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From: john@hp-pcd.UUCP (john)
Newsgroups: net.micro
Subject: Re: Re: Reading Apple II disks
Message-ID: <6200054@hp-pcd.UUCP>
Date: Tue, 9-Apr-85 12:48:00 EST
Article-I.D.: hp-pcd.6200054
Posted: Tue Apr  9 12:48:00 1985
Date-Received: Mon, 15-Apr-85 05:17:18 EST
References: <1535@decwrl.UUCP>
Organization: Hewlett-Packard - Corvallis, OR
Lines: 24
Nf-ID: #R:decwrl:-153500:hp-pcd:6200054:000:986
Nf-From: hp-pcd!john    Apr 12 09:48:00 1985

<<<

  Normal MFM recording places a flux transition at 4,6 or 8 us spacing         
depending on the data byte. Much of the expense in a MFM system is in the
Data sperator that has to decide whether a pulse that comes in at 5 us
was really a 4 or a 6 us pulse. GCR recording eliminates the patterns that
are difficult to decode so that a GCR decoder is simpler and doesn't need
VCO's or Phase locked loops to do it right.

  GCR recording trades off hardware cost for disc capacity. You can use
less expensive hardware but you only get 6 data bits in every disc byte.
MFM gives you 8 data bits for every disc byte. If you store a lot of data
then MFM is better. If not then GCR is better. GCR is also appealing to the
home market where a lower initial cost is important.

  Today there are single chip MFM controllers that contain everthing needed
to control a disc drive (including Phase locked loop) so the hardware cost
difference is dropping. 




John Eaton
!hplabs!hp-pcd!john