Path: utzoo!attcan!uunet!sdrc!thor!scjones From: scjones@thor.UUCP (Larry Jones) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: What (exactly) are MFM and RLL Modulation Techniques? Message-ID: <178@thor.UUCP> Date: 18 Sep 90 17:49:09 GMT References: <4304@trantor.harris-atd.com> <1453@gold.GVG.TEK.COM> <4320@trantor.harris-atd.com> Distribution: na Organization: SDRC, Cincinnati Lines: 29 In article <4320@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes: > What I have learned is this: When an MFM drive is pressed into > RLL service, the magnetic properties of the platters is probably OK. The > electronics must pass 26/17 = 1.53 times the bit rate on and off the > disk in RLL service compared to MFM. The transition time window is > 2/3 as wide. The drive motor speed control needs to hold the speed more > accurately so the transitions stay in the transition window. > Some MFM drives can hack these tighter requirements of RLL service. > Some can't. Actually there are a whole bunch of things that effect the timings. The platters do have some effect -- any non-uniformities in the coating will cause the recorded bits to jitter out of position. That's why most RLL disks these days have plated media, it's very uniform. As the magnetic domains are squeezed together, they tend to change shape and migrate. Write precompensation is an attempt to predict these changes and compensate for them when recording so that the playback will be more accurate. As the timing tolerance gets tighter, the precompensation needs to be more accurate. Also, the read amplification circuitry is never completely linear, so you can get phase shift and other artifacts that distort the recovered signal. The motor speed control isn't as critical since the absolute speed isn't as important as a constant speed and the mass of the disk tends to prevent any short-term changes. ---- Larry Jones UUCP: uunet!sdrc!thor!scjones SDRC scjones@thor.UUCP 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 Hello, I'm wondering if you sell kegs of dynamite. -- Calvin