FM, MFM, and GCR channel codes (was Re: stepping machanism of Apple Disk ][ drive)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sat Apr 10 10:49:10 1999

YES! and that's exactly why the pulse overlap was correctable with
write-precompensation at least in hard disks at 10x the data rate. In
general the amplitude of the pulses was sufficient to be detected by the
usual circuitry, but because the timing was quite far off due to the peak
shift introduced by the summing effect of the head/media combination.
Precompensation, which was not needed for FM, was between 188 and 125 nsec,
depending on the drives in use. On the older drives, 188 was pretty common.
The "bit-shift" was mitigated somewhat by the reduction of write-current on
the inner tracks.

Dick

-----Original Message-----
From: Eric Smith <eric_at_brouhaha.com>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Saturday, April 10, 1999 4:36 AM
Subject: Re: FM, MFM, and GCR channel codes (was Re: stepping machanism of
Apple Disk ][ drive)


>I wrote:
>
>> I thought about this for a few minutes. Ignoring rise and fall times,
>> for 250 kHz FM, I expect to see spectral peaks at 250 kHz and 500 kHz.
>>
>> For 500 kHz MFM, I expect to see peaks at 250 kHz, 375 kHz, and 500 kHz.
>
>Oops, I wasn't thinking clearly enough about how the write channel works,
so I
>was off by a factor of two. That should have been 125 and 250 kHz for the
FM
>case, and 125, 187.5, and 250 kHz for the MFM case.
>
>> Therefore, it seems to me that a channel with reasonably flat response
from
>> 250 kHz to 500 kHz should be able to handle either 250 kHz FM or 500 kHz
> ^^^^^^^^^^^^^^^^^^
>Make that
> 125 kHz to 250 kHz
>
Received on Sat Apr 10 1999 - 10:49:10 BST