Lisa "twiggy" specs (Was: selling a blank floppy disk

"Fred Cisin (XenoSoft)" <cisin(a)xenosoft.com> wrote: The Twiggy disk uses a zoned-CAV system like modern hard drives, except that it changes the spindle motor speed and keeps the data rate constant. Thus the number of sectors per track varies. There are 46 tracks for data (numbered 0 to 45), and track -1 which is used for speed calibration. I'm assuming that track 0, the outermost track for the lower head, has a radius of 2.25 inches, the same as a standard 5.25 inch drive. The Twiggy uses a track pitch of 62.5 TPI. The zone information is given in this table, which is compiled from the Lisa Hardware Manual; I've added the peak flux density which I computed in an Excel spreadsheet. 512-byte peak sectors rotation flux per rate density track track (rpm) (fcpi) ----- ------- --------- --------- 0-3 22 218.3 9721-9933 4-10 21 228.7 9550-9989 11-16 20 240.1 9588-9973 17-22 19 252.7 9552-9955 23-28 18 266.8 9509-9931 29-34 17 282.5 9463-9907 35-41 16 300.1 9414-9981 42-45 15 320.1 9453-9749 Note that for Apple's (6/8) GCR format, equivalent to (0,2) RLL, the minimum flux density is 1/3 of the maximum flux density. For MFM, the minimum is 1/2 the maximum. This means that the media has to work for the range of 3138-9989 FCPI. Standard 5.25 inch floppies spin at a constant rate, and transfer data at a constant rate, so they have a much lower flux density on the outer tracks. For 360K format, the maximum flux density ranges from 3537 FCPI on side 0 track 0 to 5877 fcpi on side 1 track 39, for an overall range of 1768-5877 FCPI. For 1.2M format, the maximum flux density ranges from 5895 FCPI on side 0 track 0 to 9870 fcpi on side 1 track 79, for an overall range of 2947-9870 FCPI. Obviously, then, my inital guess that high-density media is required would appear to be correct. Apple's (6/8) GCR is exactly 3/4 as efficient as MFM. Eric