19 Jun
2010
19 Jun
'10
2:17 p.m.
On 19 Jun 2010 at 21:22, Philip Pemberton wrote:
> The "MFM test"? As in, "are there
two '1' bits directly following
each
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.
--Chuck