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From: gerardka@hobbes.ism.isc.com (Gerard Kam)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: hard drives
Message-ID: <1991Jun27.021923.13438@ism.isc.com>
Date: 27 Jun 91 02:19:23 GMT
References: <81360@eerie.acsu.Buffalo.EDU>
Sender: usenet@ism.isc.com (Ism Usenet News)
Followup-To: comp.sys.ibm.pc.hardware
Organization: Interactive Systems Corp.
Lines: 35

In article <81360@eerie.acsu.Buffalo.EDU> kaplin@acsu.buffalo.edu (leo kaplin) writes:
>Can somebody tell me what does the RLL and MFM stand for when talking
>about hard drives.
>
>thanx, Leo

	MFM: Modified Frequency Modulation
		a method of storing digital data as a bit stream
		on an analog medium (in disk drives, this is usually
		 a ferric oxide).

	RLL: Run Length Limited
		a digital encoding which increases the length of the
		data, but allows much higher effective bit density.
		Typically used with MFM recording.
		The higher bit density is achieved because the
		recording density is set to cope with worst case
		flux reversals.  By encoding the data to new bit
		patterns that do not generate as many flux reversals,
		the new "worst case" is predictable, and allows
		a higher flux density.
		The encode on write and decode on read is done
		by the disk controller.  The drive must be capable
		of the slightly higher transfer rates.

	Disks with embedded controllers such as SCSI and IDE
	usually are MFM with RLL.  But since they have a digital
	interface, the Host computer does not have to be concerned
	with such details.

	"Bare" drives such as ST506/412 or ESDI require a
	controller of a matching type so that the serial data to/from
	the drive is interpreted properly to data bytes (records).
		
	Gerard