1 Dec
2011
1 Dec
'11
2:26 p.m.
On 2011 Dec 1, at 1:23 PM, Eric Smith wrote:
Brent Hilpert wrote:
(In reply to this and your previous message)
I'd differ on that: it is still time-quantized on read even if the
timing isn't quite the same as when it was written. The raw read data
still comes in 'time chunks', able to be directly correlated to the
raw written data. The data may not be quite the way you want it (it
includes clock transitions) and you need a transformation to get it
to the way you want, but that transformation is (or primarily is) a
discrete process or transformation. That it is possible to produce a
data-separator that recovers the data so reliably is exactly the point.
(Perhaps we have different concepts of what 'time-quantised' or
simply 'quantised' means. While there are differences, one might ask
the same question of an async serial signal for a simpler case to
examine.)
In time, it was quantized when it was recorded
and retains that
quantisation when read
(note I said discrete time slots relative to each
other).
No, it doesn't. The drive and media shift the peaks around, such
that they only have an approximate relation to the "discrete time
slots relative to each other" in which they were recorded.
While there is some room for a little looseness
in phrasing when
dealing with (signals from)
a physical medium, it's still a stretch to
call the disk signal
'analog in the time domain', esp.
when the phrase has a firmer meaning in other
applications.
It's not at all a stretch. The flux transitions definitely do NOT
have the same timing as the bits that were written, due to both
magnetic and mechanical effects. The timing is changed enough that
it is actually quite difficult to recover the original timing. If
the flux transitions, and thus the read data pulse leading edges on
the interface, were still time-quantized, this would be trivial.
The fact that there were aftermarket improved data separators sold
for the TRS-80 Model 1 Expansion Interface, because of the
unreliability of the built-in data separator of the FD1771, is one
indication of just how non-time-quantized the read data is. A
great deal of research went into developing data separators that
could restore the time quantization with reasonable reliability;
there are many published papers on this topic, and many patents.