Tim's own version of the Catweasel/Compaticard/whatever

Please see my embedded remarks below. Dick ----- Original Message ----- From: Tim Mann <mann(a)pa.dec.com> To: <classiccmp(a)classiccmp.org> Cc: allisonp <allisonp(a)world.std.com> Sent: Wednesday, July 05, 2000 11:02 PM Subject: Re: Tim's own version of the Catweasel/Compaticard/whatever In reality, you should not be able to detect the precompensation windows at all, if they'r properly applied. The purpose of the precomp is to anticipate and pre-correct for peak shift due to crowding of the bits. As I wrote before, since the peaks are of opposite sign, in cases where they are "too" close together, meaning that one pulse is still decaying while the other begins to present itself, (detectable by examining the slope of the signal) the addition of the two values subtacts from the two somewhat linearly from the theoretical peaks, forcing the peaks themselves to occur farther apart than they were written. This is helped by the lower write-current, resulting in detection of the inreasing current in the head somewhat later than if it had been written at full current, and secondly by the fact that it is written earlier at the leading end of a pulse train, and later at the trailing end. The upshot of all this is that the peak detector senses these peaks at their nominal location even though they were not written that way. Though it may appear that way, I doubt that consistently readable diskettes have a great deal of error in their position in the pulse train. It's important to keep in mind, however, that it's the level and not the edges that contain the information. But for that, the FM data/clock separators using one-shots would never have worked at all, with their wide error margin. If you sample the data at a harmonic of the rate at which its transitions were written, ever mindful of the way in which FDC's work, you should be able to do accurate prediction/correction of the sampled data in accordance with the model. It's more difficult if you write the data at 500 KHz, precomp it at 6 MHz and then recover it at 7 MHz. It just makes sense to use a harmonic of the frequencies involved. It makes the data stream larger, perhaps, but the patterns, such as they are, with their random errors due to noise, and systematic error due to mechanical components' influence, should be much more easily detectable. What's more, if you really want to eliminate unwanted signal components, you could carefully make four readings, having rotated the spindle 90 degrees in the same direction between insertions of the diskette, and then sum the data samples, aligning to index each time for rough alignment and then to the first ID address mark once it's found. That may help correllate out the random error. It should also make the mechanically generated error easily detectable. "partial