Re: Did Commodore cheat with the quad density floppies?

Am 4. Januar 2019 10:26:14 schrieb André Fachat <afachat@gmx.de>:
>
>
> Am 3. Januar 2019 20:01:30 schrieb "Mike Stein" <mhs.stein@gmail.com>:
>
>> I wonder if part of the answer to Andre's original question may be the fact 
>> that Bits per inch is not necessarily the same as Flux transitions per 
>> inch/mm...
>
> Absolutely. 300 Oersted media had 5900 flux transitions per inch, which 
> gives 2900 bpi using FM due to the many clock bits needed, or 5900 bpi 
> using MFM. QD was the same media, only was defined for 96/100 tpi instead 
> of 48 tpi.
>
> Commodore GCR 170k used 250kHz write frequency,  thus the same 5900 flux 
> transitions per inch, i.e. 4us bit cells.
> Commodore GCR 500k used 375kHz writes, which increases ftpi by 50% and 
> reduced bit cell size by 33%. Which seems to be out of spec with all Media 
> specifications I found.

And, BTW, MFM ist actually more efficient than Commodore GCR.

MFM uses 16 cells at 500kHz, i.e. 16 x 2us = 32us per byte.
Commodore GCR uses 10 cells at 250kHz, i.e. 10 x 4us = 40us.

André