Re: GCR Encoding

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

sean weintz <strap@hanh-ct.org> writes:
> Ah... so, now we know about FM, MFM and GCR. Off topic, but anyone
> know where to find good info about RLL? (since most modern non-floppy
> disks use a variant of that)

RLL is a type of GCR.  Back when PCs used RLL on drives with the
ST-506/ST-412 interface, they generally used either (1,7) RLL or
(2,7) RLL.  If I recall correctly, the numbers refer to the minimum
and maximum allowed sequences of zero (non-flux-transition) channel
bits.  Apple 13-sector GCR is a (0,1) RLL code, and Apple 16-sector GCR
is a (0,2) RLL code.

The best info I've seen on the common RLL codes was in National
Semiconductor data sheets and application notes for their disk
controller and data separator chips.  They published a mass storage
data book in the late 1980s.  Some of the application notes may
still be on their web site, though the specific chips they covered
are long since obsolete.