Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!uunet!cbmvax!cbmehq!cbmger!peterk
From: peterk@cbmger.UUCP (Peter Kittel GERMANY)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: GCR encoding on amiga
Keywords: What's GCR encoding ??
Message-ID: <429@cbmger.UUCP>
Date: 20 Sep 90 07:51:12 GMT
References: <1411@dnlunx.pttrnl.nl>
Reply-To: peterk@cbmger.UUCP (Peter Kittel GERMANY)
Distribution: comp
Organization: Commodore Bueromaschinen GmbH, West Germany
Lines: 31

In article <1411@dnlunx.pttrnl.nl> martijn@dnlunx.pttrnl.nl (Reinalda M.) writes:
>I was reading Amiga Intern about the disk controller. It said that is
>capable of doing MFM or GCR coding.
>I know about MFM, but I've never heard of GCR encoding.     

You know when you want to record to a magnetic media you must make sure
that there occur enough magnetization changes per second, even when the
data to be recorded are constant. One way to solve this is to treat
every bit and thus produce an overhead of nearly the double data size.
One way to optimize this a bit is GCR, or Group Code Recording. There
you take your data and cut it into 4-bit pieces (that seems to be the
referred "group") and translate them into 5-bit groups. In these 5-bit
groups you are indeed able to make sure that no more than 3 identical
bits follow in queue for any possible 4-bit input. So this method is
a bit more efficient than MFM.in that it has only 25 % overhead.
But it takes extra hardware (ROM in the old CBM floppies) or software
(slow, like in C64 devices) to decode this.

Provision of GCR on the Amiga is for historical reasons. It was used
in the old CBM floppies from 2020 to 8250 (did you know that Commodore
offered already 1983 floppies with 1 MB capacity each on simple DD disks?)
and on all the C64 compatible floppies. When I'm told correctly, also
Apple used GCR, perhaps in a slightly different way.

In the Amiga, there is one caveat with GCR. For hardware reasons it
must be cut in speed by half and is thus less effective than MFM on
the Amiga drives. So to my knowledge NOBODY uses it on the Amiga, sad.

-- 
Best regards, Dr. Peter Kittel  // E-Mail to  \\  Only my personal opinions... 
Commodore Frankfurt, Germany  \X/ {uunet|pyramid|rutgers}!cbmvax!cbmger!peterk