Re: CBM 8280
Date: 1999-09-05 06:37:06

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

CB>>Another problem with writing to magnetic media is hysteresis.  If you
CB>>create a magnetic field on a disk, it takes slightly more energy to
CB>>reverse the field than it took to create it.  The drive head always uses
CB>>the same field strength, so you always end up with a residual signal under
CB>>the main signal that you last wrote.  If you change the format on a disk,
CB>>you can end up with residual signals between tracks, or in sector gaps
CB>>that can interfere with reading the disk.  This can be a serious problem
CB>>for secret information.  Military security protocols specify that a disk
CB>>containing secret information must be overwriten a certain number of times
CB>>with random data in order to obliterate the residual signal.  Or, you can
CB>>bulk erase it.
If you were to tinker with the circuitry and increase the recording
current to the disk's write head would you then be successful in
formating and using more of those disks that refuse to format but from
their specs should? Has anyone tried it?
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