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From: bryce@COGSCI.BERKELEY.EDU (Bryce Nesbitt)
Newsgroups: comp.sys.amiga
Subject: Re: Reading Mac disks on the Amiga
Message-ID: <8705150540.AA04612@cogsci.berkeley.edu>
Date: Fri, 15-May-87 01:40:03 EDT
Article-I.D.: cogsci.8705150540.AA04612
Posted: Fri May 15 01:40:03 1987
Date-Received: Sat, 16-May-87 14:08:44 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 43
Summary: The secret may have already been discovered.

[Re: Reading Mac disks on the Amiga]

Sorry to bring this net-worn topic up, but it appears that someone has done
it.  Well, almost done it:

A product called "disk-2-disk" claims to read and ->write<- Commodore
4040/1541/1571 disks on the Amiga using the A1020 5 1/4 drive.	The
Commodore 5 1/4 inch disks are are similar to Mac disks in many ways:

1> Both use GCR encoding.  The methods differ only in the details of the
   implementation.

2> Both use variable density.	The Commodore format takes advantage of the
   extra area of the outer tracks by increasing the bit-rate
   electronically.  The Mac does the same thing by slowing the motor on the
   outer tracks.

There apparent success may be a result of:

1> Tricky programming.
2> Magic twoggling of Paula. (tsk, tsk.)
3> Magic inherent in the A1020 disk drive.

----------------------------------------

The company making these claims is:
[Central Coast Software, 268 Bowie Drive, Los Osos, CA 93402 805-528-4906]

I have no connection with this company other than I read their add in The
Transactor and said "hey, this may be significant".

-----------------------------------------
More technical comments not needed above:

The Commodore drives have four distinct "zones".  A two bit output register
sets the starting point for a counter and thus can change the countdown by
0,1,2 or 3 cycles.  The output is used by the bit clock.

The Mac speed value is read at intervals out of the sound buffer (!).
Unlike the electronic switching used above, the mechanical nature of this
approach requires waiting for the drive to stabilize at the new speed.
Now out of my depth and working on rumor-> the new Mac+ or Mac SE uses a
constant speed drive and manages the changes electronically.