Re: Did Commodore cheat with the quad density floppies?

From: André Fachat <afachat_at_gmx.de>
Date: Fri, 04 Jan 2019 11:57:19 +0100
Message-ID: <16818829a98.27ff.b4d1f2b66006003a6acd9b1a7b71c3b1@gmx.de>
Am 4. Januar 2019 11:48:45 schrieb Gerrit Heitsch <gerrit@laosinh.s.bawue.de>:

> On 1/4/19 10:41 AM, André Fachat wrote:
>>
>>
>> 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.
>
> But the final drive implementation still resulted in Commodore drives
> with their custom logic fitting more data onto the same media than
> compared to MFM drives using a standard controller.

That was mostly due to different speed zones so Commodore could store more 
data on outer tracks.

André
>
Received on 2019-01-04 12:01:37

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