The "cobalt" formulation for the 3.5 DD disk is chemically different than the formulation for the 3.5 HD disk. The formulation for the 5.25 DD and QD disks are chemically the same. At one time, they were different, but it is expensive to have two seperate production lines, so the higher quality formulation is used for both. The TPI and BPI are minimum values, higher values are allowed. Cheaper disks (especially no name disks) often skimp on the coating thickness and the testing. They get away with it most of the time and replaced the disks on the few percent that fail. Most people never notice the difference, unless there was important data on the disk that failed. On Wed, 1 Sep 1999, peter karlsson wrote: > > The question often occurs: What is the difference between the various > > densities since the coating seems to be the same for SD, DD and QD. > > [Scott Mueller, Upgrading and repairing PCs, eighth edition, p.554] > > 5 1/4-Inch 3 1/2-Inch > -----------------------------------------------+----------------------------- > Double Quad High | Double High Extra High > Density Density Density | Density Density Density > Media Parameters (DD) (QD) (HD) | (DD) (HD) (ED) > -----------------------------------------------+----------------------------- > Tracks Per Inch (TPI) 48 96 96 | 135 135 135 > Bits Per Inch (BPI) 5,876 5,876 9,646 | 8,717 17,434 34,868 > Media Formulation Ferrite Ferrite Cobalt | Cobalt Cobalt Barium > Coercivity (Oersteds) 300 300 600 | 600 720 750 > Thickness (Micro-In.) 100 100 50 | 70 40 100 > Recording Polarity Horiz. Horiz. Horiz. | Horiz. Horiz. Vert. > -----------------------------------------------+----------------------------- > > "Disks have two types of densities: logitudinal density and linear density. > Longitudinal density is indicated by how many tracks can be recorded on the > disk, often expressed as a number of tracks per inch (TPI). Linear density > is the capability of an individual track to store data, often indicated as a > number of bits per inch (BPI). Unfortunately, both types of densities often > are interchanged incorrectly in discussing different disks and drives. > (...) > Both the quad- and DD disks store the same linear data on each track. They > use the same formula for the magnetic coating on the disk, but the > quad-density versions represent a more rigorously tested, higher-quality > disks. The HD disks are entirely different, however." > > -- > \\// > peter - http://www.softwolves.pp.se/ > - and God said: nohup make World >& World.log & > > - > This message was sent through the cbm-hackers mailing list. > To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi. > - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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