dfitoimd: decoding Intel M2FM floppy flux images (was Re: Intel 432 floppy flux images for decoding)

[Eric Smith]

On Mon, Aug 8, 2016 at 2:33 PM, Al Kossow <aek at bitsavers.org> wrote:
> is there any correlation between creation dates of the files and the failure rate?
> i'm wondering if the heads started to clog farther into the imaging session.

I'm seeing more errors on low numbered tracks, so I really think I
just need a better data separator algorithm.

References on PLL design state that the error signal has to be low
pass filtered for loop stability, and I've seen plenty of designs for
analog PLLs, but I can't find any example ADPLL design that has low
pass filtering. The references describe using an IIR filter. I've
never had good luck with IIR filters, so I tried a 2nd order (3 tap)
FIR filter, and a range of cutoff frequencies, and didn't see any
improvement.  Maybe 2nd order doesn't have sharp enough rolloff to
help. I really need to learn more about control theory and the
Z-transform.