[cctalk] Re: FWIW CD & DVD demagnitizitation [was: Double Density 3.5" Floppy Disks]

A CD or DVD "demagnetizer" is by definition a scam and an utter fraud. Those media are non-magnetic and in any case magnetism plays no role whatsoever in reading them. Also, keep in mind that something may look like a review but it's actually a press release, perhaps slightly warmed over. They go way beyond checksums, using sophisticated ECC schemes. There's a good reason why a scratched disk can, in most cases, be read without trouble. I remember reading years ago that someone showed off the CD ECC scheme by drilling a hole into a CD (2-3 mm or so) -- it read just fine anyway. So yes, unless the disk is damaged beyond the power of the ECC, it will read correctly every time. And even if it does exceed the ECC, it will in most cases read the same, though some bits of data will be unrecoverable. You'd have to go WAY beyond the ECC limits to reach the point of undetected data errors, i.e., a misreading of the data that the ECC doesn't catch (let alone correct). That property holds for all codes, in fact. Every one of them has a set of error patterns it will detect, a set it will correct, and a set it will miss. The design challenge for codes is (a) understand the likely error patterns it will be confronted with in the wild, (b) understand the required probabilities for (1) uncorrectable and (2) undetected errors, and (c) to create a good code that delivers on these requirements efficiently. "Efficiency" is defined by coding overhead as well as implementation cost. What an amazing pile of bunk. Some years ago I was joking about the possibility that someone would sell gold plated fiber optic cables to suckers like that. Imagine my surprise when, somewhat later, I spotted Monster fiber optic cables with gold-plated connectors. paul