Disc reader project -- quick status update

Chuck Guzis cclist at sydex.com
Tue Dec 8 23:45:00 CST 2009

On 8 Dec 2009 at 18:13, Eric Smith wrote:
> The data separator has to deal with a lot more than just the variation
> in drive speed.  That variation has long-term and short-term
> components, and it also has to deal with the bit shifting that isn't
> completely avoided by write precompensation.

Indeed, as I pointed out ISV can be much worse than CSV.

Even the lowly WD9216 manages to implement long-- and short-term  
timing compensation in an 8-pin late 70's DIP.  

Bit-crowding effects can vary between media vendors, all else 
remaining the same.

The Catweasel utilities tend to use very simple algorithms for their 
MFM and FM separation (I don't know about their algorithms are for 
other formats).  I would expect that one could do considerably 
better, particularly when it comes to error recovery.  For example, a 
traditional data separator, when it hits a "glitch" in the middle of 
data is more likely than not to interchange clock and data in the 
stream.  A software algorithm can work through the "glitch" and 
reduce data loss to a much lower level.   Similarly, corrupted IDAMs 
usually result in a "Sector not found" type of error on traditional 
gear, where the placement of the IDAM and its data might be inferred 
using a clever software algorithm.

In other words, instead of being "as good as" a regular floppy 
controller, one could be considerably better.

Given that the goal of a floppy reader  is recovery of recorded 
information, what is an adequate sampling rate?  My own guess is that 
after real-world factors have been taken into account, that 8x is 
probably more than safe.  But I would be interested in hearing 
arguments to the contrary.


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