- PaulSanderson

- mscotgrove

on double density disks, there is not always a clock bit.

You sure about this? Single or double density the encoding is still MFM (assumimg a PC and not GCR on a Mac) and MFM calls for clock bits always. It's been a long time since I read a floppy at the raw level but I dont recall any difference in the way I read and pieced together double/single density disks.

It has been a long time too for me working on floppies - the following is from memory.

Single density ALWAYs has clock bit

MFM has a clock bit if there is no precceding data bit or following data bit - I think

M2FM was slightly different logic, but along the same lines where clock bits were added if there were no data bits. I think Intel were the only people who ever used M2FM

GCR was often 5 bits of disk data for 4 bits of actual data - make sure there were never more than 2 zeros in a row.

400K/800K Macs were GCR, but with 5 different recording rates for each group of 16 tracks.

Sirius and Commodore were all GCR, and some Commodore were 100tpi
_________________
Michael Cotgrove
www.cnwrecovery.com
cnwrecovery.blogspot.com/ 

I disagree

Only SD always has a clock bit
_________________
Michael Cotgrove
www.cnwrecovery.com
cnwrecovery.blogspot.com/ 

en.wikipedia.org/wiki/...Modulation

Single density is FM. Typically this died with 8" IBM disks

Some very early 5.25 disk were single density, but most were 40 track double density (360K) or 80 track, High density, 1.2MB

I am not aware of a 3.5" disk that was ever single density, always double density (720K)or high density (1.4M) Extra density was 2.8M but never caught on.


Double and high density all use MFM (with conditional clock bits).
_________________
Michael Cotgrove
www.cnwrecovery.com
cnwrecovery.blogspot.com/