Re: Did Commodore cheat with the quad density floppies?

On 03/01/2019 17:29, afachat@gmx.de wrote:

> GCR is NOT more efficient than MFM using the same bit frequency. In fact MFM
> is more efficient (writing 8 cells @ 250kHz for MFM vs. 10 cells @ 250kHz for
> CBM GCR).
>
> The reason that MFM can do this is that they actually use "half cells" at
> 250kHz, but still ensuring that flux transitions are at least 4us (i.e. a
> 250kHz cell) apart.

mfm needs 2 transitions for every bit, so 8 bits requires 16 transitions.

gcr stores 8 bits in 10 transitions. So clearly more efficient, but mfm 
can be clocked faster than gcr because it reduces the number of 0's that 
can occur in a row.

http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0194.html

      8       FAST       Value of 1 selects two microseconds per bit cell
                         (usually MFM).  Data must be valid raw MFM.
                         0 selects four microseconds per bit (usually GCR).

When Amiga Technologies started fitting high density floppy drives to 
the A1200 someone put together a package that used gcr at the higher mfm 
clock, which (if you used high density media) was supposedly reliable 
enough to store more than the standard amiga low density (880k), but 
less than the standard amiga high density (1.76mb). I never tried it, I 
had an external dual speed drive that worked with standard format high 
density disks, but I think it worked out at approximately 1.46mb.

It's here if you're interested http://aminet.net/package/disk/misc/HiDensity

Your "half cells" sounds like you're doubling the bit frequency, but 
then saying it's the same.