cclist at sydex.com
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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