My question: How do the numbers 0,1,2, and 3 relate to the densities of high, double, and single? There are 4 numbers and only 2 densities.
http://fdutils.linux.lu/Fdutils.html explains:
dtr-code rate for FM rate for MFM ============================================= 0 250kb/s 500kb/s 1 150kb/s 300kb/s 2 125kb/s 250kb/s 3 500kb/s 1000kb/s
"Single Density" is MFM, 125 kb/s. "Double Density" and "quad Density" are MFM, 250 kb/s. "High Density" is MFM at 500kb/s. The 300kb/s rate (number 1) is used on 8" drives and 1.2Mb 5.25" drives, and the 1000kb/s rate (number 3) on 2.8Mb drives.