Re: CBM 8280

From: Radioactive Warrior (
Date: 1999-09-04 13:00:07

-----Original Message-----
From: William Levak <>
>Commodore's GCR encoding was used because the timing circuits on drives at
>that time were not as reliable as they are today.  Signals are written to
>the disk using a NRZ (Non Return to Zero) encoding scheme.  If the signal
>changes, that is a one bit.  If it doesn't change, that's a zero bit.
>With that scheme, you have to know exactly when to look for a signal
>change.  If you have a long string of zeros (no signal change), a slight
>error in timing may cause it to be interpreted as too many or to few
>zeros.  The GCR scheme converts 4 bit (half byte) data into 5 bit data.
>The valid 5 bit data representations do not include any that can cause
>more tha 2 zeroes in a row.  This more reliable method allows disks to be
>used that may not work in any other machine.

>Another problem with writing to magnetic media is hysteresis.  If you
>create a magnetic field on a disk, it takes slightly more energy to
>reverse the field than it took to create it.  The drive head always uses
>the same field strength, so you always end up with a residual signal under
>the main signal that you last wrote.  If you change the format on a disk,
>you can end up with residual signals between tracks, or in sector gaps
>that can interfere with reading the disk.  This can be a serious problem
>for secret information.  Military security protocols specify that a disk
>containing secret information must be overwriten a certain number of times
>with random data in order to obliterate the residual signal.  Or, you can
>bulk erase it.

How strong a feild do those home bulk-erasers generate?  I don't own one
but I have seen my friend use his to blank audio tape.  How long does it
take for the field orient all the media particles (10 sec? 100 sec?)
Does it help at all to rotate the disk around the field many times to
'exercise' the particles or dosen't it matter?

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