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Old 06-11-2004, 04:15 AM   #1 (permalink)
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Default data encoding in hard disks

Hi
I was going through the basics of hard disks and i came across data encoding
it says that a pulse has detected by the orientation N-S followed by S-N
whereas the absence of a pulse is detected by the orientation N-S followed by N-S,

i was under the impression that the absence of a pulse would be read by no orientation.

i further read that RLL and MFM have 00 coded as pulse-absence-absence-absence followed by 4 blanks

and MFM by
pulse-absence-pulse-absence followed by four blanks.

i guess they must have some sort of mapping, right?
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Old 06-11-2004, 04:22 AM   #2 (permalink)
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Wow, this 1 is professional question, sorry that i cant answer u.
Maybe u should ask the engineer working at the HDD manufacturer.
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Old 06-11-2004, 06:27 AM   #3 (permalink)
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I'm not quite sure what your asking so this answer might not be what your looking for...

The data encoding is handled by the hard drive controller in the hard drive itself. It will be set up to read whatever protocol, MFM or RLL or PRML etc, they write. Otherwise the drive would be completely useless. Therefore there would be no need for any mapping in the controller as it will only use 1 protocol.

All data read and write requests to the hard disk are sent via the appropriate bus. IDE or ATA etc. These requests are read and interpeted by the hard disk controller, which then performs the necessary actions with the drive mechanism to either write the incoming data or read and output the read data. This means that the CPU, and hence the OS, never needs to know what protocol is used on the hard disk platters and just needs to know how to talk to the appropriate data bus.
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Old 06-11-2004, 06:31 AM   #4 (permalink)
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i was asking about the magnetic orientation on the hard disk itself

the N-S orientation,

would the absence of a mgnetic field be read as a 0 and the presence of the magnetic field be read as a 1.

i thought the presence would be an orientation should as North-South, and the absence would be an oriented section, is that right?
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Old 06-29-2004, 10:10 AM   #5 (permalink)
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It's not as simple as that. In most cases it is a sequence of different orientations that signals a 1 or a 0.

otherwise it would be hard to tell whether or not you had just read 1 zero or ten zeros!!
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