Re: it's a FAQ I know, but IIc 5.25" -> anything?

["Bryan Parkoff" <nospam@nospam.com>]

"Ed Eastman" <noone@nowhere.net> wrote in message 
news:dctcct$8eq$1@unlnews.unl.edu...
> Bryan Parkoff wrote:
>> Someone claims that PC's MFM floppy drive can read Apple II's GCR floppy 
>> disk at read only mode while it converts to create .dsk image.  Correct? 
>> I don't believe so. Please clarify more how disk2fdi program works.
>
> The MFM drive doesn't read the CGR bits, it reads the transitions 0 to 1 
> or 1 to 0.  The transition 'generate' an FM pulse that can be detected by 
> the drive's hardware.  These pulses are precisely timed and the bits 
> between pulses can be extrapolated using software.
>
> Cool, huh?
>
> More info is in the documentation and I can confirm that it works.  The 
> reason the back side does not is due to lack of an index (timing) hole 
> (reading flippy disks turned front wise) and the fact the 2nd head on a PC 
> floppy drive is offset by 3 tracks (reading the back side directly). The 
> PC drive physically can't position the 2nd head over the first three 
> tracks on the back side.
    I do know that GCR and MFM use FM encoding.  I can't remember when 
Benath to DOS 3.3 and ProDOS manual explain what FM is used in GCR encoding, 
but FM seems to be similiar.
    I am not sure what transition means.  I assume that GCR and MFM 
encodings do not have bit string on disk, but they are only pulses and 
non-pulses.  Do you claim that GCR and MFM do not have pulses / non-pulses 
in the same positions?
    PC software reads GCR encoding by detecting pulses / non-pulses before 
it begins to recognize GCR bit string into conversion?  GCR uses soft sector 
and MFM uses hard sector.  GCR can position anywhere without index hole.  It 
may mean that MFM has to start index hole while searching GCR to find the 
start position before detecting pulses / non-pulses.
    Do you have the information so it can help me to understand what 
transition and pulses / non-pulses mean?

Bryan Parkoff