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From: claudio@ethz.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: MFM format and possible improvments
Message-ID: <15@ethz.UUCP>
Date: Mon, 16-Feb-87 17:39:17 EST
Article-I.D.: ethz.15
Posted: Mon Feb 16 17:39:17 1987
Date-Received: Tue, 17-Feb-87 07:16:11 EST
References: <8702050359.AA01021@cory.Berkeley.EDU> <445@ethz.UUCP>
Reply-To: claudio@ethz.UUCP (Claudio Nieder)
Organization: CS Department, ETH Zuerich, Switzerland
Lines: 33

In article <445@ethz.UUCP> claudio@ethz.UUCP (Claudio Nieder) writes:
> ... Although I don't know very much about MFM ...

I spent this weekend learning more about MFM encoding.

>The floppy controller writes to the disk with a speed of 250000 bits/sec
>and the disk turns at 300 rpm which means that a track is 0.2 sec long 
>thus (0.2*250000)=50000 bits/track and no more. So the maximum you can 
>expect to put on a flopy is 6250 bytes/track if you use no encoding but 
>simply put every bit as it is on the disk (which, as you explained, is 
>impossible).

Argh, I was wrong. I was so used to the Apple ][ method of encoding,
where the time between two flux transitions is a multiple of the
minimum time required between two flux transitions (4, 8, 12 us), that I
didn't consider other possibilities. In MFM the time between two flux
transitions is 4, 6 or 8 us, so while the minimum time is 4 us, the time
between two flux transitions may be a multiple of 2 us. Therefor the
Amiga floppy controller must be able to write out the data at 500 kbit/s
and not only at 250 kbit/s as I assumed in my first posting.

I apologize for my misinformation.
-- 
				claudio

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