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

From: Eric Smith <eric_at_brouhaha.com>
Date: Sat Apr 10 05:30:35 1999

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 - 05:30:35 BST