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

From: Mark <mark_k_at_iname.com>
Date: Sat Apr 17 16:45:04 1999

On 10 Apr 1999 Richard Erlacher wrote:
>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.

Can someone explain exactly what precompensation is?

I remember reading about this in the Amiga hardware manual; in the ADKCON
register there are two bits which set the precompensation (none, 140ns, 280ns
or 560ns), and also one that selects GCR precompensation or MFM
precompensation. Note that the encoding method is implemented completely in
software, so you are not limited to MFM and GCR.

(There's also a control to set the data rate, 2us or 4us per bit cell. 2us is
used for MFM, apparently for GCR 4us is necessary.)



-- Mark
Received on Sat Apr 17 1999 - 16:45:04 BST