all modern tape drives. MFM = Modified Frequency Modulation GCR = Group Coded Recording RLL = Run Length Limited ----- Original Message ----- From: "Antonio Rodr�guez" <[EMAIL PROTECTED]> To: "Apple2list" <[EMAIL PROTECTED]> Sent: Sunday, November 14, 2004 07:51 PM Subject: Re: Life Span of 5.25 disks > [EMAIL PROTECTED] ha escrito: > > That's because of the data encoding schema used by PC's and Apple ]['s drives. PCs > use the (almost standard) MFM encoding, while the Apple ][ use Woz's designed GCR > (correct me if that's not the name - I'm sooo bad remembering exact data... :-( ). > It turns that, at the same media density, GCR is more reliable than MFM. > > I remember my college days, when people said 3.5" PC disks were "reliable" because > they failed less than 5.25" disks. Sometimes I had to try hard not to laugh at they > faces and say that I hadn't had a single diskette failure in more than ten years. > > Greetings, > -- Apple2list is sponsored by <http://lowendmac.com/> and... / Buy books, CDs, videos, and more from Amazon.com \ / <http://www.amazon.com/exec/obidos/redirect-home/lowendmac> \ Support Low End Mac <http://lowendmac.com/lists/support.html> Apple2list info: <http://lowendmac.com/lists/apple2.html> --> 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/apple2list%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
