ImageDisk update

Chuck Guzis cclist at
Tue Apr 10 11:47:01 CDT 2007

On 10 Apr 2007 at 3:44, Jules Richardson wrote:

> If the numbers work out, then I wonder if an improved format would be to have 
> variable-length counts: say three bits of "header", with the first bit 
> recording presence/absence of index pulse (vital, as you say), then two bits 
> which state how many bits make up the count value. That gives four possible 
> lengths of count which could be encoded....

I don't think that would be profitable.  Consider MFM recording--you 
have pulses that can occur at N, 1.5N and 2N, and for address marks 
with missing clocks 2.5N intervals.  Given the right clock selection, 
there's no real need for a lot of precision.   Consider that most 
data separator circuits can tolerate about a 20 (+/- 10) percent 
variation in speed quite easily.   A floppy drive is not a precision 
device, in my experience. 

Part of data recovery is relying on the categorization of recording 
methods.  So, for instance if, on an MFM diskette, you find a pulse 
that occurs less than about 3N/4, you're safe discarding it.  
Similarly, if you find a gap of 4N, you probably should assume a 
pulse is missing.    And this is really where the "gold" is.  You can 
be a lot smarter with software than a "dumb" data separator can be 
when it comes to figuring out how to recover from an error.

> Where are you typically finding such formats - other than the well-known 
> machines that used GCR? Is it more common in some of the factory process 
> control stuff etc. that you look at, rather than home/business systems?

Some older word processors use it and you can find it on some early 
personal/business computers as well as some industrial equiopment 
(e.g. Future Data development systems).


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