9 Dec
2009
9 Dec
'09
7:14 a.m.
On 9 Dec 2009 at 1:25, Keith M wrote:
Chuck Guzis wrote:
CSV = What's this one? Is it CONSTANT speed
variation? Comma
separated values keep ruining my google searches (Yes, I've tried
-delimited in the search.) Is this the same as MSV, motor speed
variation?
Continuous Speed Variation. See, for example:
http://www.tecno.cl/archivos/0314SNY003_ing.pdf
On 3.5" direct-drive units, I suspect that some of the ISV is motor
noise, in particular "cogging". On belt-driven systems, ISV can
arise from small eccentricities, belt defects, bearings, etc. in
addition to frictional forces.
Let me play devil's advocate. 8x500kbps would be
4mhz. Which means
that your samples are every 250ns. But the pulses come from the drive
are usually 250-300ns wide. _You could potentially miss one._ Now
the pulse WIDTH is fixed and doesn't matter, so it's not like you need
multiple samples per pulse, but I would just have to wonder about
pulse placement accuracy. (ie when did that pulse really occur, did
it happen at t=500ns, or t=750ns) Seems like a huge difference to me.
Well, pulse width has no particular importance when you're reading a
floppy; it's largely a function of the drive electronics.
You are interested in the read pulse (leading) edges and their
placement, however. The only way to miss a pulse is if two crowd
into the same sampling period. With a 500KHz clock, that's not
possible if your window is 250 usec.
So an FM bit cell using a 500 KHz clock (i.e. 8" FM) is 4 usec and
contains at most two transitions, one per 2 usec "half". At 4 MHz,
each 2 usec "half" would be sampled 8 times, or 16 samples for the
whole bit cell. That seems to be more than adquate.
With MFM and a 500 KHz clock (8" MFM), the bit cell time decreases to
2 usec, but here is at most only one transition in that 2 usec cell,
but the placement lies either at the start of the 2 usec cell or in
the middle of it. So we still have 8 samples per cell, "good enough"
for reading, I suspect.
What do you think? Am I missing something important?
--Chuck