Date   : Thu, 14 May 2009 22:32:05 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: Preservation of information (floppy discs, etc.)

> Is there some way you can get this to work with PC hardware?? How does
> Disk2FDI do it?

I think Disk2FDI uses a custom dongle connected to the parallel port. I've
never used it though (only tried to implement the finished images) - Dave
Moore would be the person to ask here.

> As an extension, you could consider whether you want to try to cut out 
> the discriminator too.? A normal FDD gives a binary bitstream, which is 
> the product of some analogue discriminator ('is this head signal a 0 or 
> a 1?'). If you knew the discriminator was deciding it was a 0 because 
> the signal was 49% then you have less confidence in that bit than at 
> 8%.?Maybe that could feature in your DSP?? 

Disk2FDI does that. Which is an absolute pain to support in an emulator.
Which is why I'm refusing to implement any other formats that aren't straight
FM/MFM in B-em or Arculator.

> I notice something on Disk2FDI mentions something about drives 
> returning a 'fake' index signal:
> http://apple2.org.za/gswv/a2zine/Docs/Disk2FDI_InfoPostings.txt
> Not sure how they would make a fake signal.

I'd guess it just periodically toggles the index pin. PC hardware can get
away with this, but I think other controllers (eg 177x) would have problems
with the index signal being in the wrong place.

Tom