Re: Write Signal on Disk II Question

[vladitx <vladitx@nucleusys.com>]

On Nov 20, 11:42 pm, ferd...@gmx.de wrote:
> The Disk ][ has a pot to turn down the sensitivity by kicking the
> MC3470 slightly out of correct bias.  One can even turn down the
> sensitivity that far that there will be NO noise at all on a blank
> track.

I have a clone with no schematics and no oscilloscope handy to do any
tweaking. Clone drive is not a 1:1 replica (although differences could
be minor).
Besides, if this is to be used by other people it shouldn't require
special tuning of the drives. The only thing serviceable by the user
should be drive speed and there are Apple2 programs which help that.

> This pot is needed to allow for 2 zeroes to be read at all. If it is
> adjusted to maximum gain the drive won't read two zeroes reliably.

Wasn't there another pot to adjust the "dead" period after a pulse?
The one that required tuning after 13 -> 16 sector conversion was
done.

> I noticed about 5% jitter on the track 0 on a freshly inited disk and
> I used clone direct-drive drives.

There are direct-drive drives? Isn't jitter from motor regulation at
so low RPM quite bad?

> If there is data on the disk, the different patterns all cause much
> more jitter. My program is able to read a freshly inited disk without
> errors. The problem only arises when there's data on the disk.
>
> At the moment I'm thinking of a real analog PLL to recover the
> original clock and use it to clock a shift register.
> BTW, a system that just syncs to the data stream is NOT a PLL. A PLL
> would be able to adjust the clock (i.e. when reading a disk which was
> written on a 320rpm drive on a 280 rpm drive it would sample at
> 220kHz). The Disk ][ controller doesn't do that (no floppy disk
> controller does!), and it's not really needed. A real PLL just
> provides more margin which might be useful in tricky situations (like
> copying copy-protected disks).

I think you have to rethink your strategy. There is no "original
clock" used to write given track. Consider the jitter in spindle motor
revolution, jitter from belt (or any other mechanical transmission),
jitter on Q3 clock because of elongation, jitter from sectors written
at different times and on different drives (speeds). Add also thermal
variations.

Although you can sample at 2 MHz you still need the equivalent of
Woz's sequencer that does the clock recovery and tolerates the various
jitters.