26 Jul
2007
26 Jul
'07
5:55 p.m.
Subject: RE: Read error correction in software
From: "Chuck Guzis" <cclist at sydex.com>
Date: Thu, 26 Jul 2007 12:50:27 -0700
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
On 26 Jul 2007 at 6:40, dwight elvey wrote:
If you implement a software PLL the key function is in the face of a
missing pulse you keep the current clock rate and clock in a zero or
or one as needed to fill the stream. The usual analog PLL tend to have
an idle rate and lacking a error signal it will either retain the current
rate or hunt back to the center rate. Since in a corrstly set PLL
the error rate and the centered rate are not widely seperated
drift is rarely a factor short term.
Allison
Your right, not too good on multiple errors.
Actually, on MFM, not even that good for single errors. Recall that
the CRC on a 256-byte sector is the 257th and 258th byte of the
sector. So, if an MFM error throws the data stream off, you don't
even get a valid CRC to work with. And MFM data errors can result in
data shifting and clock data "swapping". It might have been a whole
'nother story had the convention been that the CRC precedes the data.
I've got some ideas about simulating a PLL-type clock in my routines
developing bit cell "windows" instead of relying on the pulse-to-
pulse spacing. The latter, while being very adaptable to ISV-type
errors, is lousy for recovery of regular data errors.