# Re: Write Signal on Disk II Question

• From: "Michael J. Mahon" <mjmahon@xxxxxxx>
• Date: Mon, 19 Nov 2007 02:01:46 -0800

Alex Freed wrote:
Bryan Parkoff wrote:

I have no idea how narrow or wide between clock bit on depending of rotation speed.

For Example: "_" is zero bit. "1" is 1 bit.

___1____1________1____1____ 280 RPM
_____1______1__________1______1______ 300 RPM
_______1________1____________1________1________ 320 RPM

A little elementary school level math and a calculator can help :)
If the nominal delay between consecutive 1 bits is 4 uS at 300 RPM,
it will be 4 * 300 / 280 =~ 4.29 uS
and 4 * 300 / 320 = 3.75 uS at 320 RPM

So after you detect a 1 bit, see if there is another pulse in the interval from 3.75 to 4.3 uS. So sampling at 2 MHz will not get you
a full resolution. 4 MHz is much better: you can sample at 3.75, 4.0
and 4.25 uS.

Of course, sampling at 2MHz works fine, and tolerates an even greater
range of speed deviations within a nibble, as Sather describes very
nicely. This is important, because speed variations affect both the
writing and the reading drive, which can double the effect if they
are in opposite directions. (In the case of a disk written and read
by the same drive, only short-term speed variations are an issue.)

There is no prize for enforcing tight limits on disk speed. ;-)

The real issue is the tolerance for *two* consecutive zeroes,
since the off-speed timing error is cumulative. In practice, the
limit on consecutive zeroes is set both by read amplifier AGC and
by speed tolerance.

-michael

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