Xref: utzoo sci.electronics:18466 comp.dsp:1405 Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!newstop!eastapps!vergil!gsteckel From: gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) Newsgroups: sci.electronics,comp.dsp Subject: Re: A question about the Nyquist theorm Summary: visible lasers make denser CDs Message-ID: <4869@eastapps.East.Sun.COM> Date: 14 Mar 91 20:33:14 GMT References: <20408@shlump.nac.dec.com> <625@ctycal.UUCP> <11515@pasteur.Berkeley.EDU> <1180@aviary.Stars.Reston.Unisys.COM> <504@dcsun21.dataco.UUCP> Sender: news@East.Sun.COM Reply-To: gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware CONTRACTOR) Followup-To: sci.electronics Distribution: na Organization: Omnivore Technology, Newton, Mass. (617)969-3448 Lines: 38 In article <504@dcsun21.dataco.UUCP> mcphail@dcsun18.UUCP (Alex McPhail,DC ) writes: > >Actually, this is not true. You can not increase the density of information >on a compact disk without changing the technology. Right now, each bit >of information occupies an area 1.6 microns square (ie adjacent bits must >be seperated by at least 1.6 microns). If you attempt to compress the >data using closer seperation, the optical interference patterns will produce >intolerable noise on adjacent bits, even with oversampling. Oversampling has nothing to do with data recovery off the disk. Oversampling is a technique to make the engineering job of reconstructing the output waveform easier or cheaper. It is done after the data stream has been recovered off of the medium. >You must use >a much higher frequence laser (producing a higher energy output, thus >requiring more robust material in the compact disk, thus requiring even >higher energy writing lasers, etc., etc.) to achieve a closer seperation >of information in the compact disks. A couple of misconceptions here: 1) a higher frequency reading laser does not need any change in the read-only CD materials. A read-write CD might require some change in composition. Just because the photons have higher energy doesn't mean that 1 milliwatt of green light affects an aluminized reflector any more than 1 milliwatt of infrared. Currently 5 milliwatt orange-red semiconductor lasers are available on the surplus market, which are a good deal brighter than you need to read CDs. A green laser (semiconductor) has been announced by several companies. I expect to see them in products as soon as a standard for denser disks is hashed out. 2) read-only CDs are molded from a master, not written with a laser, and would require only good quality control to be produced with 50% smaller pits. geoff steckel (gwes@wjh12.harvard.EDU) (...!husc6!wjh12!omnivore!gws) Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line. This posting is entirely the author's responsibility.