Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!udel!rochester!kodak!uupsi!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Bizarre Fantasy: RLL Flopies?
Keywords: MFM, RLL, Floppies, Blitter, Clueless, Fantasy, etc.
Message-ID: <1991Apr2.200434.6473@grebyn.com>
Date: 2 Apr 91 20:04:34 GMT
References: <1991Apr2.061633.17980@kessner.denver.co.us>
Organization: Grebyn Timesharing
Lines: 35

In article <1991Apr2.061633.17980@kessner.denver.co.us> chris@kessner.denver.co.us (Chris Hanson [Lord Xenon]) writes:
>  I was scamming the Hardware manual the other day, looking for ways to
>improve that audio quality. But that's not what I'm posting about. I noticed
>that trackdisk.device uses the blitter to encode data into raw MFM codes,
>and back. Now, it's a simple leap of faith to wonder if therefore we
>couldn't recode that to utilize the superior RLL 2,7 encoding scheme used
>by most (NON-IBM!) hard drives. RLL 2,7 typically gives an approximate
>150% storage space increase, presumably by using only 3 bits to encode
>2 input bits, instead of MFM's piggish 4 bits out for every 2 bits in.

This is perfectly possible.  The encoding and decoding would have to be
done by the CPU as RLL 2,7 is a bit too complex.

The problem is that it would not give you any more storage capacity.

The clock vs data bit ratio of RLL 2,7 is the same as that of MFM.  It's
just that the clock rate for RLL *can* *be* increased by 50% without
recording a pair of 1 bits any closer together on the media.  This is
the real, physical limitation that the media imposes.

The Paula chip has a 2us clock period.  That's the maximum.  If you
*could* speed it up to 1.5us, then you could use RLL and record 50% more
on the same media, assuming that magnetic particle granularity doesn't
cause problems, which it might.  But you can't speed up Paula's clock.

>  Am I dreaming? Missing some crucial concept? Clueless? A genius? ;)

I would not be so rude as to call you "clueless" in public here. :-) So
let's leave it at the "crucial concept" point...

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