CB>-----Original Message----- CB>From: William Levak <wlevak@cyberspace.org> CB>... 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? - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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