Re: Write Signal on Disk II Question

[vladitx <vladitx@nucleusys.com>]

On Nov 19, 12:01 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:

> 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.)

This all thanks to the clock recovery in the sequencer (I really
admire the design).

Just for the record, I notice a good mechanical jitter in successive
revolutions both due to the spindle motor regulator and the rubber
belt. Sampling at 2 MHz is okay as long as the writing drive was not
too slow and the reading too fast, because the two consecutive zeroes
become a problem then.

Anyone remember the drive speed adjustment settings of Locksmith
6.0? ;-) There was "300 RPM" and "optimized", the latter being
somewhat slower to give better reading chance.

> 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.

One very interesting thing I noticed is that on a blank track I could
very few pulses from the MC3470, something like few hundred per
revolution. Seems like the AGC is not that picky and doesn't generate
much noise, at least on my drive.

Other interesting observation is that such non-magnetized portions of
medium make the shaper produce serrations much contrary to my
expectation that a 1usec pulse will have at least 1-2 usec pause
following.