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IBM 8" floppy "post compensation"

Al Kossow

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The BASF 6104 manual that was just posted has an interesting page
AFAIK, this is the only other drive besides the Shugart SA850 to have a switchable filter on pin 2 that
you're supposed to enable at cyl 60 when reading because IBM doesn't do write precompensation.

I have seen problems using Imagedisk on MFM IBM disks, I wonder if this is why, and if some IBM
systems used write pre-compensation and some didn't.
 

Chuck(G)

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I'm a little blurry on this point, forgive me.

Do you mean that the drive does early/late pulse adjustment (MFM precompensation) or that it does the old "reduced write current" adjustment seen with FM disks?
 

Al Kossow

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If you look at the Shugart 850 manual, there is an additional filter that can be switched in by asserting pin 2 during READ
That manual is pretty vague about when to do it, but the BASF manual says it should be used when reading IBM MFM floppies
that have NOT had pre-compensation applied.
 

Chuck(G)

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Interesting. Given that floppy writing involves writing at saturation levels, I wonder if there are some artifacts that turn up on those IBM disks.
 

BillGee

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I never understood the exact details, but the bits are much closer together on inner tracks than on outer tracks.
 

Chuck(G)

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Yabut--in MFM, one uses variable delay by looking at the last cell written, the current cell and the cell that will follow and adjusts timing slightly, because writing two flux reversals too close (spatially) together will effectively "push" the last written one a bit backwards (via interference effects), sometimes known as "bit crowding"--so you have to compensate for this while writing.

Here's a sample patent that explains this.

It does mention the use a of a "peaking amplifier" to handle issues on read, which perhaps is what the 850 statement is all about.
 

maxtherabbit

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The Qume 242 manual also has a bit about the switched filter.

Switch Filter
IBM compatibility requires an increase in the read output resolution on tracks
60 through 76.

Low Current / Switch Filter
When WRITE GATE is low, this signal line (pin 2) should be high on tracks 0
through 43, and active low on tracks 44 through 76 (low current). When data
is read, this line should be high on tracks 0 through 59, and active low on
tracks 60 through 76, in order to improve read output resolution on the inner
tracks (switch filter).
 
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