Path: utzoo!attcan!uunet!mcsun!ukc!dcl-cs!aber-cs!odin!pcg From: pcg@cs.aber.ac.uk (Piercarlo Grandi) Newsgroups: comp.arch Subject: Re: Data Storage density questions Message-ID: Date: 2 Aug 90 14:55:15 GMT References: <2635@mindlink.UUCP> <10048@pt.cs.cmu.edu> <1990Jul30.231835.13898@diku.dk> <10055@pt.cs.cmu.edu> Sender: pcg@aber-cs.UUCP Organization: Coleg Prifysgol Cymru Lines: 120 In-reply-to: lindsay@MATHOM.GANDALF.CS.CMU.EDU's message of 31 Jul 90 16:05:10 GMT "lindsay" == Donald Lindsay ??? writes: lindsay> In article <1990Jul30.231835.13898@diku.dk> njk@diku.dk lindsay> (Niels J|rgen Kruse) writes: njk> Specifically, it would be nice if we scanned a laser beam over the njk> media, rather than rotated the media. Or, at least, did the "head" njk> movement that way. The general idea is that the magnetic field is roughly spherical in emission, so you want to keep your head near the surface to cut the smallest most intense intersection with it. Coherent light instead can be focused, even from a great distance. This changes the rule of the game completely, but also causes culture shock to designers (see later). njk> There was hope for this sort of thing, a decade ago, and somehow njk> it never happened. I believe that the best spatial modulators had njk> very limited angular effect (...) I had read that Gary Kildall, The founder of Digital Research, had got into research on very fast CD-ROM readers based on flipping mirrors. Some japanese companies are also doing research on this. njk> Would you care to elaborate on this? What kind of spatial modulators? njk> Even if the angular effect is small, wouldn't it be a simple njk> matter to increase it with some sort of lens arrangement? lindsay> It used to be that one scanned a laser beam by pointing a fixed beam lindsay> at a rotating mirror - this was the technology inside laser printers. lindsay> Supermarket barcode readers used a rotating film, with a hologram lindsay> printed on it. I think that there are several technologies that can be used: Use a traditional positioning arm, but a very low mass one with a lightguide on it. Current optical drives often have the laser diode and a mirror on the head itself. I have read agument for keeping everything close to the surface, but they sound bizarre to me. A laser printer type system, with a rotating mirror for horizontal scanning and a rotating drum for vertical scanning. we have 800x400 DPI printers around, wih a paper surface of say 8.5x11 inches. This gives us around 100 inches of surface, at around 300Kb per square inch, for 30Mbit of storage, i.e. around 4 MBytes. Probably this can be improved by a factor of 100 easily. A system of piezoelectric crystals with mirrored surfaces, scanning at some dozen KHz. Current optical disc drives have an oscillating mirror on the head to cover quickly a band of tracks. A piezoelectric mirror coated crystal could do. Magnetically suspended mirrors with very low mass. IBM built a single chip magnetically operatated mirror on a chip, using sophisticated etching, suspended on two torsion bars of silicon. Its control electronics were around it on the chip. Limited deflection angle, but apparently good enough for laser printers (the intended application). Frequency can be pretty high. Fabry-Perot ineterferometers used not only to modulate, but also to encode the signal. Scanning and encoding frequencies on the order of the GHz range easily accessible (with attendant recording densities). Use of focusing holograms is a variant on the theme. Hologram like recording techniques. Long a dream; three dimensional, extreme density. There have been recent advances, mostly about pattern matching on a fairly gross scale, but also apparently somebody has found ways to use a three dimensional hologram as a storage device. lindsay> I think that the problem with limited angular effects was also lindsay> the limited angular resolution: that is, not enough bits in the lindsay> seekable address space. If that's still true, then lens lindsay> arrangements only fix the less-important limitation. This cannot be. The limited deflection is not a big problem; you can get deflection angles of 2.5 degrees, and these are enough. If you are good at optics, and have money, you can go far. You can focus very precisely from a couple yards away, and a couple yards away can mean enough surface covered at the densities you aim for. I think that there are two problems, one hard and technological, and the other harder and sociological: Limited bandwidth of recording material Reading from an optical medium requires very fast laser modulation. This is not terribly hard. What is hard is to find a recording material tha can change state very quickly, and with low power, and that does not forget the change of state that quickly. If you Have very fast scanning you can refresh, but other problems ensue. The technology requires optics prowess Most all the current mass storage designers are EE engineers, not physicists. In designing an optical scanning system you need high competence in optics. People with this competence do exists, but they don't work for mass storage companies. There is also the problem that really high quality laser optics seems to hinge on advances in phase conjugated mirrors, and those are slow to come. Another problem may be that advanced optics research, especially laser optics, is almost all military funded and very classified. If high bandwidth optical systems come of age, computer centres had better reinforce floors: granite cabinets will become common (like in a prototype IBM 370 with an optics backplane of years ago). -- Piercarlo "Peter" Grandi | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk Dept of CS, UCW Aberystwyth | UUCP: ...!mcsun!ukc!aber-cs!pcg Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk