amiga floppy disk reading experience

[Philip Pemberton]

On 13/06/10 02:21, Keith wrote:
>> You're using fixed-position analysis? That is, "if the pulse was
>> between X and Y us after the last pulse, it was a 01" and so on?
>
> I don't know what's it's technically called, but yes.

OK, same as my "proof of concept" analyser then.
What I'm doing is a histogram analysis, followed by a peak-detect, and 
then picking the centre points between the peaks as the thresholds.

Problem is, this only corrects for MSV. On a disc which has been written 
with pre-compensation, this is (usually) fine. Without precomp, or on an 
old disc which is suffering from bit-shift, it's a whole different ball 
game.

> I've read about these before. When you say the drive motor doesn't
> rotate at a constant speed, you mean that despite it having an average
> of 300 rpm, that there is some speed up and slow down throughout a
> single revolution?

That's ISV (Instantaneous Speed Variation). MSV and ISV are two 
different things. ISV is usually caused by variations in friction in the 
disc mechanism. MSV is a bit different -- that's a difference in the 
average speed and varies on a per-drive basis.

A lot of older drives (Amstrad 3" drives in particular) used motors 
which had a small speed-adjust screw. A single drive could be adjusted 
by a few percent relative to another. On modern drives, the speed is 
locked against a quartz crystal or a ceramic resonator, and is typically 
much more accurate. There's still a several-digit-PPM variation between 
drives though.

 > I've measured time between index pulses, and so far
> they appear to be 200.x ms.

200.0 is your target but some variation is normal. Hook up a frequency 
counter with a serial or HPIB output and plot the time for each rotation 
on a graph. It won't be a straight line...

> Do you have your code up someplace? Verilog or VHDL?

I can't release the software source as yet, because it's part of my 
final-year university project. The Verilog code is open, though, but I 
haven't set the version control repository as public yet.

> Thanks for all the links, they should be valuable. I'm still working on
> the basic concepts of a PLL. Once I get a better handle, I think writing
> the code for a basic one should be within my reach.

Also look up Roman Black's Bresenham Timer algorithm --
http://romanblack.com/one_sec.htm

I've been thinking about how this looks like it could be used as a PLL 
(you'd probably need two phase-accumulators, one driven by the incoming 
data), but haven't really spent much time on implementation...

-- 
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/