Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!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: <177@thor.UUCP> Date: 16 Sep 90 17:39:02 GMT References: <4304@trantor.harris-atd.com> <1453@gold.GVG.TEK.COM> <4313@trantor.harris-atd.com> Distribution: na Organization: SDRC, Cincinnati Lines: 79 In article <4313@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes: > P.S. I do not yet understand why this stuff is called Frequency Modulation. > Do you? > Is the signal recorded on the medium actually a carrier which has > been FM'ed with these data transitions that everyone speaks of? > Or is the baseband data itself some form of FM. (The MFM baseband > data stream looks a lot like Serial Minimum Shift Keying, or SMSK, > which is a binary FSK scheme that has attracted a lot of interest > in high speed digital modems of late). The recorded signal is the baseband data. The resulting signal is just simple Binary Frequency Shift Keying. In FSK, a 0 bit is represented by one frequency and a 1 bit by another, thus it is a form of frequency modulation. When dealing with digital signals, it is convenient to have the two frequencies be in a 2:1 ratio so that a zero is represented by a number of cycles of one frequency and a one by twice as many cycles of twice that frequency which keeps the durations equal. For standard FM recording, the exact counts are a half cycle for zero and a full cycle for one as shown in the following diagram: ___ _______ ___ ___ _______ ___ | |___| |_______| |___| |___| |___| |_______| | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | MFM modifies this scheme by removing all the transitions that are at bit cell boundaries except between two zero cells: ____ ___________ _______________ |___________| |_______| |___________| | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | So, it's still a frequency modulation, but it's not nearly as easy to see as in the FM case. The best way to understand these recording methods is to shift the time scale and label periods where a transition occurs with "1" and periods without a transition with a "0". This way the FM and MFM signals are labeled as follows: Data: 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | FM: ___ _______ ___ ___ _______ ___ |___| |_______| |___| |___| |___| |_______ Trans: 1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 0 | MF: ___ ___________ _______________ |___________| |_______| |___________ Trans: 1 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | Now we can map the data bits into transitions using simple tables (note that the mapping for MFM depends on the previous data bit as well as the current bit): FM MFM Data|Trans Prev|Cur|Trans ----+----- ----+---+----- 0 | 10 0 | 0 | 10 1 | 11 1 | 0 | 00 X | 1 | 01 From these tables it is easy to see that FM recording is RLL 0,1 since any data will result in transitions with between 0 and 1 zeros between ones. MFM is RLL 1,3 since the resulting transitions have between 1 and 3 zeros in a row. Since we're guaranteed that there is at least one zero between each pair of ones, we can record the signal twice as fast without increasing the number of transitions per unit length. The standard "RLL" recording is RLL 2,7 (I don't have the transition mappings or I'd show them) which means we can record the signal three times as fast as FM (1.5 times as fast as MFM) without increasing the number of transitions per unit length. However, even though the transition density doesn't increase, the location of the transitions becomes more important. For MFM, we need the location twice as accurately as FM and for RLL three times. How accurately the transitions can be recorded and detected depends on both the recording medium and the record and playback electronics. ---- 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 Let's pretend I already feel terrible about it, and that you don't need to rub it in any more. -- Calvin