Media Longitevity/Care

[Guy Sotomayor]

On Wed, 2005-03-16 at 16:01 -0800, Eric Smith wrote:
> Guy wrote:
> > Actually you want to find patterns (and they do exist) that are
> > pathological (ie worst case) for the encoder/decoder *and* for the error
> > correction being used.
> 
> Basically by the time the data has been through three layers of ECC with
> interleave, the scrambler, and the EFM modulation, there's no way to
> guarantee that your pathological pattern will actually be pathological,
> because you don't know the initial phase relationship of any of the
> stages.  In a carefully controlled lab environment with support from
> the design engineers you could do it, but as an end user of a black
> box (commercial CD/DVD writer) you can't, even armed with copies of the
> revelant standards, because you don't have enough visibility into
> the black box.

Sure you do.  You know every transformation will be performed on the
data to get it onto the media.  And in the case of CDROMs I can say with
certainty that the state of all of the stages is explicitly known.  The
derivation of the pathological pattern is based upon the transformations
that will be performed on it to get it onto the media.  It doesn't
require *any* lab environment (just the right software and suitably
compliant hardware...ie a CDwriter that allows "raw" writes).

I've done this work.  It's not easy and the math (finite fields) will
blow your mind but it is certainly possible (obviously, we have an
existence proof...the CDROMs themselves).

> 
> By comparison, coming up with the pathological test cases for FM, MFM,
> M2FM, RLL (1,7) and (2,7), etc. is a piece of cake.

Agreed, mainly because the error correction in CDROMs is very strong
(mainly because the raw error rate is relatively high).  However, it
still amazes me that CDROMs work at all!  A 1X CDROM has 6Mb/s at the
optical pickup (and don't start that it's 150KB/s or 170KB/s because
both of those numbers are wrong)!

> 
> For instance, one of the worst-case test patterns for MFM is the
> repeating byte sequence 6D, B6, DB (hexadecimal), which generates
> alternating short and long intervals between flux transitions with no
> medium length intervals.  Though most test programs just repeated the
> first two bytes.

Yes, simple interfaces with little error correction have simple
pathological patterns.

-- 

TTFN - Guy