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This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later. |
The contents of the Delay encoding page were merged into Modified frequency modulation on 5 February 2018. For the contribution history and old versions of the redirected page, please see its history; for the discussion at that location, see its talk page. |
The article says "MFM allows more than one symbol per flux transition — up to three — giving greater density of data than FM." I think this is nonsense. FM encodes a one as a 11 and a zero as 10. MFM encoding is as shown. They have the same density, given the same clocking; each takes two "possible transition" periods to encode a bit. The reason MFM achieves greater data density is it can be clocked at twice the rate (because there are never two transitions in a row). Or am I totally off-base here? See http://www.storagereview.com/guide2000/ref/hdd/geom/dataMFM.html
Nybbler 23:29, 5 March 2006 (UTC)
Nybbler 15:28, 6 March 2006 (UTC)
It would be nice if the question marks in the examples would be explained somewhere. -- 89.49.249.90 22:13, 17 August 2007 (UTC)
Can someone double check the illustrations?, when I study them in detail. They just don't seem to make sense! Esp, a "clock" signal of 11000000 seems weird. It ought to be 10101010 ? Electron9 ( talk) 17:22, 5 May 2009 (UTC)
Article begins with:
This is incomplete and/or incorrect in at least two respects:
PC's were able to format DS/DD 3½-inch or DS/QD 5¼-inch Floppies, usually marketed as 720kiB (formated 512 bytes per sector, 2 sides, 9 sectors per track, 80 tracks) with 800 KB or even more, with additional drivers (i.e. 800.com or 900.com). I do not know if it was possible to change modulation with these programs, but you could increase number of tracks and sectors per track and gain more place on disk. To read such disk there had to be one of the drivers loaded, which enabled to format it beforehand. From DS/DD 5¼-inch Floppies (Marketed at 360 kiB), it was possible to get maximum 430 kiB, but 410 kib was more reliable and more supported variant, as we found out later). 195.14.165.61 ( talk) 07:12, 25 August 2016 (UTC)
MFM was just an early system and was used for the devices built at that time, such as floppy disk drives and early hard disk drive MFM controllers, such as
WD1002. While hard disk drives, such as
Seagate ST-225 were intended for MFM, some users connected them to newer
RLL controllers to gain additional 50% of capacity. One example I was able to find at
Hard Disk Hardware Explained, Part II
I vaguely remember that bit-order on floppies (may ST-506 HDD, too) was LSB first. If anyone knows that for sure, it would be nice to put it here for reference. —Preceding unsigned comment added by 91.187.225.16 ( talk) 08:08, 31 October 2010 (UTC)
It's not obvious to me whether MFM applies to the analog or digital world. "Pure" FM is clearly an analogy phenomenon, whereas the article discusses streams of bit symbols, which is clearly digital in nature. So where does that leave MFM? Is it an analog technique that happens to have digital applications, or is it a strictly digital technique? — Preceding unsigned comment added by 70.247.162.60 ( talk) 04:18, 18 November 2015 (UTC)
The lead says:
MFM is a modification to the original digital FM (digital frequency modulation also known as delay coding)
And now since the merge, delay encoding is discussed. This delay encoding is MFM, right? So is somehow the difference between FM and MFM the difference between delay coding and delay encoding? I get the feeling delay coding is a different way of saying delay encoding, and the lead is simply wrong: FM is Differential Manchester encoding and MFM is delay coding is delay encoding. If I'm right, we should change the lead to
MFM is a modification to the original digital FM (digital frequency modulation also known as differential Manchester encoding)
It also couldn't hurt to be more explicit that delay encoding is actualy MFM; the current merger doesn't make this obvious to the reader. Digital Brains ( talk) 13:47, 5 February 2018 (UTC)
@ Myrtonos: has attempted to add fragments of recording channel design to this article which I have reverted both on the basis that they were incomplete and inappropriate to this article. The lede accurately states that the "the minimum spacing between flux transitions that is a property of the disk, head and channel design, ..." An article on recording theory might note that the isolated pulse is a function of gap length, flying height and media thickness. The isolated pulse shape along with signal to noise (including off track signal) and data separator stability all go into the code chosen which has a minimum and maximum number code zeros between code ones. This is all summarized in the lede and adding parts of this complex subject into the body is inappropriate. I note that none of this appears in the Run-length limited article which is the parent category to this article on MFM. It doesn't even appear in Magnetic recording. WikiPedia would be improved by a magnetic recording theory section possibly in Magnetic storage#Design which could be linked from this article put the partial details that Myrtonos has added simply don't belong here. TMI for this article! Tom94022 ( talk) 17:39, 26 May 2018 (UTC)
I'm investigating my old floppy disks right now, and I totally do not see a "missing sync bit" in the A1 sector marker this (and some other) article are talking about. Help? (top track, signal from the disk; bottom track, meander manually added for reference that's supposed to represent the real sync signal)-- Wesha ( talk) 22:38, 22 April 2020 (UTC)
HL LL LH HH LL LL HH HL
(or the reverse) which according to MFM rules means 1 0 1 0 0 0 0 1
but the catch is the LL LL
part, as the signal was supposed to flip there (LL HH
). --
Wesha (
talk)
17:44, 23 April 2020 (UTC)Tom94022 insists that the precursor to MFM cannot be called Digital frequency modulation. But I don't think what's described in Frequency modulation is right. How do we make this disambiguation clear to readers? Would creating a Frequency modulation (digital) redirect help? ~ Kvng ( talk) 14:47, 16 January 2021 (UTC)
I think the problem we have is the term frequency modulation has two different meanings, a rather specific meaning in magnetic storage and a more general meaning in communications and/or information theory. They really were two distinct arts until the late 1970s when the magnetic folks realized they had a communications channel and started to treat the recording playback process as one. Modified frequency modulation seems to be limited to magnetic storage and since that is the subject of this article perhaps we should limit the discussion in this article to the magnetic storage meaning. I tried to edit the article in that direction. Perhaps a hatnote pointing this out in the Frequency modulation section with a link to the more general Frequency modulation section. Tom94022 ( talk) 01:18, 26 January 2021 (UTC)
This article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | |||||||||||||||||||||
|
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later. |
The contents of the Delay encoding page were merged into Modified frequency modulation on 5 February 2018. For the contribution history and old versions of the redirected page, please see its history; for the discussion at that location, see its talk page. |
The article says "MFM allows more than one symbol per flux transition — up to three — giving greater density of data than FM." I think this is nonsense. FM encodes a one as a 11 and a zero as 10. MFM encoding is as shown. They have the same density, given the same clocking; each takes two "possible transition" periods to encode a bit. The reason MFM achieves greater data density is it can be clocked at twice the rate (because there are never two transitions in a row). Or am I totally off-base here? See http://www.storagereview.com/guide2000/ref/hdd/geom/dataMFM.html
Nybbler 23:29, 5 March 2006 (UTC)
Nybbler 15:28, 6 March 2006 (UTC)
It would be nice if the question marks in the examples would be explained somewhere. -- 89.49.249.90 22:13, 17 August 2007 (UTC)
Can someone double check the illustrations?, when I study them in detail. They just don't seem to make sense! Esp, a "clock" signal of 11000000 seems weird. It ought to be 10101010 ? Electron9 ( talk) 17:22, 5 May 2009 (UTC)
Article begins with:
This is incomplete and/or incorrect in at least two respects:
PC's were able to format DS/DD 3½-inch or DS/QD 5¼-inch Floppies, usually marketed as 720kiB (formated 512 bytes per sector, 2 sides, 9 sectors per track, 80 tracks) with 800 KB or even more, with additional drivers (i.e. 800.com or 900.com). I do not know if it was possible to change modulation with these programs, but you could increase number of tracks and sectors per track and gain more place on disk. To read such disk there had to be one of the drivers loaded, which enabled to format it beforehand. From DS/DD 5¼-inch Floppies (Marketed at 360 kiB), it was possible to get maximum 430 kiB, but 410 kib was more reliable and more supported variant, as we found out later). 195.14.165.61 ( talk) 07:12, 25 August 2016 (UTC)
MFM was just an early system and was used for the devices built at that time, such as floppy disk drives and early hard disk drive MFM controllers, such as
WD1002. While hard disk drives, such as
Seagate ST-225 were intended for MFM, some users connected them to newer
RLL controllers to gain additional 50% of capacity. One example I was able to find at
Hard Disk Hardware Explained, Part II
I vaguely remember that bit-order on floppies (may ST-506 HDD, too) was LSB first. If anyone knows that for sure, it would be nice to put it here for reference. —Preceding unsigned comment added by 91.187.225.16 ( talk) 08:08, 31 October 2010 (UTC)
It's not obvious to me whether MFM applies to the analog or digital world. "Pure" FM is clearly an analogy phenomenon, whereas the article discusses streams of bit symbols, which is clearly digital in nature. So where does that leave MFM? Is it an analog technique that happens to have digital applications, or is it a strictly digital technique? — Preceding unsigned comment added by 70.247.162.60 ( talk) 04:18, 18 November 2015 (UTC)
The lead says:
MFM is a modification to the original digital FM (digital frequency modulation also known as delay coding)
And now since the merge, delay encoding is discussed. This delay encoding is MFM, right? So is somehow the difference between FM and MFM the difference between delay coding and delay encoding? I get the feeling delay coding is a different way of saying delay encoding, and the lead is simply wrong: FM is Differential Manchester encoding and MFM is delay coding is delay encoding. If I'm right, we should change the lead to
MFM is a modification to the original digital FM (digital frequency modulation also known as differential Manchester encoding)
It also couldn't hurt to be more explicit that delay encoding is actualy MFM; the current merger doesn't make this obvious to the reader. Digital Brains ( talk) 13:47, 5 February 2018 (UTC)
@ Myrtonos: has attempted to add fragments of recording channel design to this article which I have reverted both on the basis that they were incomplete and inappropriate to this article. The lede accurately states that the "the minimum spacing between flux transitions that is a property of the disk, head and channel design, ..." An article on recording theory might note that the isolated pulse is a function of gap length, flying height and media thickness. The isolated pulse shape along with signal to noise (including off track signal) and data separator stability all go into the code chosen which has a minimum and maximum number code zeros between code ones. This is all summarized in the lede and adding parts of this complex subject into the body is inappropriate. I note that none of this appears in the Run-length limited article which is the parent category to this article on MFM. It doesn't even appear in Magnetic recording. WikiPedia would be improved by a magnetic recording theory section possibly in Magnetic storage#Design which could be linked from this article put the partial details that Myrtonos has added simply don't belong here. TMI for this article! Tom94022 ( talk) 17:39, 26 May 2018 (UTC)
I'm investigating my old floppy disks right now, and I totally do not see a "missing sync bit" in the A1 sector marker this (and some other) article are talking about. Help? (top track, signal from the disk; bottom track, meander manually added for reference that's supposed to represent the real sync signal)-- Wesha ( talk) 22:38, 22 April 2020 (UTC)
HL LL LH HH LL LL HH HL
(or the reverse) which according to MFM rules means 1 0 1 0 0 0 0 1
but the catch is the LL LL
part, as the signal was supposed to flip there (LL HH
). --
Wesha (
talk)
17:44, 23 April 2020 (UTC)Tom94022 insists that the precursor to MFM cannot be called Digital frequency modulation. But I don't think what's described in Frequency modulation is right. How do we make this disambiguation clear to readers? Would creating a Frequency modulation (digital) redirect help? ~ Kvng ( talk) 14:47, 16 January 2021 (UTC)
I think the problem we have is the term frequency modulation has two different meanings, a rather specific meaning in magnetic storage and a more general meaning in communications and/or information theory. They really were two distinct arts until the late 1970s when the magnetic folks realized they had a communications channel and started to treat the recording playback process as one. Modified frequency modulation seems to be limited to magnetic storage and since that is the subject of this article perhaps we should limit the discussion in this article to the magnetic storage meaning. I tried to edit the article in that direction. Perhaps a hatnote pointing this out in the Frequency modulation section with a link to the more general Frequency modulation section. Tom94022 ( talk) 01:18, 26 January 2021 (UTC)