Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!sdrc!thor!scjones From: scjones@thor.UUCP (Larry Jones) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: Perstor controllers -- info from the horse's mouth Summary: What the horse didn't tell you ;-) Message-ID: <138@thor.UUCP> Date: 25 Aug 90 15:16:01 GMT References: <38331@shemp.CS.UCLA.EDU> Organization: SDRC, Cincinnati Lines: 42 In article <38331@shemp.CS.UCLA.EDU>, wales@valeria.cs.ucla.edu (Rich Wales) writes: > I asked how it was that Perstor could get reliable performance at such > high data densities, even on non-RLL drives. Their answer is that their > ADRT technique encodes the data more compactly, but still uses the same > flux density as MFM. The disk is =not= stressed beyond MFM specs; their > method works even on drives that won't take RLL at all. ADRT is =not= a > "compression" technique, so it should work no matter what a given sector > contains. I was, unfortunately, unable to get any more details on the > ADRT method (it's proprietary) -- but their sales brochure says it's > based on the "IBM 3370 DK" technique. ADRT is just Perstor's name for a particular version of RLL encoding. An RLL encoding simply maps a number of bits into a larger number of bits and guarantees that strings of repeating zeros and ones in the result will be between some minimum and maximum value. If the minimum is greater than 1, you can record the resulting bit string faster without having the flux changes be any closer together than normal. So, by picking your encoding carefully, you can record more bits in the same amount of space without increasing the flux density. The tricky part is that you can't let the maximum get too big, or you can't keep your clock synchronized and you start to loose track of exactly were the bits are. Also, as the minimum gets larger and you record faster, you need to know the location of the flux changes a lot more accurately to know exactly which bit it corresponds to. The traditional RLL encoding makes the tradeoff such that the maximum flux density is somewhat higher than for MFM and the timing of the flux changes is also somewhat higher. Perstor's encoding makes a different tradeoff that keeps the maximum flux density to be just a teeny tiny bit higher than MFM, but the timing is much more critical. This turns out to be a win for Perstor since many more non-RLL rated disks are capable of meeting their timing requirements than are capable of handling traditional RLL's higher flux density. ---- Larry Jones UUCP: uunet!sdrc!thor!scjones SDRC scjones@thor.UUCP 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 Girls are like slugs -- they probably serve some purpose, but it's hard to imagine what. -- Calvin