Xref: utzoo comp.sys.ibm.pc.hardware:7612 comp.multimedia:344 comp.misc:12098 Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!masscomp!peora!tarpit!bilver!bill From: bill@bilver.uucp (Bill Vermillion) Newsgroups: comp.sys.ibm.pc.hardware,comp.multimedia,comp.misc Subject: Re: Diffs between DD & HD disks Message-ID: <1991Apr12.181737.13482@bilver.uucp> Date: 12 Apr 91 18:17:37 GMT References: <571@lysator.liu.se> <1991Apr9.042503.18670@bilver.uucp> <1991Apr9.065832.16253@cs.ucla.edu> Organization: W. J. Vermillion - Winter Park, FL Lines: 89 In article <1991Apr9.065832.16253@cs.ucla.edu> chao@oahu.cs.ucla.edu (Chia-Chi Chao) writes: >In article <1991Apr9.042503.18670@bilver.uucp> bill@bilver.uucp (Bill Vermillion) writes: >>Typically the DD's are about 600 oersted coercivity while the HDs >>are in the 700 oersted range. >>You can get by with this (sometimes) better than those who tried to >>use 360's at 1.2 meg in the 5.25" world. Those are about 300 >>oersteds for the DD's and 600 for the HD's. As a matter of fact >>the 5.25" HD media is just about identical to the 3.5" DD media. >I totally agree with these points. 3.5" DD disks can be formatted to HD most >of the time because the coercivities between DD and HD are much smaller than >those between 5.25" DD and HD. Look at the MEI/Micro catalog. They have these >numbers listed. >The following was obtained from one of the newsgroups a few years(?) ago. That >is the most technical explanation I have seen. >============================================================ >>From: tim@j.cc.purdue.edu (Timothy Lange) >Organization: PC Learning Resource Center, Purdue University >I know that HD drives can deal with DD floppies by changing the recording >frequency to match the oxide used on the media and the track to track ^^^^^^^^^^ >distances. It is not just the frequency of the data transfer that changes. The write current is increased to be able to record on the HD media. The coercivity is the key. The higher the coercivity the stronger the write current needed. (Think of the root of the word coerce, sometimes used to mean force). It takes more "persuasion" to "force" the data onto an HD disk. The problems with trying to write on a media not supported are two-fold. Trying to write 360k type data onto and HD disk results in too little current being applied and the disk can not be "coerced" into accepting the data. The opposite happens when trying to write HD (1.2 megs) onto a standard disk. Too much current is being applied. The standard media has a lower corecivity and the higher write current causes self-erasure. This is because the higher current generates a higher magnetic field. These are not finite fields, but decrease with distance. A bit will be written to a disk, the disk rotates enought to place the next bit into position. The higher current on the HD write extends the magenetic field far enough to partially (or wholly) erase the previously written bit. You can typically get the first 20 to 30 tracks on a std media written with an HD format, but as the writing circumference becomes smaller as the head moves closer to the center of the disk, the bits are physically spaced closer together and self-erasure starts. Disks are designed to be reliable at the smallest circumference. Another thing to note with 3.5" disks writing at 135 tpi is that the track length at the outer track is not as drastically different from the inner track as it is in the 5" media. On an older format, 8" floppies, which were written at 48 tpi (same as the 5" disks), the 77 tracks were quite long. The shortest track on those was about the length of the longest on the 5" disk. The problems with putting more information on the 8" disks, with such things as more tracks per inch, was that the media was so large, that centrifugal force came into play and caused the disk to stretch outward slightly causing track mis-alginment. The only people who successfully got around this (to my knowledge) were the folks at Bernouli, who put an embedded servo on their disks to get 10 megs on 8" inch media. Embedding a servo on a data track takes up valuable space. In the 3.5 inch world Brier is recording the servo track BENEATH the data track to get 25 MEGS on a 3.5 floppy. They can do this because on of the properties of magnetic media is that the shorter the wavelength the closer to the surface of the media the signal is written. By using a lower frequency for the servo track, the higher frequency data track can be written in the same place and both can read. Got a bit off the track, but I think it was pertinent. bill -- Bill Vermillion - UUCP: uunet!tarpit!bilver!bill : bill@bilver.UUCP