While that may be true in theory, I'm going to guesstimate that 99 and 44/100 percent of the applications of the ST506/412 out there used either MFM, MMFM (maybe) or 2,7 RLL for recording. While anyone could throw together a bunch of TTL chips to record any old way on a floppy (and did), implementing a hard disk controller with random logic wasn't a simple matter and almost all manufacturers, by the time of the ST506 used commodity solutions. That's not to say that there aren't any bizarre recording schemes out there for ST506s; just that I've never run into one. For example, I've got a few mutually incompatible 2,7 RLL PC-AT style controllers, but they differ more in logical details (address marks, header layout, ECC codes) than they do in modulation methods or clock rate. While it may have been possible to, say, employ FM recording on a ST412, I've never seen it done in practice. There *were* some RLL variants that attempted to push things past 2,7 (Perstor?) on a 412 interface, but they were never very reliable or popular. Did any application of a 506 even use zoned recording (i.e. using a faster data clock on the outer cylinders)? Cheers, Chuck