Software-based floppy disc data separator

Chuck Guzis cclist at
Sat Jun 19 16:17:49 CDT 2010

On 19 Jun 2010 at 21:22, Philip Pemberton wrote:

>> The "MFM test"? As in, "are there two '1' bits directly following 
> other in the encoded data stream?"

Far more basic--just look at the bitstream frequency peaks.  FM will 
have 2 peaks (11 and 10) at t and 2t.  MFM has 3 peaks (10, 100, and 
1000) at t, 1.5t and 2t and so on.

> Are those similar to the Mitsumi Quickdisk (nee Famicom Disk System)
> discs? Basically a continuous stream of binary data, a SEEK_TO_START
> input and a HEAD_AT_START status output?

The same.  Mechanically, it's like a self-completing auto winshield-
wiper mechanism.  The head positioner is coupled to the spindle motor 
through a clutch mechanism.  Pop the clutch and the head sweeps 
across the disk in a big spiral, then quickly returns to its starting 
point.  Encoding is left to the OEM, but there's usually a sectoring 
scheme of some sort and the bitrate is pretty low (~ 50Khz).  A 
typical capacity is about 60KB using MFM.

> 32-sector, soft-sectored, 64 tracks. So probably not a standard track
> pitch then... you're probably pretty screwed if you don't have a 651
> sitting on a shelf.

Usually hard-sectored--the sector holes are punched in the outer edge 
of the disk, though I suppose there's nothing to prevent one from 
ignoring the sector pulses and soft-sector using only the index pulse 
as a reference.

"Having one on the shelf" is essential.  For example, you might have 
what looks to be a 5.25" floppy, but if it's a 2MB disk written in a 
Drivetec drive, you'll need the drive to recover the data.  


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