Tarbell is making me insane

On 13 Nov 2007 at 11:34, Grant Stockly wrote: Very close--I've got 3M's published specs on all of these, if anyone cares. And 3.5 DSHD media is fairly close to DS2D; the coercivity is somewhat higher, requiring increased write currents, but not as drastic as the difference between 5.25" DSDD and DSHD. Of course--for at least two reasons. The first is that the linear path is shorter; you're packing more bits per unit length. The second is that the surface velocity with respect to the head is lower. Since the induced write current is a function of the square of the velocity (another one of those v**2 laws that are so common in physics), the signal is weaker. The methods of getting around this are several. One can vary the rotational speed of the drive so the linear velocity remains relatively constant (Apple Lisa and the Victor 9000 come to mind). The upshot here is that outer tracks will hold more than inner ones, so a translation layer is necessary in software when attempting to determine the address of a block. One can also vary the speed of the bit clock, slowing it as the center of the disk is approached. This doesn't elminate the problem of a lower read signal on the inner tracks, but it helps. One can reduce the write current on the inner tracks (8" drives do this on receipt of the TG43 signal) to make the transitions a bit cleaner (oxide coated media has a tendency to "blur" or "shift" the edges of domains if they're close together). Finally, one can play some tricks, varying the timing of bit transitions during writing depending on the pattern of adjacent bits to make things look better when reading--in other words, if two bits are close together, one can make the transition of the first one a bit earlier or the transition of the second one a bit later. This is mostly done with MFM recording, not FM. I don't know enough about your controller to diagnose things, but perhaps this will help a bit. Cheers, Chuck