Path: utzoo!attcan!uunet!unsvax!jimi!tahoe!apple!goofy.apple.com!esmith From: esmith@goofy.apple.com (Eric Smith) Newsgroups: comp.arch Subject: Re: CD-ROM documents (was Paperless Office) Message-ID: Date: 30 Nov 90 20:59:39 GMT References: <0094070F.B370E000@KING.ENG.UMD.EDU> <1181@dms.UUCP> Sender: usenet@Apple.COM Organization: Frobozz Magic Widget Company Lines: 57 In-reply-to: albaugh@dms.UUCP's message of 30 Nov 90 00:33:33 GMT In article <1181@dms.UUCP> albaugh@dms.UUCP (Mike Albaugh) writes: > Which might, of course, revive the raging flame-war from > sci.electronics about the increase in error rate with time on this > supposedly "read only" media. I don't mind the occasional drop out > in ZZ top, and probably wouldn't even see the "interpolation" that > audio CDs do when they are a little more confident that the errors > were "small", but I don't really want to deal with "just a few little > errors" in, say, "as" or "ld". :-) > BTW: I know about error correcting codes, etc, but the story from > the audiophiles is that the little "correction light" (_If_ you have > one) tends to light more and more often with age (of the disk), which > perhaps explains why newer decks tend not to have them. "Don't lets alarm > the users, now" :-). And a n-bit correction scheme with n-1 _known_ bad > bits gets us a bit close to the edge... I agree that one wrong bit is too many, but the error correction in CD-ROM is up to the challenge. Keep in mind that CD-Audio players have two levels of error correction (real honest-to-God correction, as in get the original data back). Error concealment (interpolation) is only used when the error correction fails. Many CD-Audio players don't implement the maximum possible error correction, because the manufacturers deem it acceptable to have error concealment used with moderate frequency. CD-ROM incorporates one additional layer of error correction above and beyond the error correction of CD-Audio. CD-ROM drives don't use concealment (except when playing audio CDs), but they still use the error correction of CD-Audio, and usually implement it fully. CD-ROM drives also typically have better tracking than CD-Audio players, which reduces the error rate considerable from its already low level. Most uncorrectable errors on CDs are the result of pressing defects. CD-ROMs are less succeptible to this as described above. I expect that more care is taken when CD-ROMs are pressed, although I have no evidence to support this. Every case of bit-rot on an Audio CD that I have seen has been due to improper handling of the CD. Most people don't realize that the label side of the CD is actually much more succeptible to damage than the "data" side. There is only an extremely thin layer between the aluminum and the label of the disk. *Don't* write on or scratch the top of a disk within the data area! (I write my name immediately around the center hole where no data is stored). The error correction for CD-Audio is interleaved. On a hard disk, a burst error of sufficient length can wipe out a block. On a CD, the bytes that make up a block are not consecutive on the disk, so a burst will really just cause a single correctable errors in each of many blocks. The error correction is capable of dealing with multiple errors per block. When you add the third layer of correction, the error rate for CD-ROM becomes lower than that of any other current data storage medium (that I've heard of). Would you rather trust your archives to floppies? Hard disk? Quarter or half inch tape? I'll take CD-ROM any day. -- Eric L. Smith Opinions expressed herein do not necessarily reflect those esmith@apple.com of my employer, friends, family, computer, or even me! :-) Brought to you by Super Global Mega Corp .com