amiga floppy disk reading experience

Eric Smith eric at
Thu Jun 10 17:46:05 CDT 2010

Keith wrote:
> Without sounding too obvious here, the time between the pulses (which 
> more or less define the data) were grossly out of spec.  The DD pulses 
> should nominally be 4us, 6us, and 8us apart before pre-write 
> compensation.  Most good disks are slightly faster, and normal times 
> for these ranges are:
> 4us: 3.2-4.2us.  Many around 3.75us
> 6us: 5.5-6.2us.
> 8us: 7.5-8.2us
> (notice margins around 1-1.3us)
> My original microcontroller implementation was 3.2-4.2, 5.2-6.2, and 
> 7.2-8.2.
> When my current FPGA controller would have a problem, I'd notice that 
> there were problems right on a boundary.  So maybe pulses were coming 
> in at 3.1us apart instead of 3.2.  Or maybe 4.3 instead of 4.2.  So I 
> kept bumping the intervals apart, making a larger range of pulse times 
> acceptable --- the XOR sector checksums were passing, so I was likely 
> making the right choices.  The bits were ending up in the right buckets.
> But as I went through some of these disks, I ended up with the 
> difference between ranges(and basically my noise margin) being reduced 
> smaller and smaller.  Some to the point where an incoming pulse time 
> might fall darn smack in the middle of the noise margin.  Which bucket 
> does THAT one go into?
> My approach has been very successful(easily 95%+), but it makes me 
> wonder about Phil's DiscFerret dynamic adaptive approach where a 
> sample of the incoming data defines the ranges.
This is exactly why good floppy controller hardware uses a PLL for data 
recovery, rather than one-shots, simple state machines, and the other 
approaches that were taken to save money, board space, etc.

My experiments with floppy data recovery in software used a simple DPLL, 
and I found that it tracked the data much better than any simple 
threshold scheme.

The best data separators actually used PLLs with two feedback paths with 
different characteristics, one to track longer-term variation due to 
motor speed, and one to track short-term effects.


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