amiga floppy disk reading experience

On 13 Jun 2010 at 0:03, Eric Smith wrote: I'll agree that it takes a mental stretch, but there is a "lock". and a "loop" is easy enough to incorporate. Play along with me on this one. I'll use a Catweasel for my model, since it's most familiar to people. Suppose there is a Catweasel reading a disk at 50t (the reference frequency), where t is the assumed "short" inerval of an MFM stream. That means that the center points of the sampling windows are 50t, 75t and 100t. Let's also assume some finite sample window size that allows for capture of a sample on either side of the window center, plus or minus a fixed number of counts. Note that there is a 1-count "jitter" in the Catweasel hardware, where a pulse arriving at 50t can be reported as 49t, as well as 50t; let's assume that the windows are wide enough to accomodate that. What's lacking is "loop" or feedback term to adjust window centers to track variations in speed. Without it, this is no better than the average UART receiver. However, there is also an error quantity. Just as one can take the error signal from a traditional PLL and estimate the spindle speed of the drive, so can one sum the intervals reported by the Catweasel and compare it with an ideal and obtain the spindle speed of the drive. On a per-sample basis, one can compare the value of the sample with its corresponding window center to obtain an error value. Thus, one can take this error value and adjust the sampling window centers appropriately, giving a "loop" and a "lock" to the incoming data stream. Thus, one has a reference, a lock, and a loop based on an error value. Maybe not a PLL in the strict sense, but something that shares the characteristics of one. --Chuck